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Sizes and Long-term Trends of Duck Broods in Maine, 1955–2007
Michael L. Schummer, R. Bradford Allen, and Guiming Wang

Northeastern Naturalist, Volume 18, Issue 1 (2011): 73–86

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2011 NORTHEASTERN NATURALIST 18(1):73–86 Sizes and Long-term Trends of Duck Broods in Maine, 1955–2007 Michael L. Schummer1, R. Bradford Allen2,*, and Guiming Wang1 Abstract - Productivity is a primary parameter used in waterfowl population models; however, few long-term metrics of reproductive output exist for eastern North American waterfowl. We used 52 years of brood survey data from throughout Maine to determine mean Class III brood sizes for Lophodytes cucullatus (Hooded Merganser), Anas platyrhynchos (Mallard), Anas rubripes (American Black Duck), Aix sponsa (Wood Duck), Aythya collaris (Ring-necked Duck), and Bucephala clangula (Common Goldeneye). Using model selection with theoretic-information approaches, we also investigated effects of wetland type, mean ambient temperature during nesting and brood rearing, and year (1955–2007) on trends in brood sizes. Brood sizes declined throughout the survey period for American Black Ducks (-0.88 ducklings/brood), Wood Ducks (-0.91), Ring-necked Ducks (-1.75), and Common Goldeneyes (-1.45). Declines in brood sizes in Maine are consistent with that of other metrics of productivity (e.g., age ratios of harvested waterfowl) for breeding ducks in Maine and may be cause for concern, especially given that declines in brood sizes were observed across a range of species with highly disparate life-history strategies. Declines in age ratios of hunter-harvested ducks could be indicative of range-wide declines in productivity resulting from decreased breeding propensity, nest success, clutch size, or duckling survival. Our findings may suggest that declines in productivity observed in age ratios of hunter-harvested ducks are, at least in part, related to conditions during the breeding season. Thus, understanding factors influencing productivity on breeding grounds are of primary concern for long-term conservation of breeding waterfowl populations in Maine. Introduction Productivity, indexed by the numbers of immatures:numbers of adults (i.e., age ratios), is a vital metric used in population modeling and management for waterfowl (Williams and Johnson 1995). Changes in waterfowl productivity are of interest to waterfowl managers because of influences on population dynamics, harvest potential, and hunter success (Conroy et al. 2002, USFWS 2007a, Williams and Johnson 1995). In addition, long-term changes in productivity are potentially indicative of large-scale environmental change (Reynolds et al. 2001, Zimpfer and Conroy 2006). With the exception of breeding-ground and national harvest surveys, few long-term metrics of waterfowl production exist, especially in eastern North America (Wilkins et al. 2007, Zimpfer and Conroy 2006). Changes in waterfowl production often are monitored using age ratios generated from duck wings collected from a subset of hunters (i.e., national 1Department of Wildlife and Fisheries, Box 9690, Mississippi State University, Mississippi State, MS 39762. 2Maine Department of Inland Fisheries and Wildlife, 650 State Street, Bangor, ME 04401. *Corresponding author - brad.allen@maine.gov. 74 Northeastern Naturalist Vol. 18, No. 1 harvest surveys; USFWS 2007b, Zimpfer and Conroy 2006). However, bias can exist in harvest-derived productivity estimates because of changes in hunter effort, hunting-season frameworks, and other extrinsic variables influencing hunter behavior and success (Nichols et al. 1995, Szymanski and Afton 2005, Williams et al. 1996). Further, age ratios from species harvested or banded in comparatively low numbers can be less reliable indicators of productivity, partially because it is not possible to account accurately for differential harvest vulnerability (i.e., Bucephala clangula L. [Common Goldeneyes], Aythya collaris Donovan [Ring-necked Ducks]; Geissler 1990, Johnson et al. 1992, USFWS 2007b). Declining productivity of some species of waterfowl (e.g., Anas rubripes Brewster [American Black Duck]), relatively high harvest of waterfowl with limited banding effort, large confidence intervals on population estimates, and incomplete population modeling efforts (i.e., Ring-necked Ducks) have been cause for concern among Atlantic Flyway waterfowl managers for decades (Conroy and Eberhart 1983; Conroy et al. 2002; D. Eggeman, Florida Fish and Wildlife Conservation Commission, Tallahassee, fl, pers. comm.). Clearly, analyses of additional independent datasets that either corroborate or contradict trends in productivity resulting from national harvest surveys and breeding-population estimates are needed in the Atlantic Flyway. Here, we analyze data from brood surveys conducted across Maine from 1955–2007. Our objectives were to determine mean brood size by species and whether brood size of each species changed from 1955–2007. Methods Study area Maine is found within Bird Conservation Regions 14 (Atlantic Northern Forest) and 30 (New England/Mid-Atlantic Coast) and is a transitional zone from northern hardwoods in the south to spruce-fir forests in the north, with relatively nutrient-poor soils characteristic of post-glaciation (USNABCIC 2000). Castor canadensis Kuhl (American Beaver; herein Beaver)-created wetlands and permanent lakes and ponds provide the majority of breeding waterfowl habitat (Longcore et al. 2006, Ringleman and Longcore 1982a, USNABCIC 2000). Brood surveys were conducted throughout Maine in a variety of wetland types (e.g., inland fresh meadow, inland shallow fresh marsh, inland deep fresh marsh, inland open fresh marsh, shrub swamp, wooded swamp, bog, coastal salt meadow; see McCall 1972 for detailed descriptions). Brood surveys Waterfowl brood surveys were conducted annually in June and July 1955– 2007, excluding 1974–75 when personnel were not available (P. Corr, B. Allen, Maine Department of Inland Fisheries and Wildlife, Bangor, ME, pers. comm.). Wetlands surveyed were distributed across a range of wetland types throughout Maine, and an average of 40 wetlands were surveyed per year (range = 20–63, median = 39). The composition of wetland types surveyed annually averaged 2.3% inland fresh meadow, 17.5% inland shallow fresh 2011 M.L. Schummer, R.B. Allen, and G. Wang 75 marsh, 20.6% inland deep fresh marsh, 37.8% inland open fresh water, 10.7% shrub swamp, 4.6% wooded swamp, 3.7% bog, 1.8% coastal salt meadow, and 1.0% unknown wetland type (McCall 1972). An attempt was made to survey wetlands once during the first week of June and again during the first week of July. However, because of weather and other logistical issues, surveys often occurred throughout June and July. The two survey dates of the same wetland took into account potential yearly differences in brood-rearing period based on nesting conditions, as well as, early (i.e., American Black Duck) and late (i.e., Ring-necked duck) nesting waterfowl. Surveys were conducted by canoe or from stationary blinds during 2-hr observation periods at dawn or dusk. When a brood was observed, species, number of ducklings, and age class of ducklings (Class I, II, or III; Gollop and Marshall 1954) were recorded. These same criteria were used to individually identify broods and record repeat broods in subsequent visits to the same location (Longcore et al. 1998). Observers only recorded brood size if ducks could be counted reliably in relatively open habitats. We did not use repeat brood observations for analyses because data were compiled in a manner that did not allow us to determine which repeat broods corresponded to original brood observations. We only used Class III broods for analyses because Class III ducklings normally survive to fledge (Ringelman and Longcore 1982b). We included Anas platyrhynchos L. (Mallard), American Black Duck, Aix sponsa L. (Wood duck), Lophodytes cucullatus L. (Hooded merganser), Ring-necked Duck, and Common Goldeneye broods in our analyses. Wetland types were classified following McCall (1972). We calculated April–May and June–July daily mean temperatures using data from the United States Historical Climatology Network (HCN; Quinlan et al. 1987, Williams et al. 2006) from Acadia National Park (44.35ºN, 68.27ºW), Farmington (44.68ºN, 70.15ºW) and Holton (46.20ºN, 67.83ºW). Model development and selection We applied mixed generalized linear models (Pinheiro and Bates 2000) to annual brood size by species, incorporating year, April–May daily mean temperature, June–July daily mean temperature, and wetland type of brood habitat as explanatory variables. Cold temperatures during nesting and brood rearing may decrease duck productivity and duckling survival (Bolduc et al. 2008, Johnson et al. 1992, Rotella and Ratti 1992, Zimpfer and Conroy 2006). We accounted for potential inter-annual variation in brood-rearing conditions by including April– May and June–July daily mean temperatures as covariates in models. We used linear regression analyses to determine if April–May or June–July daily mean temperatures changed throughout the 52-year brood survey period (P ≤ 0.10). We included a pond identification number (ID) as a random factor to account for temporal autocorrelations (Everitt 2005, Faraway 2006). In our preliminary analysis, we included order-1 autocorrelative (AR-1) error structure in the mixed models; however, the AR-1 covariance matrices did not improve the Akaike information criterion values of the models. Therefore, we did not include AR-1 error structure 76 Northeastern Naturalist Vol. 18, No. 1 in the final analyses. We used the Poisson link function for brood size. The package lmer (Bates et al. 2008) of statistical software R was used to conduct analyses of mixed models (R Development Team 2006). We inferred the effects of year, April–May mean temperature, June–July mean temperature, and wetland type on brood sizes, using model selection with theoretic-information approaches (Burnham and Anderson 2002). A total of 16 models with all possible combinations of the 4 explanatory variables were built. Akaike weight of evidence (wi ) was calculated for each of the 16 models. The sums of Akaike weights over all models containing the variable were used to assess the relative importance of an explanatory variable (Burnham and Anderson 2002). The index value ranges from 0 to 1. The greater the index value, the more important the variable is to brood sizes. We further examined the P-values of the coefficients measuring the fixed effects of explanatory variables. If P ≤ 0.05, we concluded that the effect of the variable was significant. Results Mean (±SE) Class III brood sizes (bs) for 1955–2007 were 4.55 ± 0.10 for American Black Ducks, 3.63 ± 0.15 for Wood Ducks, 4.14 ± 0.19 for Common Goldeneyes, 4.10 ± 0.23 for Hooded Mergansers, 4.46 ± 0.31 for Ring-necked Ducks, and 3.96 ± 0.30 for Mallards (Fig. 1, Table 1). April–May daily mean temperatures increased throughout the 52-year brood survey period, but June– July daily mean temperatures showed no linear trend (Fig. 2). Excluding Mallards, median brood sizes were lower than mean brood sizes for all species surveyed, and brood sizes of all species resembled a Poisson distribution. None of the explanatory variables had an overwhelming support, measured by Akaike weights (wi ≥ 0.80), from data in all six species (Table 2). Nevertheless, year (yr; 0–52, 0 = 1955, 52 = 2007) and June–July temperature (jjtemp) were relatively more important to the brood sizes of Mallards, American Black Ducks, and Common Goldeneyes (Fig. 3). Year influenced brood size of Wood Ducks and Ring-necked Ducks more compared to other model variables (Fig. 3). Brood size of Mallards was positively related to June–July temperatures (bs = exp(-1.33 + 0.15*jjtemp) [P = 0.05, n = 55]). Brood sizes of American Black Ducks decreased with years (yr), but increased with increasing June–July temperatures (bs = exp(0.31 - 0.004*yr) + (0.07*jjtemp) [P = 0.02 for years; P = 0.02 for June–July temperature, n = 521]), as did brood size of Common Goldeneyes (bs = exp(0.63 - 0.006*yr) + (0.12*jjtemp) [P = 0.040 for years; P = 0.006 for June–July temperature, n = 179]). Brood sizes of Wood Ducks (bs = exp(1.22 - 0.006*yr) [P = 0.04, n = 206]) and Ringnecked Ducks (bs = exp(1.46 - 0.01*yr) [P = 0.003, n = 69]) declined over years. Linear declines in brood sizes from 1955–2007 were 0.91 ducklings per brood for Wood Ducks and 1.75 ducklings per brood for Ring-necked Ducks. Model-estimated linear declines in American Black Ducks and Common Goldeneyes were 0.88 and 1.45 ducklings per brood, respectively, when using the 52-year mean of 17.7 ºC for jjtemp. We did not identify significant variables (P ≤ 0.05) for Hooded Mergansers (n = 139). 2011 M.L. Schummer, R.B. Allen, and G. Wang 77 Discussion Our results suggest a decline in Class III brood sizes for several species of ducks in Maine from 1955–2007. The negative trends in brood sizes across dabbling and diving ducks species are similar to other indices of duck productivity (Zimpfer and Conroy 2006; K. Richkus and B. Raftovich, US Fish and Wildlife Service, Laurel, MD, unpubl. data). However, because few independent metrics Figure 1. Annual mean (± SE) Class III brood sizes calculated from raw data for selected ducks breeding in Maine, 1955–2007. 78 Northeastern Naturalist Vol. 18, No. 1 Table 1. Annual number of Class III duck broods observed during June and July waterfowl surveys in Maine, 1955–2007. NS = not surveyed. American Common Hooded Ring-necked Year Black Duck Wood Duck Goldeneye Merganser Duck Mallard 1955 23 6 1 2 0 0 1956 6 0 0 2 2 0 1957 7 1 3 1 1 0 1958 4 1 0 0 1 0 1959 8 3 0 1 0 0 1960 19 1 0 3 0 0 1961 13 2 1 2 1 0 1962 19 0 0 0 0 0 1963 6 3 1 2 1 0 1964 6 2 1 3 1 0 1965 20 9 4 3 2 2 1966 23 3 0 1 0 0 1967 7 0 4 1 0 0 1968 12 3 6 0 4 0 1969 15 3 3 2 1 1 1970 11 0 1 0 3 0 1971 8 2 6 0 8 0 1972 6 1 6 1 1 0 1973 11 0 0 1 1 0 1974 9 1 0 1 0 0 1975 NS NS NS NS NS NS 1976 NS NS NS NS NS NS 1977 5 1 1 2 0 0 1978 3 2 0 1 0 0 1979 3 3 1 4 1 0 1980 13 5 4 8 0 2 1981 4 4 4 5 1 0 1982 3 1 6 3 1 2 1983 6 3 5 2 0 2 1984 5 6 6 2 0 1 1985 23 0 11 3 3 1 1986 27 9 9 7 3 2 1987 21 2 9 2 2 0 1988 21 6 9 3 0 5 1989 9 11 2 8 0 6 1990 14 14 7 7 0 0 1991 26 13 5 4 5 3 1992 9 4 4 2 2 3 1993 9 6 9 9 2 2 1994 8 7 5 6 1 3 1995 7 5 7 2 4 0 1996 9 7 2 2 0 3 1997 7 9 3 0 0 0 1998 7 10 2 5 1 0 1999 8 3 3 2 2 4 2000 5 5 0 2 1 4 2001 2 5 1 5 1 2 2002 8 8 8 4 1 2 2003 7 2 4 1 8 0 2004 5 2 4 2 0 3 2005 7 7 2 4 0 1 2006 6 2 4 1 3 0 2007 1 3 5 5 0 1 Total 521 206 179 139 69 55 2011 M.L. Schummer, R.B. Allen, and G. Wang 79 were available to explain patterns observed within the brood survey dataset, we cannot unambiguously identify the underlying causes of declines in brood sizes. Nonetheless, we relate our findings to available literature on productivity of ducks in Maine and pose potential hypotheses for long-term declines in brood sizes in the context of stimulating continued monitoring and research of duck productivity in this region. We restrict our comparison of sizes and trends in Class III brood sizes to species for which similar available data exist (i.e., American Black Duck, Ring-necked Duck). Our raw estimate of 4.55 ± 0.10 ducklings per Class III brood for American Black Ducks (1955–2007) is similar to those found for the St. Lawrence Estuary (x̅ = 4.50, range = 4.20–5.10; Reed 1970), south-central Maine (x̅ = 5.26; Ringleman and Longcore 1982b), and northern Maine (x̅ = 3.90–4.60; 1993–1994; Longcore et al. 1998), but our model-predicted Class III brood size of 3.82 for 2007 is considerably lower. In 1993 and 1994, our model predicting Class III brood sizes (3.98 and 4.50 ducklings, respectively) were similar to those recorded by Longcore et al. (1998). Our data combined with previous estimates are consistent with the hypothesis of declining Class III brood sizes for American Black Ducks in Maine. Mendall (1958) estimated Figure 2. Mean temperature and linear trend analyses (± 95% CI), April–May and June–July in Maine, 1955–2007, calculated using data from the United States Historical Climatology Network (HCN; Quinlan et al. 1987, Williams et al. 2006) from Acadia National Park (44.35ºN, 68.27ºW), Farmington (44.68ºN, 70.15ºW) and Holton (46.20ºN, 67.83ºW). 80 Northeastern Naturalist Vol. 18, No. 1 Table 2. Model selection of explanatory variables influencing Class III broods sizes of selected ducks breeding in Maine, 1955–2007, using theoretic-information approach. Dashes denote a wi of zero. Hooded American Common Ring-necked Merganser Mallard Black Duck Wood Duck Goldeneye Duck ModelA AIC wi AIC wi AIC wi AIC wi AIC wi AIC wi Null 205.61 0.106 67.17 0.102 595.12 0.011 255.58 0.121 370.04 - 103.49 0.037 Yr 207.46 0.042 68.75 0.046 592.43 0.043 253.59 0.327 369.91 - 98.59 0.430 AMtemp 205.85 0.094 66.65 0.132 595.56 0.009 256.26 0.086 365.14 0.001 105.64 0.013 Wet 206.75 0.060 65.63 0.220 592.83 0.036 257.48 0.047 354.14 0.194 105.06 0.017 JJtemp 204.84 0.156 72.95 0.006 604.68 - 266.84 - 378.38 - 107.03 0.006 Yr+AMtemp 207.85 0.035 67.91 0.070 590.83 0.097 254.96 0.165 362.93 0.002 100.30 0.183 Yr+JJtemp 208.71 0.023 66.39 0.151 588.91 0.252 255.20 0.146 355.28 0.110 100.42 0.172 Yr+Wet 206.88 0.056 74.71 0.002 602.12 - 264.22 0.002 376.29 - 104.10 0.027 AMtemp+JJtemp 206.56 0.066 66.85 0.120 593.46 0.026 258.20 0.033 353.78 0.232 107.20 0.006 AMtemp+Wet 205.64 0.105 73.02 0.006 604.97 - 267.13 - 374.50 - 108.79 0.003 JJtemp+Wet 206.27 0.076 69.99 0.025 601.70 - 268.64 - 357.57 0.035 108.97 0.002 Yr+AMtemp+JJtemp 208.52 0.025 67.49 0.087 587.49 0.513 256.64 0.071 353.38 0.284 102.10 0.074 Yr+AMtemp+Wet 207.47 0.042 74.65 0.002 600.06 0.001 265.41 0.001 369.85 - 106.06 0.010 Yr+JJtemp+Wet 208.22 0.029 71.14 0.014 597.81 0.003 265.62 0.001 356.89 0.049 105.68 0.012 JJtemp+AMtemp+Wet 206.80 0.059 71.65 0.011 602.22 - 268.98 - 358.16 0.026 110.73 0.001 Yr+AMtemp+JJtemp+Wet 208.32 0.027 72.83 0.006 595.64 0.009 266.91 - 356.26 0.067 107.61 0.005 ASymbols: Yr = year, AMtemp = April–May temperature, JJtemp = June–July temperature, and Wet = wetland type. Null models contain an intercept only. 2011 M.L. Schummer, R.B. Allen, and G. Wang 81 a mean Class III brood size of 5.20 for Ring-necked Ducks using observations from throughout Maine, 1939–1954 (n = 141). Our raw estimate of 4.46 ± 0.31 Class III ducklings per Ring-necked Duck brood (1955–2007) is only slightly lower, but greater than that reported by McAuley and Longcore (1988; x̅ = 3.40 ± 0.26). Our model predicted estimates of Class III brood size ranged from 3.19–3.25 from 1983–1985, which is only slightly lower that those reported by McAuley and Longcore (1988) for the same period. Our model-predicted Class III brood size for Ring-necked Ducks in 2007 was substantially lower (2.56 ducklings). Our data and modeling effort for Ringnecked Ducks, although derived from a relatively small sample size (n = 69), are consistent with other published literature and support our conclusion that Class III brood sizes have declined in Maine. Declines in age ratios of hunter-harvested American Black Ducks may be indicative of range-wide declines in productivity from decreased breeding propensity, clutch size, nest success, or duckling survival (Conroy et al. 2002, Zimpfer and Conroy 2006). At a finer scale, in Maine, we also found declines in Class III brood sizes for American Black Ducks. Specific reasons for declines in productivity of American Black Ducks are unknown but include Figure 3. Relative importance of explanatory variables of Class III brood size for selected ducks breeding in Maine, 1955–2007. 82 Northeastern Naturalist Vol. 18, No. 1 both breeding-ground and non-breeding-season hypotheses (DiBona 2007, Merendino and Ankney 1994, Zimpfer and Conroy 2006). However, we also documented declines in brood sizes of Wood Ducks, Ring-necked Ducks, and Common Goldeneyes, which have different wintering ranges and habitat needs from American Black Ducks (Bellrose 1980). Age ratios from Wood Ducks, Common Goldeneyes, and Ring-necked Ducks harvested in Maine also declined during the same period as the Maine brood survey (1961–2007; K. Richkus and B. Raftovich, unpubl. data). This trend indicates that declines in brood sizes of American Black Ducks and other ducks in Maine may be linked with environmental conditions on breeding grounds. The only explanatory habitat variable that was included in our models was wetland type, which generally ranked low in variable importance. Possible reasons for declining productivity in ducks investigated or discussed previously for eastern North America include density dependence (Zimpfer and Conroy 2006), suitability of brood-rearing wetlands (Deifenbach and Owen 1989), acidification of wetlands (Longcore et al. 2006), climate change (Browne and Dell 2007, Jacobson et al. 2009), spring body condition of nesting hens (Devries et al. 2008), and species competition (i.e., Mallard-American Black Duck and Hooded Merganser-Wood Duck interactions; Ankney et al. 1987, Hepp and Bellrose 1995, McAuley et al. 2004). Because of the cursory nature of our brood surveys, we cannot link declines in brood sizes to any of the above factors. Further study is required to understand factors influencing broods sizes and potential impacts of brood sizes on population dynamics of eastern North American waterfowl. Further, we were unable to account for total brood loss and therefore only document apparent long-term declines in brood sizes for some species of ducks nesting in Maine. However, a reduction in brood sizes is consistent with a density-dependence mechanism, which has been suggested as a limiting factor for American Black Ducks (Zimpfer and Conroy 2006), Wood Ducks (Haramis and Thompson 1985), and Common Goldeneyes (Pöysä and Pöysä 2002). Breeding population estimates of ducks in Maine and elsewhere in eastern North America have generally been stable to declining (MDIFW 2007, USFWS 2008). If density-dependent factors have played a role in duckling survival in Maine, and these effects were caused by declining habitat suitability as suggested by Diefenbach and Owen (1989) and Zimpfer and Conroy (2006), declines in brood sizes may have resulted from declining wetland availability and quality. Many of Maine’s wetlands are managed for stable water levels (R.B. Allen, pers. observ.), which may have resulted in reduced productivity over several decades (Middleton 1999). Also, residential development has increased human encroachment on wetlands in Maine (Widoff 1988), which may cause reductions in use by some species of ducks (Diefenbach and Owen 1989). Development also has increased human-Beaver conflicts in residential areas, resulting in further control of Beaver-induced flooding (W. Jakubas, Maine Department of Inland Fisheries and Wildlife, Bangor, ME, pers. comm.), possibly reducing the availability of suitable brood-rearing habitat via Beaver-created wetlands (McCall et al. 1996). 2011 M.L. Schummer, R.B. Allen, and G. Wang 83 Beaver-created wetlands are considered more suitable brood-rearing habitat with greater overhead cover and invertebrate substrate than open oligotrophic systems (Longcore et al. 2006, McCall et al. 1996, Nummi and Hahtola 2008). Our findings should be useful in population-model development for eastern North American ducks species for which few estimates of productivity exist. Our results also are consistent with available literature and data suggesting that productivity for several species of ducks is declining in northeastern North America (Conroy et al. 2002; K. Richkus and B. Raftovich, unpubl. data). Further, we found that declines occurred across a wide-range of species, potentially suggesting large-scale environmental and habitat change throughout Maine. We suggest that future research should focus on elucidating factors influencing clutch size and duckling survival for various sympatric ducks breeding in Maine and throughout eastern North America. Acknowledgments Waterfowl surveys were funded by the Maine Department of Inland Fisheries and Wildlife (MDIFW). 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