Scientific
Papers
Dr.
Richard Holmes began studying birds at Hubbard Brook in 1969,
and since then he and his colleagues have published a tremendous
number of scientific papers on the bird community as a whole
and on species like the Black-throated Blue Warbler in particular.
Below is a selection of papers that are relevant to the ideas
and data discussed here. They are listed in chronological order
and appear with abstracts written by the authors of the various
papers, summarizing the goals of their study, the method used,
and their main findings.
RT Holmes, RE Bonney, and
SW Pacala (1979) Guild structure of the Hubbard Brook bird community:
a multivariate approach. Ecology 60 (3): 512–520.
We examined the similarities and
differences in the foraging patterns of 22 insectivorous bird
species during their breeding season in the Hubbard Brook Experimental
Forest, New Hampshire, USA. Using multivariate techniques (clustering
of hyperdimensional Euclidean distances, principal components
analysis, and Varimax rotated factor analysis), we distinguish
4 groups of species or guilds, each of which exploits food resources
in a distinctly different way. Partitioning occurs primarily
by (1) foraging height and height—related characters,
(2) foraging locations within the forest canopy, and (3) differential
use of tree species, foraging substrates and foraging maneuvers.
The results indicate that the importance of vegetation height
to bird species diversity is related (1) to foraging opportunities
which differ along a gradient from ground level to the upper
canopy and which are roughly indexed by measures of foliage
height diversity (FHD), and (2) to the presence of the supporting
branch and bole framework which provides a major distinct foraging
region. We suggest that foraging opportunities vary with height
in a forest and are influenced by the physical and chemical
characteristics of the plant species, which in turn affect the
kinds and distributions of foraging substrates, the ways in
which birds search for and find food, and the abundances of
food resources. The implications of these findings for understanding
the structure of forest bird communities are discussed.
RT Holmes, JC Schultz, and
P Nothnagle (1979) Bird predation on forest insects: an exclosure
experiment. Science 206 (4417): 462–463.
Exclusion experiments show that
birds significantly reduce densities of larval Lepidoptera on
forest understory vegetation. When insect densities are already
low, bird predation may act both as a population regulator and
as a strong agent of natural selection.
RT Holmes and SK Robinson
(1981) Tree species preferences of foraging insectivorous birds
in a northern hardwoods forest. Oecologia 48: 31-35.
Birds searching for insects in the
canopy of a northern hardwoods forest depart significantly from
random in their use of tree species, even when these trees are
generally similar in life form. All 10 foliage-dwelling bird
species in the Hubbard Brook forest showed preferences for Yellow
Birch, most had an aversion to Beech and Sugar Maple, and a
few had special preferences for conifers or White Ash. Birds
that glean prey from leaves had stronger tree species preferences
than those that often hover for their prey, and were more influenced
by tree species differences in foliage structure. The less common
bird species and those for which northern hardwoods are marginal
habitat had the most pronounced tree-species preferences. Food
densities which are higher on Yellow Birch and specific adaptations
to foraging in trees with particular foliage structures are
considered major factors responsible for the observed tree species
preferences. The implications of these findings for bird community
structure and for forest management practices are discussed.
RT Holmes, TW Sherry, and
FW Sturges (1986) Bird community dynamics in a temperate deciduous
forest: long-term trends at Hubbard Brook. Ecological Monographs
56 (3): 201-220.
Changes in species composition and
abundance of birds breeding in an unfragmented temperate deciduous
forest in New Hampshire, USA, were studied intensively during
16 consecutive breeding seasons, 1969-1984. The number of species
breeding in the 10-ha study area in any one year varied from
17 to 28, and averaged 24. Total numbers of individuals breeding
on the 10-ha plot ranged from 214 to 89, with many species (70%)
declining during the 16-yr period. Overall, there was significant
positive covariation among population trends of all species,
suggesting a major, perhaps single, factor affecting population
levels, such as weather or food supply. No two species, however,
had identical patterns of change across all 16 yr. Thus, populations
in this forest fluctuated largely independently of one another,
suggesting a different combination of regulatory factors for
each species. The major factors pulsing or stressing bird populations
in the field included: (1) changes in food abundance due largely
to irruptions of defoliating Lepidoptera (most bird species,
but especially vireos and warblers), (2) harsh late spring and
summer weather (Scarlet Tanager, American Redstart), (3) changes
in habitat structure related to forest succession (Least Flycatcher,
Philadelphia Vireo), (4) interspecific interactions, particularly
interference competition (Least Flycatcher-American Redstart,
Red-eyed Vireo-Philadelphia Virreo), and (5) mortality during
winter for both resident and migrant populations (Hermit Thrush,
Dark-eyed Junco, permanent resident species such as woodpeckers
and nuthatches. Some, but not all, irruptions of defoliating
Lepidoptera significantly influenced many bird populations in
this forest and contributed to the observed positive covariation
in abundances. These irruptions produced pulses of food for
breeding birds, but occurred at long and variable time intervals.
Between outbreaks, food may regularly limit reproductive output
of these forest birds, and low food abundance, along with the
effects of predators, weather, and other mortality factors,
contributes to high variability in bird reproductive success,
which ultimately influences population size. Competition may
also become important during these periods of food scarcity.
We therefore propose that birds in these temperate deciduous
forests experience periods of prolonged food limitation, interrupted
by relatively brief periods of superabundant food, a pattern
contrasting with that described for other temperate systems
(e.g., by Wiens 1977). From this examination of bird community
dynamics on one site over many years, it is apparent that each
species responds to its environment in a unique way, as determined
by a variety of influences on its populations. Some of these
operate on a local scale (e.g., vegetation structure, food abundance,
interspecific competitors), while others function at regional
(e.g., some weather effects) and global (e.g., winter events)
geographic scales. These influences also differ as to their
temporal persistence and predictability, and thus are difficult
to detect in short-term studies. This pluralistic view of community
structure for birds in temperate forests represents a compromise
between nonequilibrial and competitionist models, and argues
against the existence of a tightly organized community at any
one spatial or temporal scale.
RT Holmes and TW Sherry
(1988) Assessing population trends of New Hampshire forest birds:
local vs. regional patterns. Auk 105 (4): 756-768.
We examined the changes in abundance
between 1969 and 1986 of 19 forest-dwelling, mostly migratory
bird species breeding in New Hampshire at 2 different scales:
one local (an intensively studied 10-ha plot in unfragmented
forest) and the other regional (Breeding Bird Surveys statewide).
Twelve of the 19 species exhibited similar trends at both scales.
Eight neither increased nor decreased, and 4 (Least Flycatcher,
Winter Wren, Wood Thrush, and Swainson's Thrush) declined significantly.
Others increased, decreased, or remained steady at one or the
other scale. Overall, more species declined than increased both
locally (8 vs. 1) and regionally (5 vs. 1). Comparisons of these
patterns, combined with results of intensive studies at the
local level, suggest that changes in food abundance and in vegetation
structure related to forest succession on the breeding grounds,
along with other processes that influence bird reproductive
success and survivorship, are the most plausible explanations
for most of the observed trends. Winter mortality was also identified
as affecting breeding abundances, but only in short-distance
migrant and permanently resident species. We have no evidence
to indicate that the numbers of long-distance migrants were
affected by events in their Neotropical wintering areas, although
this possibility is difficult to assess from breeding-ground
data. We urge caution in attributing declines of breeding forest
migrant birds to tropical deforestation or similar causes until
we either can eliminate alternate explanations that involve
breeding-season events or have available critically needed demographic
information on migrant populations in their wintering areas.
NL Rodenhouse (1992) Potential
effects of climatic change on a Neotropical migrant landbird.
Conservation Biology 6 (2): 263-272.
Significant climatic changes due
to the "green-house" effect are predicted to occur
in breeding and wintering areas of neotropical migrant landbirds
within 50 years. A simulation model was used to quantitatively
evaluate the impact of predicted climatic changes on the annual
productivity [number of fledgling · (1/pair ·
season)] of Black-throated Blue Warblers (Dendroica caerulescens)
breeding in northern hardwoods forests. Results suggested that
where mean summer rainfall increases by 20%, greater egg and
nestling mortality may reduce annual breeding productivity by
as much as 26%. Warmer temperatures, however, will tend to enhance
food abundance for insectivorous birds and lengthen the avian
breeding season. These changes will act to increase breeding
productivity, but they can only compensate for about a 10% increase
in precipitation. Precipitation, however, is predicted to decline
over most of the range of Black-throated Blue Warblers. Where
mean temperature is warmer and average summer precipitation
is lower by as little as 10%, breeding productivity of Black-throated
Blue Warblers may be enhanced by up to 25%. Because significant
regional differences in climatic change are likely, and because
migratory bird species will differ in their response to climatic
change, the emphasis of avian research should be on intensive
regional studies of the factors influencing avian reproduction
and survival.
NL Rodenhouse and RT Holmes
(1992) Results of experimental and natural food reductions for
breeding Black-throated Blue Warblers. Ecology 73 (1): 357-372.
We examined effects of natural and
experimentally created reductions in food abundance on the reproductive
ecology of Black-throated Blue Warblers (Dendroica caerulescens).
The study was carried out between 1982 and 1985 on four 30-ha
plots of temperate deciduous forest within and near the Hubbard
Brook Experimental Forest, New Hampshire, USA. Experimental
food reduction using aerially sprayed Bacillus thuringiensis
created significant differences in caterpillar biomass between
one sprayed and two unsprayed plots throughout the breeding
season in 1983. Where caterpillar abundance was reduced, Black-throated
Blue Warblers made significantly fewer nesting attempts and
diets of neslings included fewer caterpillars. Clutch size,
hatching success, and numer of young fledging per nest did not
differ among the food reduction site and controls. Also, the
reduced number of nesting attempts per pair on the food reduction
site in 1983 did not significantly lower production of young
per pair. No detectable differences in caterpillar biomass between
sprayed and unsprayed sites were created in 1984 or 1985 because
natural caterpillar abundances were already low. Natural declines
in food abundance for Black-throated Blue Warblers occurred
from 1982 through 1985, primarily because of a decrease in caterpillar
abundances. Significant reductions in number of young fledging
per nest, nestling growth rates and survival, and number of
nests attempted per pair corresponded with the natural decline
in food abundance. Of those measures, a reduced number of nesting
attempts per pair lowered annual production the most. When effects
of food limitation were calculted separately from those of nest
predation, food limited annual breeding productivity to below
that needed to balance annual mortality in at least one of the
four years of this study. These findings indicate that neotropoical
migrant bird species are probably limited periodically by food
when breeding in north-temperate habitats.
RT Holmes, PP Marra, and
TW Sherry (1996) Habitat-specific
demography of breeding Black-throated Blue Warblers (Dendroica
caerulescens): implications for population dynamics. Journal
of Animal Ecology 65: 183-195.
The distribution of individuals
among habitats and their relative success in those habitats
can have important consequences for population dynamics. To
examine these processes for a long-distance migratory bird species,
we studied the population structure, age-specific reproductive
output, and local survival of black-throated blue warblers (Dendroica
caerulescens, Gmelin) in two breeding habitats differing
in shrub density within northern hardwoods forests in New Hampshire,
USA. On forest plots with dense shrubs, warblers occurred at
higher densities, and fledged significantly more young per capita
per season than those occupying areas with lower shrub density.
This differential productivity was due to higher reproductive
output, mainly through double-brooding, of older (>= 2 years
of age) individuals, which were disproportionately more abundant
in high shrub density sites. Clutch initiation dates, clutch
sizes, and predation rates at individual nests did not differ
significantly between habitats. Mean body mass of nestlings
on day 6 following hatching were higher on average on plots
with high shrub density, but differences were not significant.
Annual return rates, as indices of local survival, did not differ
between habitats for older males or for females. Yearling males,
however, returned in subsequent years at a significantly lower
rate to low shrub density plots, a result of either lower survival
or, more likely, dispersal to more suitable habitat in their
second year of breeding. Parental age and habitat suitability
interact in that older individuals, through their experience
and/or dominance, acquire sites of higher quality, which results
in higher reproductive output and probably higher survival.
These differences between habitats in density, reproductive
performance and local survival are consistent with an ideal-despotic/preemptive
distribution of individuals, and suggest that this population
could be regulated by the availability, distribution, and extent
of high and low quality breeding habitats.
TS Sillett, RT Holmes, and
TW Sherry (2000) Impacts of a global climate cycle on population
dynamics of a migratory songbird. Science 288: 2040-2042.
Progress toward understanding factors
that limit abundances of migratory birds, including climate
change, has been difficult because these species move between
diverse locations, often on different continents. For black-throated
blue warblers (Dendroica caerulescens), demographic rates in
both tropical winter quarters and north temperate breeding grounds
varied with fluctuations in the El Niño Southern Oscillation.
Adult survival and fecundity were lower in El Niño years
and higher in La Niña years. Fecundity, in turn, was
positively correlated with subsequent recruitment of new individuals
into winter and breeding populations. These findings demonstrate
that migratory birds can be affected by shifts in global climate
patterns and emphasize the need to know how events throughout
the annual cycle interact to determine population size.
RT Holmes and TW Sherry
(2001) Thirty-year bird population trends in an unfragmented
temperate deciduous forest: importance of habitat change. Auk
118 (3): 589–609.
Abundances of forest birds in an
unfragmented, undisturbed, and relatively mature temperate deciduous
forest at the Hubbard Brook Experimental Forest, New Hampshire,
changed markedly between 1969 and 1998. Total numbers of birds
(all species combined) declined from 210–220 individuals/10
ha in the early 1970s to 70–90/10 ha in the 1990s. Of
the 24 regularly occurring species, 12 decreased significantly
(four to local extinction), three increased significantly, and
nine remained relatively constant in abundance. Nine of the
12 declining species were Neotropical migrants. Most species
exhibited similar trends on Breeding Bird Survey (BBS) routes
in New Hampshire during the same 30 year period and on three
replicate study sites in nearby sections of the White Mountains
from 1986–1998. Probable causes of trends were diverse
and differed among species. Most could be accounted for by individual
species' responses to events occurring primarily in the local
breeding area. The most important local factor affecting bird
abundance was temporal change in forest vegetation structure,
resulting from natural forest succession and local disturbances.
Four species that declined markedly and in some cases disappeared
completely from the study plot (Least Flycatcher, Empidonax
minimus; Wood Thrush, Hylocichla mustelina; Philadelphia
Vireo, Vireo philadelphicus; and American Redstart,
Setophaga ruticilla) appear to attain peak abundance
in early or mid successional forests. Species preferring more
mature forests, such as Black-throated Green Warbler (Dendroica
virens) and Ovenbird (Seiurus aurocapillus), increased
significantly in abundance over the 30 year study. Other important
factors influencing bird abundances were food availability and
events in the migratory and winter periods. Nest-predation rates,
although varying among years, showed no long-term pattern that
would account for population declines, and brood parasites were
absent from this forest. Findings from this study demonstrate
that major changes in bird abundances occur over time even in
undisturbed and relatively mature forests, and illustrate the
need for considering habitat requirements of individual species
and how habitat suitability changes over time when trying to
assess the causes of their long-term population trends. The
results also imply that any conclusions about the effects of
other factors affecting forest bird abundances, such as increased
nest predation or brood parasitism associated with habitat fragmentation,
must also account for successional changes that may be affecting
habitat suitability.
TS Sillett and RT Holmes
(2002) Variation in survivorship of a migratory songbird throughout
its annual cycle. Journal of Animal Ecology 71 (2): 296-308.
Demographic data from both breeding
and non-breeding periods are needed to manage populations of
migratory birds, many of which are declining in abundance and
are of conservation concern. Although habitat associations,
and to a lesser extent, reproductive biology, are known for
many migratory species, few studies have measured survival rates
of these birds at different parts of their annual cycle. Cormack-Jolly-Seber
models and Akaike's information criterion model selection were
used to investigate seasonal variation in survival of a Nearctic
- Neotropical migrant songbird, the black-throated blue warbler,
Dendroica caerulescens. Seasonal and annual survival were estimated
from resightings of colour-ringed individuals on breeding grounds
in New Hampshire, USA from 1986 to 2000 and on winter quarters
in Jamaica, West Indies from 1986 to 1999. Warblers were studied
each year during the May-August breeding period in New Hampshire
and during the October-March overwinter period in Jamaica. In
New Hampshire, males had higher annual survival (0.51 ±
0.03) and recapture probabilities (0.93 ± 0.03) than
did females (survival: 0.40 ± 0.04; recapture: 0.87 ±
0.06). In Jamaica, annual survival (0.43 ± 0.03) and
recapture (0.95 ± 0.04) probabilities did not differ
between sexes. Annual survival and recapture probabilities of
young birds (i.e. yearlings in New Hampshire and hatch-year
birds in Jamaica) did not differ from adults, indicating that
from the time hatch-year individuals acquire territories on
winter quarters in mid-October, they survive as well as adults
within the same habitat. Monthly survival probabilities during
the summer (May-August) and winter (October-March) stationary
periods were high: 1.0 for males in New Hampshire, and 0.99
± 0.01 for males in Jamaica and for females in both locations.
These annual and seasonal survival estimates were used to calculate
warbler survival for the migratory periods. Monthly survival
probability during migration ranged from 0.77 to 0.81 ±
0.02. Thus, apparent mortality rates were at least 15 times
higher during migration compared to that in the stationary periods,
and more than 85% of apparent annual mortality of D. caerulescens
occurred during migration. Additional data from multiple species,
especially measures of habitat-specific demography and dispersal,
will improve our understanding of the relative impacts of the
breeding, migratory, and winter periods on population dynamics
of migratory birds and thus enhance future conservation efforts.
J Jones, PJ Doran, and RT
Holmes (2003) Climate and food synchronize regional forest bird
abundances. Ecology 84 (11): 3024-3032.
Analysis of synchrony in population
fluctuations can help to identify factors that regulate populations
and the scales at which these factors exert their influence.
Using 15 years of data on the abundances of songbirds at four
replicate forest sites in New Hampshire, USA, we addressed two
main questions: (1) Are forest bird populations synchronous
at the scale measured (tens of kilometers), and if so, (2) what
environmental factors are responsible for the synchrony? Nine
of the 10 bird species we examined exhibited significant spatial
synchrony across the four sites. Within nesting and foraging
species groups, tree nesters and foliage gleaners exhibited
the highest spatial synchrony. Long-distance (Neotropical) migrants
exhibited higher spatial synchrony than did short-distance migrants
or year-round residents. Synchrony within and among six species
of long-distance, migratory, insectivorous birds was correlated
with synchronous fluctuations in the abundance of lepidopteran
larvae, a primary food type during the breeding season, which
in turn have been shown to be influenced by El Niño/La
Niña global climate patterns. Abundances of year-round
resident species were related to another large-scale climatic
phenomenon, the North Atlantic Oscillation. Winter weather can
have both direct (e.g., via temperature-mediated mortality)
and indirect (e.g., via winter food availability) effects on
year-round resident species. We do not believe that predation
on adults or nests accounted for the observed synchrony. Dispersal
among regional populations in this system may have played a
role but is likely a product of the influence of regionally
synchronous caterpillar fluctuations on bird reproduction. Long-term
regional population trends may have contributed to the observed
synchrony for some species, but we do not consider these trends
to be primary factors. Our findings of population synchrony
support the importance of food and climate in influencing forest
bird abundances and have broad implications for potential responses
of bird and insect populations to climate change.
NL Rodenhouse, TS Sillett,
PJ Doran, and RT Holmes (2003) Multiple density-dependence mechanisms
regulate a migratory bird population during the breeding season.
Proceedings of the Royal Society of London B 270: 2105-2110.
The mechanisms regulating bird populations
are poorly understood and controversial. We provide evidence
that a migratory songbird, the black-throated blue warbler (Dendroica
caerulescens), is regulated by multiple density-dependence
mechanisms in its breeding quarters. Evidence of regulation
includes: stability in population density during 1969-2002,
strong density dependence in time-series analyses of this period,
an inverse relationship between warbler density and annual fecundity,
and a positive relationship between annual fecundity and recruitment
of yearlings in the subsequent breeding season. Tests of the
mechanisms causing regulation were carried out within the Hubbard
Brook Experimental Forest, New Hampshire, during 1997-1999.
When individuals from abutting territories were experimentally
removed in a homogeneous patch of high-quality habitat, the
fecundity of focal pairs nearly doubled, revealing a locally
operating crowding mechanism. A site-dependence mechanism was
indicated by an inverse relationship between population size
and mean territory quality, as well as by greater annual fecundity
on the sites that were most frequently occupied and of highest
quality. These site-dependence relationships were revealed by
intensive monitoring of territory quality and demography at
the landscape spatial scale. Crowding and site-dependence mechanisms,
therefore, acted simultaneously but at different spatial scales
to regulate local abundance of this migratory bird population.
TS Sillett, NL Rodenhouse,
and RT Holmes (2004) Experimentally reducing neighbor density
affects reproduction and behavior of a migratory songbird. Ecology
85 (9): 2467-2477.
Because populations of territorial
birds are relatively stable compared to those of other animal
taxa, they are often considered to be tightly regulated. However,
the mechanisms that produce density-dependent feedbacks on demographic
rates and thus regulate these populations are poorly understood,
particularly for migratory species. We conducted a three-year
density-reduction experiment to investigate the behavioral mechanisms
that regulate the abundance of a Nearctic–Neotropical
migrant passerine, the Black-throated Blue Warbler (Dendroica
caerulescens), during the breeding season. We found that
the number of young fledged per territory, territory size, and
the proportion of time males spent foraging were significantly
greater on territories around which neighbor density was experimentally
reduced compared to control territories. Territory quality,
proportion of nests depredated per territory, and male countersinging
rates were not statistically different between treatments. These
results indicate that individuals with more neighbors (i.e.,
in neighborhoods with greater conspecific density) have reduced
breeding productivity. The results also suggest that a crowding
mechanism that mediates interactions among territory-holders
could generate the density dependence needed to regulate local
abundance, at least in areas of homogeneous, high-quality habitat.
The effect of the neighbor-density reduction on warbler fecundity
and behavior varied with annual fluctuations in weather and
food availability, and was strongest in 1997, an El Niño
year, when conditions for breeding were least favorable. This
variation in our experimental results among years implies that
density dependence due to crowding may have its strongest impact
on local abundance when environmental conditions are relatively
poor.
LR Nagy and RT Holmes (2004)
Factors influencing fecundity in migratory songbirds: is nest
predation the most important? Journal of Avian Biology 35 (6):
487-491.
Nest predation often is reported
to be the major cause of nest failure in birds and by extension
the predominant influence on annual fecundity. However, other
factors such as the ability of some individuals to renest after
a failed nest or to lay additional clutches after successful
ones, both of which may in turn be influenced by food availability,
can also contribute to individual fecundity. We evaluated the
relative importance of nest predation, multiple brooding, renesting,
and food availability on mean annual fecundity of a migratory
songbird population, using AICc from multiple regression models.
In a 16-yr study of black-throated blue warblers Dendroica caerulescens,
these four variables combined explained 87% of the annual variance
in fecundity. Individually, however, food abundance explained
more of the variability (35%) than did predation (29%), double-brooding
(19%), or renesting (15%). These results suggest that measuring
food availability may be essential to understanding annual differences
in reproductive output. They also demonstrate that failing to
account for the contribution of food availability, renesting,
and multiple brooding in studies of avian demography can result
in serious underestimates of mean annual fecundity, potentially
biasing calculations of population growth rate, source-sink
dynamics, habitat quality, or population viability.
LR Nagy and RT Holmes (2005)
Food limits annual fecundity of a migratory songbird: an experimental
study. Ecology 86 (3): 675-681.
In short-lived species, fecundity
strongly influences population size. For those species with
multiple breeding attempts per breeding season, variance in
fecundity is best explained by the number of breeding attempts.
For birds, multiple brooding may be influenced by food availability.
Here, we report results of a food supplementation experiment
that tests the role of food as a mechanism driving variation
among individuals in the frequency of multiple brooding in a
Neotropical migrant songbird, the Black-throated Blue Warbler
(Dendroica caerulescens). Supplementally fed females
produced more second broods, spent less time foraging and more
time loafing, and stayed closer to their nests than did control
females. Fed and control females did not differ in the number
or mass of young fledged from the first nesting attempt. Supplemental
food increased the probability that females would initiate second
broods in both a low and an average food year, suggesting that
this population is food limited during the breeding season in
most years. Our results thus demonstrate that food availability
can strongly influence annual fecundity in migratory bird species
breeding in temperate forests, which, in turn, affects annual
recruitment rates and population size.
LR Nagy and RT Holmes (2005)
To double brood or not? Individual variation in the reproductive
effort in Black-throated Blue Warblers (Dendroica caerulescens).
Auk 122 (3): 902-914.
Individuals within a population
vary in important fitness components, such as reproductive success.
In general, females can maximize the number of young they produce
by altering either the number of young per breeding attempt
or the number of breeding attempts per season. In short-lived
species, and especially in small passerine birds, number of
breeding attempts per season varies markedly among individuals.
Here, we evaluated factors influencing whether female Blackthroated
Blue Warblers (Dendroica caerulescens) initiated additional
nests after a successful breeding attempt (i.e. double-brooded).
The percentage of females that laid a second clutch after successfully
fledging a first brood ranged from 0 to 87% and averaged 53%
(n = 7 years). Multiple logistic regression and AICc model selection
indicated that double-brooded females bred in territories with
greater food availability and produced heavier nestlings than
single-brooded females. Female age, male age, date of first
breeding attempt, and number of young in the first clutch were
not included in the best-fit model. Older females, however,
produced heavier fledglings, and females mated to older males
occurred on territories with greater food availability, indicating
that age contributed to individual variation in reproductive
output. Because the proportion of females that produce multiple
broods within a season can have a substantial effect on the
annual fecundity of a population, variation among females and
among the territories they occupy (i.e. habitat quality) are
key factors influencing population dynamics in this and other
multibrooded, shortlived species.
RC Dobbs, TS Sillett, NL
Rodenhouse, and RT Holmes (2007) Population density affects
foraging behavior of male Black-throated Blue Warblers during
the breeding season. Journal of Field Ornithology 78 (2): 133-139.
Foraging behavior often reflects
food availability, a resource that may increasingly limit breeding
birds as intraspecific crowding increases. Measuring foraging
behavior, therefore, provides a way to investigate effects of
population density on food limitation, an important link in
understanding how crowding functions to regulate populations.
We quantified three components of foraging behavior (prey attack
rate, foraging speed, and relative use of morphologically constrained
attack maneuvers) for male Black-throated Blue Warblers (Dendroica
caerulescens) breeding under experimentally manipulated
density conditions. Building on the previous work showing the
density of conspecific neighbors affects territory size, reproductive
success, and the time budgets of males (Sillett et al. 2004,
Ecology 85: 2467–2477), we further show that density affects
male foraging strategies. Although not differing in attack rate
or foraging speed, male Black-throated Blue Warblers on territories
with reduced neighbor densities used energetically expensive
aerial attack maneuvers significantly less frequently than males
in control (high-density) territories during both the incubation
period and when provisioning nestlings and fledglings. We conclude
that males altered their foraging behavior to compensate for
density-related reductions in time available for foraging and
that population density may constrain the time available for
foraging.
RT Holmes (2007) Understanding
population change in migratory songbirds: long-term and experimental
studies of Neotropical migrants in breeding and wintering areas.
Ibis 149 (2): 2-13.
Effective conservation and management
of migratory bird species requires an understanding of when
and how their populations are limited and regulated. Since 1969,
my colleagues and I have been studying migratory songbird populations
in their breeding quarters at the Hubbard Brook Experimental
Forest in north-central New Hampshire, USA, and since 1986,
in their winter quarters in the Greater Antilles (Jamaica).
Long-term data on the abundance and demography of these populations,
coupled with experimental tests of mechanisms, indicate that
processes operating in the breeding area (e.g. density-dependent
fecundity, food limitation) are sufficient to limit and regulate
the local abundance of these species. At the same time, limiting
factors operating in the non-breeding season (e.g. climate-induced
food limitation in winter quarters and especially mortality
during migration) also have important impacts on migrant populations.
Furthermore, recent studies have shown that limiting processes
during the winter period can carry over into the breeding season
and affect reproductive output. These findings clearly demonstrate
that to understand changes in abundance of long-distance migrant
species requires knowledge of events operating throughout the
annual cycle, which presents a challenge to researchers, managers
and others concerned with the welfare of these species.
NL Rodenhouse, SN Matthews,
KP McFarland, JD Lambert, LR Iverson, A Prasad, TS Sillett,
and RT Holmes (2008) Potential effects of climate change on
birds of the Northeast. Mitigation and Adaptation Strategies
for Global Change 13: 517-540.
We used three approaches to assess
potential effects of climate change on birds of the Northeast.
First, we created distribution and abundance models for common
bird species using climate, elevation, and tree species variables
and modeled how bird distributions might change as habitats
shift. Second, we assessed potential effects on high-elevation
birds, especially Bicknell’s thrush (Catharus bicknelli),
that may be particularly vulnerable to climate change, by using
statistical associations between climate, spruce-fir forest
vegetation and bird survey data. Last, we complemented these
projections with an assessment of how habitat quality of a migratory
songbird, the black-throated blue warbler (Dendroica caerulescens)
might be affected by climate change. Large changes in bird communities
of the Northeast are likely to result from climate change, and
these changes will be most dramatic under a scenario of continued
high emissions. Indeed, high-elevation bird species may currently
be at the threshold of critical change with as little as 1°C
warming reducing suitable habitat by more than half. Species
at mid elevations are likely to experience declines in habitat
quality that could affect demography. Although not all species
will be affected adversely, some of the Northeast’s iconic
species, such as common loon and black-capped chickadee, and
some of its most abundant species, including several neotropical
migrants, are projected to decline significantly in abundance
under all climate change scenarios. No clear mitigation strategies
are apparent, as shifts in species’ abundances and ranges
will occur across all habitat types and for species with widely
differing ecologies.
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