Brood Provisioning and Nest Survival of Ardea herodias (Great Blue Heron) in Maine
Margaret M. Meserve Auclair, Kathryn A. Ono, and Noah G. Perlut
Northeastern Naturalist, Volume 22, Issue 2 (2015): 307–317
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2015 NORTHEASTERN NATURALIST 22(2):307–317
Brood Provisioning and Nest Survival of Ardea herodias
(Great Blue Heron) in Maine
Margaret M. Meserve Auclair1,*, Kathryn A. Ono1, and Noah G. Perlut2
Abstract - From 1983–2009, the number of coastal breeding pairs of Ardea herodias (Great
Blue Heron [GBHE]) in Maine declined by 64%, and the number of occupied islands on
which these birds bred declined by 40%. The Maine Department of Inland Fisheries and
Wildlife listed the GBHE as a species of special concern in 2007, and expanded its annual
monitoring to include inland colonies in 2009. To assess regional demographic differences,
we compared the relationship between brood provisioning and nest survival of GBHEs in
1 coastal and 1 inland colony. In terms of brood-provisioning within the 2 colonies, the
inland colony had significantly greater rates for the first 2 weeks post-hatch, but the coastal
colony had greater rates in subsequent weeks. These differences did not affect either nest
fate (≥1 chick fledged) or daily nest survival at the inland or coastal colony. In both colonies,
the maximum number of nestlings observed at a nest was positively correlated with
the number that subsequently fledged. Daily nest survival was positively associated with an
increasing number of nestlings, earlier hatch dates, and increased brood-provisioning rates
for 1–2-week-old chicks. Our results suggest that the number of nestlings per nest can be
used as a proxy for nest survival in GBHE colonies in the northeastern part of their range.
Furthermore, because nest survival was influenced by brood-provisioning rates during the
first 1–2 weeks post-hatch, our results suggest that the most sensitive time for disturbance
of GBHEs in the northeastern part of their range may be earlier in the nesting stage than
previously thought.
Introduction
Between 1960 and 2008, the human population inhabiting the coast of Maine
increased by 62.2%, and housing units in coastal counties increased 106.4% (US
Census Bureau 2008). From 1983–2009, the number of coastal breeding pairs of
Ardea herodias L. (Great Blue Heron [GBHE]) in Maine declined by 64% (2.46%
annually), and the number of occupied islands on which these birds breed declined
by 40% (1.54% annually) (D’Auria 2009). Likewise, between 1983 and
2009, the Breeding Bird Survey (BBS) indicated a 66.3% decline of GBHEs in
Maine (2.55% annually; Sauer et al. 2012). In 2007, the Maine Department of
Inland Fisheries and Wildlife (MDIFW) listed the GBHE as a species of special
concern. Starting in 2009, MDIFW began monitoring wading birds, including
GBHE, by conducting both aerial and ground surveys of locations determined by
the historical distribution of GBHE colonies as well as information from citizen
scientists and state biologists.
1Department of Marine Sciences, University of New England, 11 Hills Beach Road, Biddeford,
ME 04005. 2Department of Environmental Science, University of New England,
11 Hills Beach Road, Biddeford, ME 04005. *Corresponding author - mmeserve@une.edu.
Manuscript Editor: Peter Paton
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Available evidence suggests that GBHEs select a specific colony location based
on proximity to a productive food source (Gibbs 1991, Gibbs et al. 1987, Kirsch
et al. 2008) and distance from potential human and predator disturbances (Carlson
and McLean 1996, Norman et al. 1989, Parnell et al. 1988, Todd et al. 1982, Vos
et al. 1985). Islands in Maine with or without a GBHE colony tend to be in similar
proximity to profitable foraging areas; however, occupied islands were farther from
human populations (Gibbs 1991, Gibbs et al. 1987). With increasing human development
in coastal areas, potential colony locations that are both free of disturbance
and close to productive food sources are becoming rare. Although data have been
presented about coastal breeding sites, little is known about factors affecting the
population dynamics of inland colonies in Maine, primarily due to logistical challenges
the state had in monitoring these inland colonies.
In 2012, we initiated a preliminary study to understand demographic differences
between a coastal and an inland GBHE colony in Maine. We assumed that coastal
birds were moving to increase provisioning rates of chicks and predicted that
feeding rates of chicks would be higher at inland colonies than coastal colonies.
Furthermore, we predicted that higher brood-provisioning rates would result in
higher nest-survival rates at the inland colonies.
Methods
Study area
During the 2012 breeding season, there were 87 active GBHE colonies in Maine,
with 940 breeding pairs (D’Auria, MDIFW, Bangor, ME, pers. comm.). In 2012,
the MDIFW reported 13 coastal colonies with an average of 27.6 nests per colony
(± 32.2 SD) and 74 inland colonies with an average of 8.7 nests per colony (± 10
SD). We focused on 1 inland colony (IN) in southern Maine between Long and
Highland Lakes in Bridgton, ME (44°4'44.56''N, 70°42'25.55''W) and 1 near-shore
coastal-island colony (CO) in Brunswick, ME (43°51'45.29N, 69°54'21.17''W);
both colonies were on private property (Fig. 1). We selected these 2 sites due to the
large number of breeding pairs (inland: 56, coastal: 40), comparable acreages (IN:
2.4 ha; CO: 1.35 ha), and accessibility.
Field methods
We placed 5 Trekker® T-200 ground blinds (173 cm L x 173 cm W x 165 cm H),
3 at CO and 2 at IN, to conduct focal behavioral observations. To limit disturbance,
we set up the blinds in March before birds arrived at the sites. We assigned each
nest a unique number and conducted nest observations using a Nikon® ProStaff
fieldscope (82-mm body, 20–60x zoom eyepiece) and, depending on the distance of
the nest from the blind, 10 x 42 or 8 x 40 Nikon Monarch ® binoculars.
Brood-provisioning observations
Four observers (M.M. Meserve and 3 volunteer field technicians) monitored all
active nests in both the IN and CO colonies; 2 observers were in a blind during all
observation periods. Observation periods lasted for 6 h every 3 days from 14 April
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to 25 May, 9 h every 4 days from 26 May to 5 June, and 6 h every 3 days from 6
June to 24 July (Table 1). A comprehensive review of human-disturbance effects on
nesting colonial waterbirds suggested limiting our surveys of GBHE colonies to no
more than once every 3 d (Carney and Sydeman 1999); due to our small sample size
(2 colonies) and the observed sensitivity during the 26 May–5 June time period,
Figure 1. Location of 2 Great Blue Heron colonies monitored in southern Maine in 2012.
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we utilized a 4-d rotation to minimize our disturbance at the colonies (Carney and
Sydeman 1999, Vennesland and Butler 2004), while still allowing for adequate data
collection. We continued observations until the last fledgling left each colony: 13
July for CO, and 24 July for IN.
We habituated the birds to our presence by slowly approaching the area of the
observation blind and stopping if a bird flushed or began to “chortle” or “cluck”
(Vennesland and Norman 2006). Once the birds settled, observers continued to the
blind. During each focal observation, we recorded the nest number, status (active
or inactive), and stage (incubating, nestlings, fledglings), as well as the number and
stage of nestlings (1–2, 2–4, 4–6, or 6–8 weeks old). Each observer conducted 3-h
focal-observation periods for each nest in succession. Observers rotated through
nests numerically and only skipped a nest if it was inactive (no longer contained
any nestlings); we classified a nest as inactive when there was no evidence of adults,
nestlings, or fledglings for an entire observation day.
Daily nest survival
We used the Program MARK nest-survival module (Rotella et al. 2004, White
and Burnham 1999) to evaluate daily nest survival (DNS), determine if daily nest
survival differed between the IN and CO colonies, and identify what ecological and
behavioral factors best explained variation in DNS. We considered a nest to be successful
if it fledged ≥1 young. We tested the effect of 7 covariates on DNS: hatch
date; average number of feeding trips per hour across the entire nestling stage (average
brood-provisioning rate [AvgBP]); average brood-provisioning rate within
the first 2 weeks (AvgBPa), 2–4 weeks (AvgBPb), 4–6 weeks (AvgBPc), and 6–8
weeks (AvgBPd) post-hatch; and total number of hatchlings (defined as the maximum
number of nestlings seen in a nest at any stage). Age was determined using
the nestling illustrations from Vennesland and Norman (2006). We ran all possible
one-way, interactive, and additive models, and ranked competing models by their
corrected (for small sample size) AICc values. AICc is a second-order correction for
AIC that is computed as,
AICc = -2(log-likelihood) + 2k + (2k[k + 1]) / (n - k - 1),
where n = number of observations and k = number of parameters (Burnham and
Anderson 2004). We then calculated ΔAIC for each model, which measured the difference
in AICc between model i and the best-fitting model. We also calculated the
Table 1. Nest-observation rotation schedule used to monitor an inland (IN) and coastal (CO) Great
Blue Heron colony in Maine. am = morning, pm = afternoon.
Day number
Schedule 1 2 3 4 5 6 7 8 9 10 11
6-hour IN CO - IN CO - IN CO - IN CO
Time of day am am - pm pm - am am - pm pm
9-hour IN CO - - IN CO - - IN CO -
Time of day am am - - pm pm - - am am -
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AICc weight (wi), interpreted as the probability of any model being the best model
in the model set. We considered models with ΔAICI < 2 to have substantial support
in explaining variation in the data (Burnham and Anderson 2004).
Statistical analysis: Brood-provisioning rates and effects of number of nestlings
present
We conducted separate one-way analysis of variance (ANOVA) tests to assess
any differences between the CO and IN colonies in: nestling age (using the age of
the oldest chick), age-specific brood-provisioning rates, and the number of nestlings
per nest. To test differences between AvgBPa, AvgBPb, AvgBPc, and AvgBPd,
we used Kruskal-Wallis tests because these data did not fit the assumptions of normality
and equal variance needed for an ANOVA. We conducted an ANOVA with
pairwise comparisons for both IN and CO colonies to determine the relationship
between brood-provisioning rates by nestling age class. We used Pearson productmoment
correlations to look for relationships between number of nestlings within
a nest, brood-provisioning rates, and number of chicks fledged w ithin a nest.
Results
Daily nest survival
Nest survival for the IN colony was 68% (95% CI = 50–81%) and 49% (43–55%)
for the CO colony. Daily nest survival was 0.996% (95% CI = 0.998–0.993%) and
0.993% (0.994–0.991%) for the IN and CO colonies, respectively. Two competing
models best explained variation in daily nest survival (Table 2). The interaction
between the number of nestlings and hatch date best explained variation in DNS,
and had 1.7 times more support than the second-ranked model (Table 2). The additive
model including the number of nestlings and AvgBPa was the second-ranked
model (Table 2). The number of nestlings in a nest was an additive or interactive
factor in the highest-ranked models, accounting for wi = 0.610. Post-hatching daily
nest survival increased as both the number of nestlings increased and hatch date
decreased (Fig. 2A, B). Post-hatching daily nest survival also increased with the
combination of a higher AvgBPa and an increased number of nestlings (Fig. 2C, D).
Table 2. Our 5 highest-ranked program MARK nest-survival models for Great Blue Herons breeding
in Maine, 2012, listed in order of their Akaike weights (wi).
Evidence
Model Deviance ΔAICc K wi ratio
Number of nestlings*Hatch date 158.46 0.000 4 0.286 3.497
Average brood-provisioning rate for 1–2-week-old 161.53 1.076 3 0.167 5.988
chicks + Number of nestlings
Average brood-provisioning rate for 1–2-week-old 160.92 2.473 4 0.083 12.048
chicks* Number of nestlings
Average brood-provisioning rate for 1–2-week-old 161.16 2.703 4 0.074 13.514
chicks* Hatch date
Average brood-provisioning rate for 1–2-week-old 163.52 3.058 3 0.062 16.129
chicks+ Site
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Brood-provisioning rates
We observed 21 nests at the CO colony between 14 April and 13 July, and 29
nests at the IN colony between 17 April and 24 July. We conducted a total of 411 observation
hours (IN = 216 h, CO =195), and observed 64 brood-provisioning events
at the IN colony and 59 at the CO colony. Both IN and CO colonies had similar
hatch dates, numbers of chicks per nest, and showed the highest feeding rates in the
youngest age class, and these rates consistently decreased with increasing nestling
age (Fig. 3). AvgBPa in the IN colony was significantly greater than AvgBPb (t27 =
-4.767, P < 0.001); AvgBPc (t20 = -4.672, P = 0.0001), and AvgBPd (t32 = -5.057,
P < 0.001). There was no significant difference in the brood-provisioning rates
as the season progressed for the CO colony (F1,3 = 0.806, P = 0.496). We found
no correlation between the brood-provisioning rate for a nest and the number of
nestlings within that nest (IN: R2
22 = 0.067, P = 0.76; CO: R2
13 = 0.461, P = 0.08;
combined: R2
37 = 0.305, P = 0.06). There was no correlation between colony and
brood-provisioning rate (R2
44 = 0.269, P = 0.07) or between brood-provisioning rate
and nest fate (R2
44 = -0.196, P = 0.19).
Both the IN and CO colonies fledged 1.1 young per nest (IN = 1.11, CO = 1.09).
The average number of nestlings per nest did not differ between colonies—IN:
mean = 2.6, SD = 1.1, CO: mean = 3.1, SD = 1.0 (F1,33 = 1.379, P = 0.249; Fig. 4).
The number of nestlings within a nest was positively associated with the number
that fledged for both colonies (IN: R2
27 = 0.173, P = 0.025; CO: R2
19 = 0.438, P =
0.001; combined: R2
48 = 0.545, P < 0.001; Fig. 5).
Figure 2. Daily nest-survival rates (DSR) of Great Blue Heron nests in southern Maine
based on the interactive models of hatch date (day of season) (A) and number of nestlings
(B), and the additive models of brood provisioning rate of 1–2-week-old chicks (C) and
number of nestlings (D). The dotted lines indicate the 95% confidence intervals.
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Discussion
We predicted lower daily nest survival at coastal colonies compared to inland
colonies to explain the decline of GBHE nesting in coastal Maine, potentially as a
consequence of lower brood-provisioning rates at coastal sites. However, we did
Figure 3. Average brood-provisioning rates during 4 time intervals at 2 Great Blue Heron
colonies (IN = inland, CO = coastal) in southern Maine in 2012. Error bars indicate standard
error, letters denote statistical differences within sites based on an ANOVA with pairwise
comparisons, and * indicates a significantly greater provisionin g rate between sites.
Figure 4. Percent of Great Blue Heron nests with 1–5 nestlings at an inland (IN: n = 29) and
coastal (CO: n = 21) colony in Maine in 2012.
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not detect any variation in daily nest survival between the 1 coastal and 1 inland
colony we studied. Instead, variation in daily nest survival across all nests was best
explained by the interaction between the number of nestlings and hatch date. We
also found that higher brood-provisioning rates within the first weeks post-hatch
were positively associated with daily nest-survival rates. Our results are contrary to
past studies that documented differences in wading-bird reproductive success and
breeding phenology between 2 different habitat types. For example, Ardea cinera
L. (Grey Heron) in northern Poland nest in both inland and coastal colonies, and
Jakubas (2011) found that coastal Grey Herons began nesting and hatching earlier
than their inland counterparts. Frederick et al. (1992) found that fledging success of
Egretta thula Molina (Snowy Egret) and Egretta tricolor Muller (Tricolored Heron)
in southern Florida was higher in freshwater rather than saline habitats. Although
we found no variation in nest survival between colonies, we acknowledge that we
monitored only 2 colonies. Therefore, we suggest future studies investigate more
colonies to assess whether other factors (i.e., colony size, landscape characteristics)
may affect nest survival at coastal versus inland colonies.
We observed 64 brood-provisioning events at the inland site (n = 29 nests) and
59 at the coastal site (n = 21 nests). The highest brood-provisioning rates occurred
in the first 1–2 weeks post-hatch at both the inland and coastal colonies; feeding
rates thereafter declined as the season progressed, a pattern reported in past
research (Brandman 1976, Collazo 1981, Pratt 1970). The average brood-provisioning
rates for 2–8-week-old chicks at the inland colony appeared lower than those
at the coastal colony (Fig. 3); however, feeding rates during these periods did not
explain variation in daily nest survival at either colony. Our results might indicate
that there was a difference in the quality and/or quantity of food delivered between
the colonies, which is a topic for further study.
Figure 5. Relationship between the number of Great Blue Heron nestlings and fledglings
at an inland (IN) and coastal (CO) colony in southern Maine in 2012 (IN: R2 = 0.173, df =
27, P = 0.025, n = 29; CI: R2 = 0.438, df = 19, P = 0.001, n = 21). The lines on the graph
represent the linear regression of the data (IN: y = 0.4342x ; CO: y = 0.7039x - 0.0183) .
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Our top-ranked model to assess daily nest-survival rates indicated that GBHE
daily nest-survival rates would increase if they hatched their young earlier in the
season and had more nestlings in the nest. In this study, we were unable to assess
clutch size due to nest heights. However, if the number of nestlings in a nest was an
accurate reflection of clutch size, our results agree with others who found that pairs
that produce more eggs have more fledglings (GBHE: Pratt and Winkler 1985, Grey
Heron: Millstein et al. 1970, Ardea purpurea L. [Purple Heron]: Tomlinson 1975,
and Great Egret: Morrison and Shanley 1978). Furthermore, Butler (1993) found
that food availability determined when a female GBHE laid her eggs (Butler 1993,
Perrins 1970). Future studies should explore the relationship between clutch size,
food availability, and colony location.
The second-ranked model showed that daily nest survival increased when adults
fed their young more frequently in the first 2 weeks post-hatch, and when eggs began
hatching earlier in the season. Jakubas (2005) found that the number of feeding
visits to colonies of Grey Herons was the most important factor affecting breeding
success. In California, Pratt (1970) also saw an increase in brood-provisioning
frequency during the first 1–2 weeks of life in GBHEs. A study of the energy requirements
in hand-reared GBHE nestlings found the greatest energy requirements
for growth were between 10 and 29 d post-hatch (Bennett et al. 1995). The discrepancy
in the period of time of greatest provisioning demand between Bennett et al.
(1995) and Pratt (1970) may be due to the fact that Bennett et al. (1995) studied
hand-reared chicks. Our results indicate the most important time period for higher
brood-provisioning rates as the first 1–2 weeks post-hatch.
To fully understand the regional population dynamics of Great Blue Herons,
monitoring efforts should focus on key times within the breeding season when accurate
nestling numbers and ages can be gathered; specifically after the first 1–2
weeks post hatch. Ideally, monitoring efforts should encompass both coastal and
inland habitats in order to better understand variation in regional population dynamics,
and particular attention should be paid to the hatching times in these colonies.
The decline of GBHEs along the coast of Maine could be an indicator of a greater
disruption in Maine’s coastal ecosystem. With a total of 411 hours of colony monitoring
and 123 observed brood-provisioning events, this study is the most thorough
examination of parental care and nestling behavior of GBHEs in the northeastern
part of its range. Our work provides a more accurate and current estimate of the
most sensitive times during the GBHE breeding season in the northeastern part of
their range, as well as a way to accurately and non-invasively assess nest survival.
These data can serve as a model for monitoring regional waterbird populations including
those in Maine.
Acknowledgments
Thanks to D. D’Auria, Maine Department of Inland Fisheries and Wildlife, for her support
and collaboration. We thank the Knedler and Knowles families for access to their land.
G. Nau, E. Brzycki, and L. Manchen provided invaluable field assistance. This work was
supported by the National Science Foundation GK-12 Fellowship [DGE-0841361] and the
Biological and Marine Sciences Graduate Program at the University of New England.
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