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Status and Trends of Birds in an Extensive Western Massachusetts Forest
Bradford G. Blodget, Randy Dettmers, and John Scanlon

Northeastern Naturalist, Volume 16, Issue 3 (2009): 423–442

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2009 NORTHEASTERN NATURALIST 16(3):423–442 Status and Trends of Birds in an Extensive Western Massachusetts Forest Bradford G. Blodget1,2, Randy Dettmers3,*, and John Scanlon1 Abstract - We report forest bird population trends from 1987−2005 at a 280.7- ha site in Chester, MA. Our study site consisted primarily of mid-successional (70−90 yr old) forest in a landscape of similar, non-fragmented forest. Using point counts at 100 stations, we detected 94 species in early June over all years. Mean numbers of species and detections/yr were 57 and 1104, respectively. Species richness and total detections were stable (P ≤ 0.05). We analyzed trends for the 36 most frequently detected species. Four species exhibited significant increasing trends and 10 significant declining trends; 22 species were stable or exhibited no significant trend (P ≤ 0.05). Whether a species was a resident, short-distance, migrant, or Neotropical migrant did not appear to be a determinative factor for trends. Trends were more readily explained by (I) areal and structural expansion of mature forest conditions and (II) forestry treatments that affected 70 ha (25%) of the study site. We report few changes in species abundances that could not be plausibly explained by intra-site habitat changes and no mysterious or alarming changes in the first 19 years of this study. Introduction Although the decline of Neotropical migrants at many small, isolated forest sites in eastern North America since the 1950s is incontrovertible (Askins 2000, Askins et al. 1990, Robbins et al. 1989), establishing whether these declines are attributable to site-level changes in conditions or to larger processes affecting entire populations has been more difficult. Hence, there continues to be a critical need to conduct long-term studies (Hall 1984a, b; Johnston and Hagan 1992; Terborgh 1992), particularly in extensive forest tracts in generally non-fragmented landscapes (Askins et al. 1990). Within heavily forested landscapes in the Northeast, such studies are rare. In mature deciduous forestlands in the eastern United States, Johnston and Hagan (1992) found only 13 breeding-bird census data sets spanning a decade or more in the published literature. Only six exceeded 20 years. Most were aperiodic. Only one of the 13 long-term sites was in the New York/New England area. This site, a breeding-bird census on a 10-ha mature northern hardwood forest plot within Hubbard Brook Experimental Forest in West Thornton and Woodstock, NH, has been conducted annually since 1969 (Holmes et al. 1986; Holmes and Sherry 1988, 2001). 1Massachusetts Division of Fisheries and Wildlife, 1 Rabbit Hill Road, Westborough, MA 01581-3337. 2Current address - 1246 Main Street, Holden, MA 01520-1020. 3US Fish and Wildlife Service, 300 Westgate Center Drive, Hadley, MA 01035-9589. *Corresponding author - randy_dettmers@fws.gov. 424 Northeastern Naturalist Vol. 16, No. 3 We established an open-ended, forest bird population-trend study to test the hypothesis that populations of forest-nesting Neotropical migratory passerines in non-fragmented forest in western Massachusetts are stable and that fluctuations in their numbers may often be explained by changes in intra-site conditions. In this paper, we document the bird community on our study site over a 20-year period (1986−2005) and examine trends for the period from 1987−2005 using an index to abundance derived from 100 point counts conducted along transects. We believe our study site and extensive sampling scheme are large enough to potentially buffer effects of intra-site perturbations as well as factors influencing detectability at stations. Such an approach has been recommended by Bart et al. (1998) to help keep the index ratio (number detected:number present) relatively constant across years and to minimize the biases in trend estimates. However, acknowledging that not all perturbations and factors influencing detectability at stations can be adequately buffered by appropriate sampling design, we have incorporated certain likely sources of variability into our data analysis. Our objectives were to: (1) establish baseline data on the occurrence and abundance of forest-nesting bird species on a relatively remote and ecologically stable forested site; and (2) establish and implement a long-term monitoring system to detect (a) long-term population trends and (b) short-term population changes in response to natural and humancaused perturbations. Field-site Description Our approach to study-site selection was governed both by our interest in tracking populations of forest-nesting Neotropical passerines in a nonfragmented forest and by a desire to create an open-ended, long-term study. We sought a site meeting the following prerequisites: (1) non-fragmented forest except for transient openings created by natural events and modest forestry activities, (2) located in a similarly non-fragmented forest landscape, (3) buffered from most human activities other than forestry, and (4) stable as indicated by a high percentage of the surrounding landscape being in conservation ownership or control. The study site is a 280.7-ha polygon within the 1250-ha Hiram H. Fox Wildlife Management Area (HyFox) in Chester, MA (42°20'N, 72°54'W). Most of the area was cleared for pasture and cropland in the 1700s and then abandoned in the mid- to late 1800s. Reforestation of the abandoned fields and pastureland commenced about 1850, and sawtimber forest became reestablished by the early to mid-1900s (Foster 1992). The site is characterized by moderate and occasional steep slopes interspersed with both upland flats and abrupt ledge outcrops at elevations of 210−387 m. Aspects are primarily east and west. 2009 B.G. Blodget, R. Dettmers, and J. Scanlon 425 HyFox is situated in a landscape characterized by large tracts of nonfragmented forest and sparse human population. In 1999, 87.6% (28,885 ha) of the surrounding four towns (Chester, Chesterfield, Huntington, and Worthington) was forest or vegetated wetland (MGIS 2006). In 2000, the four towns had a combined human population of 5953, with a mean density of 18.2/km2 (US Census Bureau 2001). Approximately 36.2% of the four-town land area was protected open space in 2003 (MGIS 2006). Forest on the study site consists primarily of northern hardwoods (155 ha; 55.1%), northern hardwoods−hemlock−white pine (87 ha; 31%), and northern hardwoods−white pine (13 ha; 4.6%) (Massachusetts Division of Fisheries and Wildlife [MDFW], Westborough, MA, unpubl. 2000 land cover data). A 1986 inventory of forest trees revealed a well-diversified, 70–90 yr old, mid-successional (small sawtimber) forest. The distribution of tree size classes was <1% seedling forest (mean dbh <1.27 cm), 18% sapling/ polewood forest (mean dbh = 1.27−28.96 cm), and 82% sawtimber forest (mean dbh >28.96 cm) (MDFW, unpubl. data). The study site is almost exclusively upland forest (267.6 ha; 95.3%), except for seven beaver flowages and associated non-forested wetlands along Moss Meadow Brook (9.5 ha; 3.4%) and 11 scattered units of palustrine forest (3.6 ha; 1.3%). Coniferous swamp (1.5 ha) and red maple−northern hardwood swamp (1.3 ha) are the dominant palustrine forest types (Fig. 1). Figure 1. Forest cover types and cutting history on the HyFox study site and surrounding areas during the study period in relation to transects and point-count stations. 426 Northeastern Naturalist Vol. 16, No. 3 From 1987−1990, 70 ha (25%) of the study site received forestry treatment: 15 ha of large pole and sawtimber forest were clearcut (5% of study area; 10 separate clearcuts averaging 1.5 ha each) and 55 ha of sawtimber forest were partially cut under a shelterwood system that removed 40−50% of the basal area, but retained evenly distributed, large diameter, seed trees (Smith 1986). Two adjacent clearcuts totaling 3.6 ha established in 1987 were briefly held in seedling stage by a prescribed burn program in 1989−1995, but had largely reverted to sapling/small polewood stage by 2000. The remaining eight clearcuts (11.4 ha) were established in 1988−1990 and were allowed to succeed naturally (Fig. 1). Reflecting the forestry treatments, the distribution of tree size-classes on the study site in 1993 was about 13% seedling forest, 12% sapling/ polewood forest, and 75% sawtimber forest. An assessment in 2000 revealed a mix of less than 1% seedling forest, 17% sapling/polewood forest, and 83% sawtimber forest on the study site (MDFW, unpubl. data). Virtually all the seedling forest had advanced to the sapling/polewood stage and about two-thirds of older polewood forest had attained small sawtimber status. In addition to forestry disturbance, the only other discernible forest changes (1986−2005) resulted from a microburst in 1995 that felled or damaged individual trees at 18 of 100 stations, resulting in small (less than 0.1 ha) openings in some areas. Methods Sampling design Although our study began in 1986, preceding the appearance of point-count standards, our survey methodology substantially follows the recommendations of Ralph et al. (1993, 1995). Using a systematic sampling design, we established 100 stations along 13 parallel transects, each with a random starting point (Fig. 1). Transects are 200 m apart and perpendicular to the prevailing topography. The mean inter-station distance along transects is 124 m (range = 83.5−226.1 m). As a consequence of being nonconforming with point-count standards for the spacing of stations (150−200 m), our sampling design results in varying degrees of overdetection of species, especially the more vociferous ones, along transects. We interpret the number of detections for a species as an index value, not an actual number of individuals present, and presume the overdetection effects to be sufficiently similar across years so as not to produce bias that would affect the outcome of our statistical analysis for trends. Initial results from randomization exercises (R. Dettmers, unpubl. data; following methods described in Manly 2007) suggest that even modest changes between years in the relationship between the actual number of individuals present and the number of detections due to overdetection effects from our sampling design should not significantly alter the results of our statistical analysis for trends. We consider the sampling transects (200 m apart) to represent independent samples of the HyFox study area and claim no statistical 2009 B.G. Blodget, R. Dettmers, and J. Scanlon 427 inference for population trends beyond this area. We present comparisons of our trends with regional trends based on breeding-bird survey (BBS) data (Sauer et al. 2005) for discussion purposes only and acknowledge that our trends do not represent regional population trends. Survey specifications Surveys were conducted during the nesting season from 1−14 June on cool, calm mornings (temperatures = 7−22 °C [42−72 °F], winds <10 km/hr, and with no precipitation) between 0530−0900 hrs. Stations were visited once annually and the sampling order varied from year to year. Stations were visited by a single observer for exactly 3 min. Three observers were used each year, and there were four different observers during the study period. All birds detected (seen and heard) at unlimited distances were recorded, exercising care to avoid recording the same individual twice at a station. There was no waiting period at stations. No “pishing,” tape recorders, or attracting devices were used. Station dwell time could be extended, at the observer’s discretion, for up to 1 min to compensate for noise such as aircraft or flowing water. If noise became unacceptable due to wind or precipitation, work was suspended. Data analysis A list of species detected at the 100 stations over 20 years (1986−2005) was compiled, with each species classified according to its migratory group (north temperate resident, short-distance migrant, or Neotropical migrant), residency, and presumed nesting status on the study site (Supplementary Appendix 1, available online at https://www.eaglehill.us/nena/nenasuppl- files/n16-3-Dettmers-s1, and, for BioOne subscribers, at http:// dx.doi.org/10.1656/N764.s1). In assigning species to the Neotropical migrant group, we followed Rappole et al. (1983), except that Neotropical migrants that also overwinter over significant areas of the North Temperate Zone (American Ornithologists’ Union 1998) were classified with the short-distance migrant group. Pilot-year (1986) data were not included in our analyses to eliminate the “novice effect” (Kendall et al. 1996), and no sampling was conducted in 1998. We prepared descriptive statistics for all species detected (Supplementary Appendix 2, available online at https://www.eaglehill.us/nena/nena-supplfiles/n16-3-Dettmers-s2, and, for BioOne subscribers, at http://dx.doi. org/10.1656/N764.s2). In this paper, we present descriptive statistics and population-trend analyses for 36 species (those with a mean of ≥4 detections/ yr for the 100-stations), organized according to their migratory group (see above). We used the total number of detections/yr from the 100 stations as our metric of relative abundance for the descriptive statistics, which are provided solely for the purposes of making relative comparisons among species—not for statistical tests. We calculated descriptive measures of frequency of occurrence (number of years detected and mean stations/yr) and abundance (median, range, and mean [±SD] detections/yr) for each species. 428 Northeastern Naturalist Vol. 16, No. 3 For population trend analysis of individual species, we considered transects (Fig. 1) as the primary sampling units. For analysis, we combined stations from the two northernmost transects because of the small number of stations on these transects. For each transect (n = 12), we calculated the total number of detections for each species in each year and used this as our index of abundance for assessing trends in abundance over time. Stations were not used as sampling units because some were <150 m apart, and vociferous individuals recorded at one station could be recorded at a subsequent station. Our analysis accommodated transect effects, as well as observer effects, time of day, and survey date, to account for some of the factors known to influence detection probabilities. Specifically, we fit an overdispersed Poisson regression model of the index of transect abundance as a function of year, transect, observer, time of day, and date of survey: Log(Transect Index) = β0 + β1*year + β2*transect + β3*observer + β4*time + β5*date Regression analyses were performed using PROC GENMOD (SAS Institute 1989), with the DIST option set to the Poisson distribution, the LINK option set to the log function, and using the SCALE option to specify an overdispersed model. Transect and observer variables were treated as categorical variables, and all others were treated as continuous variables. A trend for a species was considered significant if β1 differed significantly from zero (P ≤ 0.05), based on F-tests. We calculated % annual change using the transformation: ([exp(β1 - 0.5*variance)] - 1)*100, where variance is the square of the standard error of β1 (Holmes and Sherry 1988). In addition to analyzing population trends for each species, we analyzed trends in total number of species detected and total detections. We calculated total number of species detected and total detections across all stations surveyed in a year and tested for significant trends in these two parameters over time by fitting an exponential curve to these data using the regression method described by Holmes and Sherry (1988). For comparison at the regional level, we compared population trends of our 36 tested species with the same species on 52 BBS routes in the Northern New England Physiographic Region (BBS Region 27) for years 1987−2005 (Sauer et al. 2005). BBS trends were calculated on the basis of the linearroute regression method (Peterjohn et al. 1997), which yielded bootstrap estimates for each species of the median “trend” from which the average percentage annual change was calculated. Statistical significance was determined with z-tests. Results Description of baseline bird community Overall, 94 species were detected on our study site, including 16 (17%) north temperate residents, 35 (37%) short-distance migrants, and 43 (46%) 2009 B.G. Blodget, R. Dettmers, and J. Scanlon 429 Neotropical migrants. Classified by status, 15 were permanent residents, 73 summer residents, and six passage migrants. At least 74 species likely nested, including 17 species of wood-warblers (Parulidae) and seven species of flycatchers (Tyrannidae) (Supplementary Appendix 1, available online at https://www.eaglehill.us/nena/nena-suppl-files/n16-3-Dettmers-s1, and, for BioOne subscribers, at http://dx.doi.org/10.1656/N764.s1). Twenty-eight species were detected in all 18 yrs, 39 in either 17 or 18 yrs, 16 species in only 1 yr, and 20 in either 1 or 2 yrs. Twenty-two species (23%) had ≥10 mean detections/yr., 50 (53%) had ≤3 mean detections/yr, and 36 species occurred at trace levels (≤1.0 mean detections/yr). We present descriptive statistics for the 36 most abundant species (≥4 mean detections/yr) in Table 1. Vireo olivaceus (Red-eyed Vireo) and Seiurus aurocapilla (Ovenbird), with mean detections/yr of 187.6 and 165.3, respectively, were the most frequently detected species and accounted for 32% of detections. Neotropical migrants accounted for 75% of detections. Ranked by mean detections/ yr, the top 10 species accounted for 64% of all detections. Another 10 species made limited appearances in the lists of the 10 most frequently detected species for each year (Table 2). Several species exhibited pronounced spikes in abundance in single years or over several years following forestry treatments, but were absent or present in relatively low abundance in most years, resulting in mean values ≥25% greater than median values due to highly skewed distributions in annual abundances (e.g., Corvus brachyrhynchos [American Crow], Dendroica pensylvanica [Chestnut-sided Warbler], D. magnolia [Magnolia Warbler], Oporornis philadelphia [Wilson] [Mourning Warbler], Zonotrichia albicollis [Gmelin] [White-throated Sparrow] and Molothrus ater [Brown-headed Cowbird]). Many of the less frequently detected species exhibited considerable year-to-year fluctuation, with low mean and median detections and high standard deviations. However, even the 10 most abundant species had standard deviations that averaged 34.7% (range = 10.2−90.4%) of mean detections. Trends in species richness and total detections The mean number of species detected annually was 57 (range = 49−64) and was stable over time (slope = -0.003; P = 0.34). From 1990 (the last year of forestry activity) through 1997 (8 yrs), the mean number of species recorded annually was 60.4 compared to means of 55.3 and 55.0 in the three preceding and five subsequent survey years, respectively. The mean number of total detections/yr was 1104 (range = 957−1272) and remained stable (slope = -0.002; P = 0.61). A short-term increase in total abundance was evident from 1990−1997, with 1157.5 mean total detections/ yr compared to 1066 and 1050 in the three preceding and five subsequent survey years, respectively. Nineteen-year population trends Of the seven resident species analyzed, six species were stable or exhibited no significant trend, and one species, Certhia americana (Brown 430 Northeastern Naturalist Vol. 16, No. 3 Table 1. Frequency of occurrence (number of yrs and number of stations/yr) and abundance (median and mean detections, range, and variability in abundance) of 36 forest-nesting birds detected at 100 stations within Hiram Fox Wildlife Management Area, Chester, MA, 1987−1997 and 1999−2005. Frequency Years Mean stations/yr Abundance Bird species (n =18) (n =100) Median (range) Mean ±SD North temperate residents Picoides pubescens (L.) (Downy Woodpecker) 17 4.4 4.5 (0–12) 4.83 ± 3.57 Picoides villosus (L.) (Hairy Woodpecker) 17 4.6 4.0 (0–11) 4.72 ± 2.74 Cyanocitta cristata (L.) (Blue Jay) 18 20.2 20.5 (4–50) 24.72 ± 12.34 Corvus brachyrhynchos Brehm (American Crow) 14 4.2 3.0 (0–16) 4.28 ± 4.44 Poecile atricapillus (L.) (Black-capped Chickadee) 18 25.9 31.0 (10–66) 33.83 ± 14.26 Sitta canadensis Latham (White-breasted Nuthatch) 18 5.9 5.0 (2–14) 5.94 ± 3.02 Certhia americana Bonaparte (Brown Creeper) 18 11.4 9.0 (1–28) 10.44 ± 6.79 Short-distance migrants Sphyrapicus varius (L.) (Yellow-bellied Sapsucker) 18 38.8 40.5 (18–92) 45.28 ± 19.60 Vireo solitarius (Wilson) (Blue-headed Vireo) 18 13.3 13.5 (4–27) 13.72 ± 6.24 Troglodytes troglodytes (L.) (Winter Wren) 18 15.3 13.0 (2–38) 15.61 ± 9.34 Catharus guttatus (Pallas) (Hermit Thrush) 18 23.3 22.0 (12–58) 26.11 ± 12.63 Turdus migratorius L. (American Robin) 18 15.0 16.0 (7–32) 17.22 ± 7.32 Dendroica coronata (L.) (Yellow-rumped Warbler) 18 7.8 7.5 (3–14) 7.61 ± 3.07 Junco hyemalis (L.) (Dark-eyed Junco) 16 8.0 7.0 (0–28) 8.50 ± 8.40 Molothrus ater (Boddaert) (Brown-headed Cowbird) 18 4.9 3.5 (1–14) 5.28 ± 4.18 Carduelis tristis (L.) (American Goldfinch) 17 5.2 5.0 (0–12) 5.33 ± 3.07 2009 B.G. Blodget, R. Dettmers, and J. Scanlon 431 Table 1, continued. Frequency Years Mean stations/yr Abundance Bird species (n =18) (n =100) Median (range) Mean ±SD Neotropical migrants Contopus cooperi (L.) (Eastern Wood-Pewee) 18 26.7 25.5 (22–41) 27.89 ± 6.28 Empidonax minimus (Baird and Baird) (Least Flycatcher) 18 11.0 14.0 (1–27) 14.00 ± 8.26 Miarchus crinitus (L.) (Great Crested Flycatcher) 17 5.1 5.0 (0–11) 4.94 ± 2.73 Vireo olivaceus (L.) (Red-eyed Vireo) 18 97.7 182.5 (132–238) 187.56 ± 28.01 Catharus fuscescens (Stephens) (Veery) 18 41.5 50.5 (36–70) 50.83 ± 8.96 Hylocichla mustelina (Gmelin) (Wood Thrush) 18 37.2 45.0 (28–66) 43.83 ± 9.16 Dumetella carolinensis (L.) (Gray Catbird) 18 4.7 4.0 (1–13) 4.89 ± 3.41 Dendroica pensylvanica (L.) (Chestnut-sided Warbler) 18 20.6 18.5 (1–77) 30.22 ± 27.33 Dendroica magnolia (Wilson) (Magnolia Warbler) 17 6.8 4.5 (0–23) 7.33 ± 6.32 Dendroica caerulescens (Gmelin) (Black-throated Blue Warbler) 18 15.6 18.5 (2–30) 17.17 ± 8.51 Dendroica virens (Gmelin) (Black-throated Green Warbler) 18 48.4 55.0 (27–111) 63.78 ± 27.95 Dendroica fusca (Muller) (Blackburnian Warbler) 18 24.1 27.5 (6–49) 26.61 ± 13.54 Mniotilta varia (L.) (Black-and-white Warbler) 18 33.3 33.0 (15–51) 33.83 ± 9.81 Setophaga ruticilla (L.) (American Redstart) 18 38.8 53.5 (20–82) 52.61 ± 18.32 Seiurus aurocapilla (L.) (Ovenbird) 18 91.0 167.0 (131–196) 165.33 ± 16.93 Seiurus noveboracensis (Gmelin) (Northern Waterthrush) 17 4.1 4.0 (0–14) 4.61 ± 3.94 Geothlypis trichas (L.) (Common Yellowthroat) 18 19.6 21.5 (4–41) 22.33 ± 9.54 Wilsonia canadensis (L.) (Canada Warbler) 18 9.8 9.5 (3–24) 9.67 ± 5.27 Piranga olivacea (Gmelin) (Scarlet Tanager) 18 27.4 27.5 (16–43) 28.5 ± 9.15 Pheucticus ludovicianus (L.) (Rose-breasted Grosbeak) 18 9.9 9.5 (3–22) 9.94 ± 5.32 432 Northeastern Naturalist Vol. 16, No. 3 Creeper), declined significantly (Table 3). Nine short-distance migrants were analyzed, of which six were stable or exhibited no significant trend. Sphyrapicus varius (Yellow-bellied Sapsucker) increased significantly. Junco hyemalis (Dark-eyed Junco) and Catharus guttatus (Hermit Thrush) declined significantly. The junco exhibited the steepest negative regression slope of all species tested (Table 3). Of the 20 Neotropical migrants, half were stable or exhibited no signifi- cant trends. Magnolia Warbler increased significantly and had the steepest positive regression slope of all species tested. Significant increases were also found for D. virens (Black-throated Green Warbler) and Red-eyed Vireo (Table 3). Empidonax minimus (Least Flycatcher), Chestnut-sided Warbler, Mniotilta varia (Black-and-white Warbler), Setophaga ruticilla (American Redstart), S. noveboracensis (Northern Waterthrush), Geothlypis trichas (Common Yellowthroat), and Wilsonia canadensis (Canada Warbler) all declined significantly (Table 3). Six species that did not reach our threshold for statistical analysis made remarkable short-term appearances on our study site from 1987−1997. Primarily early successional species, these included E. alnorum Brewster (Alder Flycatcher), V. flavifrons Vieillot (Yellow-throated Vireo), Vermivora pinus (L.) (Blue-winged Warbler), Mourning Warbler, White-throated Sparrow, and Passerina cyanea (L.) (Indigo Bunting). Pipilo erythrophthalmus Table 2. Species appearing in the annual lists of the top-ten most frequently detected species, along with the maximum and minimum detections in a year, the mean detections/yr, and the number of yrs detected. Detections are from 100 stations within Hiram Fox Wildlife Management Area, Chester, MA, 1987−1997 and 1999−2005. Highest Yrs. in rank top-10 Yrs. Species Max. Min. Mean achieved group detected Red-eyed Vireo 238 132 187.56 1 18 18 Ovenbird 196 131 165.33 1 18 18 Black-throated Green Warbler 111 27 63.78 3 17 18 American Redstart 82 20 52.61 3 17 18 Veery 70 36 50.83 3 18 18 Yellow-bellied Sapsucker 92 18 45.28 3 14 18 Wood Thrush 66 31 43.83 5 17 18 Black-capped Chickadee 66 10 33.83 3 11 18 Black-and-white Warbler 51 21 33.83 4 14 18 Chestnut-sided Warbler 77 1 30.22 3 8 18 Scarlet Tanager 43 16 28.50 6 9 18 Eastern Wood-Pewee 41 22 27.89 8 6 18 Blackburnian Warbler 49 6 26.61 4 7 18 Hermit Thrush 58 12 26.11 5 4 18 Blue Jay 50 4 24.72 4 6 18 Common Yellowthroat 41 4 22.33 6 4 18 American Robin 30 7 17.22 10 1 18 Black-throated Blue Warbler 30 2 17.16 10 3 18 Winter Wren 38 2 15.61 8 2 18 Blue-headed Vireo 27 4 13.72 10 1 18 2009 B.G. Blodget, R. Dettmers, and J. Scanlon 433 (L.) (Eastern Towhee), though not captured in our sample, was known to be present on the study site in 1992 (B.G. Blodget, pers. observ.). In addition, Table 3. Population trend estimates for 36 forest-nesting bird species, arranged by migratory group membership and ranked from greatest positive to greatest negative regression slope, at Hiram Fox Wildlife Management Area, Chester, MA, 1987−1997 and 1999−2005 (n=18) and compared with Breeding Bird Survey (BBS) Routes in Physiographic Region 27 (Northern New England), 1987−2005 (n =19). NC denotes no change. Hiram Fox Wildlife Management Area BBS Region 27 (1987−1997, 1999−2005) (1987−2005) Regression % annual % annual Species slope P-value change1 change1 North temperate residents American Crow 0.036 0.250 3.6 1.7*** Hairy Woodpecker 0.003 0.910 0.2 -1.7 White-breasted Nuthatch 0.002 0.930 0.2 3.9* Black-capped Chickadee -0.001 0.914 -0.2 -0.4 Blue Jay -0.023 0.135 -2.3 0.5 Downy Woodpecker -0.048 0.064 -4.7 2.7* Brown Creeper -0.082 0.0001 -7.9*** -4.0 Short-distance migrants Yellow-bellied Sapsucker 0.041 0.0001 4.2*** 5.4*** Yellow-rumped Warbler 0.006 0.752 0.6 -0.8 Brown-headed Cowbird 0.003 0.913 0.2 -4.0*** American Robin -0.005 0.742 -0.5 -0.2 Blue-headed Vireo -0.017 0.321 -1.7 2.1 Winter Wren -0.019 0.272 -1.9 -2.2* American Goldfinch -0.029 0.219 -2.9 3.3*** Hermit Thrush -0.039 0.008 -3.9** -1.3 Dark-eyed Junco -0.164 0.0001 -15.1*** -3.7** Neotropical migrants Magnolia Warbler 0.129 0.0001 13.7*** -5.2 Black-throated Green Warbler 0.056 0.0001 5.8*** 3.3** Black-throated Blue Warbler 0.018 0.247 1.8 -0.1 Red-eyed Vireo 0.016 0.0001 1.6*** 1.1** Blackburnian Warbler 0.015 0.198 1.5 -2.3 Wood Thrush 0.014 0.185 1.4 -4.6*** Great Crested Flycatcher 0.006 0.822 0.5 -1.7 Rose-breasted Grosbeak 0.0003 0.987 NC -1.4 Veery -0.001 0.917 -0.1 -2.9*** Ovenbird -0.006 0.114 -0.6 -0.3 Gray Catbird -0.018 0.359 -1.8 -1.8*** Eastern Wood-Pewee -0.020 0.085 -2.0 -4.3*** Scarlet Tanager -0.020 0.108 -2.0 NC American Redstart -0.028 0.003 -2.8** -4.0*** Black-and-white Warbler -0.044 0.0001 -4.3*** -4.2*** Canada Warbler -0.045 0.016 -4.4* -4.2* Northern Waterthrush -0.047 0.047 -4.6* -4.9** Chestnut-sided Warbler -0.057 0.0009 -5.6*** -2.8*** Common Yellowthroat -0.065 0.0001 -6.3*** -1.8*** Least Flycatcher -0.076 0.0001 -7.3*** -7.3*** 1Trend estimates are expressed as % annual change. Asterisks indicate probability that slopes differ from zero (*P ≤ 0.05, **P ≤ 0.01, ***P ≤ 0.001). 434 Northeastern Naturalist Vol. 16, No. 3 two species associated with mature forest conditions appeared late in the study period: E. virescens (Vieillot) (Acadian Flycatcher) was detected in 2004 and 2005, and D. pinus (Wilson) (Pine Warbler) was discovered on the study site in 2004 and captured in our sample for the first time in 2005. Overall trends and comparison with BBS data Of the 36 species we tested, slopes for 14 species were statistically significant (P ≤ 0.05), with four increasing and 10 declining slopes (Table 3). The increasing species included one short-distance migrant and three Neotropical migrants. The declining species included one north temperate resident, two short-distance migrants, and seven Neotropical migrants. Comparison of trends at HyFox with those reported for BBS Region 27 revealed 24 species with significant trends at one or both geographic scales (Table 3). Eleven species had coincident trends. Three species, of which two were Neotropical migrants (Red-eyed Vireo and Black-throated Green Warbler) and one was a short-distance migrant (Yellow-bellied Sapsucker), increased significantly at both geographic scales. Eight species, of which seven were Neotropical migrants (Least Flycatcher, Chestnut-sided Warbler, Black-and-white Warbler, American Redstart, Northern Waterthrush, Common Yellowthroat, and Canada Warbler) and one was a short-distance migrant (Dark-eyed Junco), showed significant declining trends at both geographic scales. While there were no conflicting significant trends, 13 species had significant trends at only one geographic scale (Table 3). Discussion While there was a moderate amount of forestry activity on our study site in 1987−1990 (Fig. 1), we emphasize that our study was not designed to experimentally test the effects of forestry treatments on bird populations, but as an open-ended monitoring scheme to track trends in forest bird populations. We recognized the inevitability of intra-site perturbations, including possible forestry activities from time to time, and attempted to buffer the effects of such perturbations by the sheer size of our study site and the extensive nature of our sampling scheme. We believed that in a long-term trend study this approach might reduce the possibility that intra-site effects would obscure extra-site effects such as the effect of tropical deforestation on Neotropical migrants. However, we found that trends observed for most species on our study site could be plausibly explained by intra-site changes in the forest. We found no consistent trends among species within migratory groups, observing significant declining trends for species within all migratory groups as well as significant increasing trends for species within all groups except the north temperate residents. Rather, we concluded that the trends appear to reflect two intra-site factors: (I) a dynamic forest ecosystem, characterized by a vigorous, mid-successional (70−90 yr old) forest steadily increasing in biomass (see changes in measurements of tree size classes over 2009 B.G. Blodget, R. Dettmers, and J. Scanlon 435 time in the Field-site Description) and (II) forestry treatments in 1987−1990 that affected 70 ha (25%) of the study site and a burn program in 1989−1995 that affected 3.6 ha of the treated area. While forestry activities appeared to drive predictable short-term responses in a suite of early successional species, similar to responses documented by previous studies on the effects of forestry activities on early successional birds (DeGraaf 1985, Hagan et al. 1997, Webb et al. 1977), 80% of our treated areas were shelter-wood cuts that retained 50%–60% of the original basal area with evenly distributed, large-diameter trees. Only 20% of the treated areas (5% of the entire study area) were clearcut. We feel this moderate amount of forestry activity did not preclude detection of long-term trends in species responding to the much larger process of overall maturing forest conditions on the study site as a whole. Given the apparent importance of intra-site forest changes and the lack of consistent trends within migratory groups, we find it difficult to make any interpretations regarding extra-site effects after the first 20 years of this study. We offer the following plausible explanations for linking observed trends to intra-site factors. We suggest that factor I is a reasonable explanation for the increases in Yellow- bellied Sapsucker, Red-eyed Vireo, and Black-throated Green Warbler. The increase in Yellow-bellied Sapsucker at HyFox would seem consistent with an increasingly mature forest and a growing abundance of suitable trees for feeding, drumming, and nesting. Structural expansion of the forest canopy consistent with forest maturation may have facilitated the increases in Redeyed Vireo and Black-throated Green Warbler. Forest canopy on the study site typically consists of dominant, 75−90 yr old trees with full crowns. Morse (1993) noted that Black-throated Green Warbler was responding, among other factors, to the continuing process of “afforestation” over much of its range. Holmes and Sherry (2001) attributed a significant increase for this species— as well as significant increases for D. coronata (Yellow-rumped Warbler) and Ovenbird⎯at Hubbard Brook to ongoing, gradual succession toward structurally diverse, mature forest. The overall trend toward less-disturbed, mature forest conditions at Hy- Fox, as well as locally and regionally, may also explain significant declining trends for Black-and-white Warbler, Canada Warbler, and Northern Waterthrush (Table 3, Fig. 2). These three species also declined significantly at the BBS Region 27 level (Table 3). Black-and-white Warbler is especially partial to mid- and late-successional forests (Kricher 1995; B.G. Peterjohn, US Geological Survey, Laurel, MD, pers. comm.), yet Hagan et al. (1997) found Black-and-white Warbler attained its maximum abundance in regenerating shelterwood cuts in an industrial forest in Maine. Canada Warbler and Northern Waterthrush have steadily declined in the region since the late-1960s (Sauer et al. 2005), possibly in response to forest succession and loss of palustrine wetlands (Baird 1990, Conway 1999, Easton 1995, Lambert and Faccio 2005). A number of studies in the northeast have shown these two species to be asssociated with regenerating forest 436 Northeastern Naturalist Vol. 16, No. 3 patches and forests with dense vegetation near ground level (Craig 1985, DeGraaf 1985, Hagan et al. 1997, Webb et al. 1977). However, we did not witness such a response on our study site, possibly because the forestry treatment areas were in dry uplands. Sufficiently cool, moist conditions seem to be an important common denominator in the selection of habitat by Canada Warbler (Bonney, Jr. 1988) and Northern Waterthrush (Eaton 1995). Factor II effectively set back succession on a quarter of our study site, triggering an array of responses by different species. We assigned these species to three groups: (A) early successional forest species whose numbers were either initially high or which increased during and soon after the forestry treatments and then subsequently declined, (B) species that exploited regenerating forest conditions at some time during the study period, and (C) mature forest species that declined to lower population levels and did not recover during the study period. Most of the Group A species had already returned to population levels approaching those of undisturbed forest by the end of the study period. As Webb et al. (1977) found, the response of no two species is exactly the same and most species that either increase or decrease following forestry activity will tend to return to population levels characteristic of undisturbed forest. Figure 2. Trends of mid-successional (70–90 yr old) forest species in response to a general ongoing pattern of forest maturation on the HyFox study site in Chester, MA, 1987−2005. No data collected in 1998. Note differences in scale on y-axes. 2009 B.G. Blodget, R. Dettmers, and J. Scanlon 437 Group A included Least Flycatcher, Chestnut-sided Warbler, Common Yellowthroat, American Redstart, and Dark-eyed Junco. Declines exhibited by these five species over the study period were predictable since early successional conditions stemming from the forestry treatments in the early years of the study waned (Table 3, Fig. 3). The declining trends for these species coincided with trends in BBS Region 27, reflecting the region-wide trend toward increasingly less-disturbed, mature forest conditions. Other species that did not show significant trends were assigned to Group A. These included Turdus migratorius (American Robin), Dumetella carolinensis (Gray Catbird), and Brown-headed Cowbird, all of which exhibited periods of elevated detections we believe were in response to open brushy conditions created by the forestry activities. Group A also included six species (Alder Flycatcher, Yellow-throated Vireo, Blue-winged Warbler, Mourning Warbler, White-throated Sparrow, and Indigo Bunting) that did not meet our threshold for statistical analysis yet displayed apparent positive responses to the forestry treatments at various times from 1989 through 1997. Fleeting appearances by these species contributed to a mean 10% Figure 3. Trends of species in response to successional changes associated with forestry and burn treatments at the HyFox study site in Chester, MA, 1987−2005. No data collected in 1998. Note differences in scale on y-axes. 438 Northeastern Naturalist Vol. 16, No. 3 increase in species richness (range = 0−16.8%) for 1990−1997 compared to the mean species richness for the surrounding survey years. Peaks in both the number of species detected (63 in 1990 and 1997, and 64 in 1995) and total detections (1229 in 1995, and 1272 in 1997) were positively driven by forestry treatments. Group B was represented by Magnolia Warbler, which exploited the forestry treatment areas starting in 1997 after most of the early successional forest species in Group A had peaked and, in some cases, disappeared (Fig. 3). Although Magnolia Warbler occupies a variety of habitats, it attains maximum densities in recently logged, dense, young coniferous-deciduous forests (Morse 1989) and is one of the first species to exploit young white pine-northern hardwood associations (Kibbe 1985). In 2005, the majority (12 of 13) of Magnolia Warbler detections were at 36 stations in or within 100 m of forest treatment areas. This may explain the species’ significant positive trend at HyFox compared to a non-significant declining trend seen in BBS Region 27 (Table 3). Hylocichla mustelina (Wood Thrush) may also be a species that benefited from forestry treatments. This species declined significantly in BBS Region 27 (Table 3) and across its range (Sauer et al. 2005), but was stable at HyFox (Table 3). Wood Thrush is generally considered an area-sensitive species, preferring extensive, mature forest tracts for nesting (Roth et al. 1996). However, Hunter et al. (2001) found that fledgling and molting adult Wood Thrushes move from mature forest to patches of disturbed habitat that may provide critical food and cover resources not typically associated with nesting sites. Forestry disturbance within the extensive forest at HyFox may have provided such optimal conditions compared to the increasingly extensive and undisturbed mature forest conditions found in the region. Group C included Brown Creeper and Hermit Thrush, whose populations on the study site may have been negatively affected by the forestry treatments (Fig. 3). Neither of these two species declined significantly in BBS Region 27 (Table 3). Brown Creeper is recognized as a species of late-successional forests, needing large trees and snags (Hejl et al. 2002). Removal of varying amounts of mature timber from 25% of the study site could have reduced the amount of suitable habitat within the site for this species. While Hermit Thrush often appears to be tolerant of or even to benefit from forestry disturbance within large forest tracts (Hagan and Grove 1999, Rosenberg et al. 2003, Webb et al. 1977), other studies have found just the opposite (Freedman et al. 1981, Welsh and Healy 1993). Jones and Donovan (1996) suggest that the impact of forest management on Hermit Thrush populations varies substantially with location. In summary, it would appear that natural and human disturbances during the study period did not reverse upward population trends of many mature-forest species on our study site. For example, Red-eyed Vireo, Ovenbird, and Black-throated Green Warbler together comprised 34.7% of all detections in the first 4 yrs compared to 46.2% in the last 4 yrs of the 2009 B.G. Blodget, R. Dettmers, and J. Scanlon 439 study, reflecting an overall increase in mature-forest conditions and a bird community increasingly dominated by a small number of mature-forest species. Forestry treatments, however, temporarily enhanced bird species diversity, benefiting or attracting 16 bird species (Groups A and B). The treatments also may have caused declines in at least two species (Group C), but importantly, did not result in any bird species being eliminated from the study site. Trends for most species could be plausibly explained by intra-site changes in the forest, which is similar to the conclusion reached by Holmes and Sherry (2001) from their study of an unfragmented, mature deciduous forest in New Hampshire. Migratory group membership did not appear to be important in determining trends. Bird population trends for the 19-yr period we analyzed at HyFox generally aligned well with regional trends reported for BBS Region 27. Our results suggest that despite much speculation about potential impacts of tropical deforestation on neotropical migratory bird populations, intra-site factors continue to be primary determinants of forest bird population trends on our study site and possibly at other large, non-fragmented forest tracts in BBS Region 27. Our results also appear to be consistent with studies suggesting negative relationships between degree of forest fragmentation at the landscape level and population trends of forest bird species (Boulinier et al. 2001, Donovan and Flather 2002), and to support the hypothesis presented by Askins et al. (1990) that landscapes with large, unfragmented tracts help sustain populations of forest bird species. We believe our long-term monitoring approach, utilizing a relatively large, stable study site and an extensive sampling scheme, to be a useful model for tracking forest bird populations. Our study, which is ongoing, demonstrates that even as individual species respond to the ongoing process of forest maturation, interrupted from time to time by short-term disturbances in the forest, overall bird communities in large non-fragmented forests remain remarkably stable. Such results suggest large, unfragmented tracts should be targets for habitat conservation efforts designed to maintain healthy populations of the full suite of forest-associated bird species in New England. These sites appear capable of sustaining mature-forest species even when modest amounts of disturbance are introduced on a periodic basis to provide habitat for species associated with early successional forest stages. Acknowledgments We thank the Massachusetts Division of Fisheries and Wildlife for supporting this long-term study and extend special appreciation to the late T. Keefe for his support of our study. We also thank T. Gola, D. St. James, and C. Welsh for assistance in conducting point counts. We extend our thanks and appreciation to B. Davis, B. Hawthorne, A.M. Kittredge, and J. Livingston, who helped maintain the transect system in good condition. We are indebted to S. Langlois, who spent many hours patiently working with us to bring our data management under control, to L. Newlands 440 Northeastern Naturalist Vol. 16, No. 3 for assistance with data compilation, and to information specialists J. Bell and J. DeNormandie, who managed GPS data and accessed GIS data files for developing text maps. B.G. Peterjohn offered valuable dialogue and insight. We also thank R.A. Askins, R.M. DeGraaf, T.W. French, W.R. Meservey, G. Ritchison, F. 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