2008 NORTHEASTERN NATURALIST 15(1):87–96
Lipid Content of Nearctic-Neotropical Migratory Passerines
Killed During Stopovers in a New York City Park
Chad L. Seewagen*
Abstract - Urban parks often represent the only stopover habitats available to migrating
birds encountering expansive metropolitan areas. Green spaces remaining within
cities may therefore be valuable to migrants; yet studies of migrants in this context are
few. I examined the lipid content of birds killed by window collisions in spring and
autumn in a small recreational park in New York City to assess the energetic condition
of migratory passerines utilizing an urban habitat as a stopover site. I compared
chemically determined fat content (expressed as a lipid index: g fat / g lean dry mass)
and visible subcutaneous fat scores between seasons, autumn age classes, and birds
grouped by family and foraging guild. Average total body fat (as % of dry mass) was
29.4% in spring and 24.1% in autumn; few lean birds were found in either season. Birds
in spring were significantly fatter than in autumn. In spring and autumn, no differences
in fat content (i.e., fat scores and lipid indices) were observed between warblers and
thrushes. In spring, there were no differences in fat content between warbler foraging
guilds, whereas in autumn, ground/understory-foraging warbler species were fatter than
warbler species associated with arboreal foraging. In autumn, the fat content of immature
birds was comparable to that of adults. It could not be determined whether the high
fat content of birds found here was acquired during stopovers in the study site or if birds
arrived with substantial fat stores remaining from previous stopovers. The likelihood of
each scenario and the value of urban parks to migratory birds are discussed.
Migration places great energy demands on birds, and stopover habitats
where depleted fat reserves can be quickly restored are critical to successful
migrations (Moore et al. 1995). Recent broad-scale habitat loss along
migration flyways has increased concern about the conservation of many
migratory species that already face pressures from anthropogenic changes
on their breeding and wintering grounds. In the northeastern United States,
urban land covers approximately one third of the region, considerably limiting
the amount of stopover habitat available to birds migrating through this
area (Dettmers and Rosenberg 2000). Here, the habitats remaining within
cities may be of particular importance to en route migrants (Dettmers and
Rosenberg 2000, Mehlman et al. 2005). Yet the use of urban habitats by
migrating birds remains poorly understood, and it is uncertain whether such
areas can serve as adequate stopover sites. The high densities at which birds
often occur in these small green spaces and the prevalence of exotic vegetation
may prohibit migrants from adequately gaining body mass as a result
of intense competition for limited resources. With increasing urbanization,
municipal parks and similar fragments will account for a growing proportion
*Department of Ornithology, Bronx Zoo/Wildlife Conservation Society, 2300 Southern
Boulevard, Bronx, NY 10460; email@example.com.
88 Northeastern Naturalist Vol. 15, No. 1
of the stopover sites available to migrating birds. Investigating migrant utilization
of existing urban habitats is an important step towards understanding
the effect that further urban sprawl will have on bird migration.
The goal of this study was to examine the energetic condition of transient
birds utilizing an urban habitat as a stopover site. I measured the lipid content
of Nearctic-Neotropical migratory passerines killed by window collisions during
spring and autumn stopovers in a small recreational park on the heavily
urbanized island of Manhattan, New York City, NY. The birds’ lipid content
was considered indicative of their energetic condition (Morton et al. 1991).
Chelsea Park is a rectangular, 1.4-ha recreational park located between
27th and 28th Streets and 9th and 10th Avenues (40°45'N, 73°59'W) in the borough
of Manhattan. An artificial turf sports field accounts for approximately
1/3 of the park’s total area; the remaining area consists of asphalt basketball
and handball courts, a children’s playground, a two-story government
office building, and a historical monument surrounded by park benches
and annual flower beds. No natural or artificial water bodies are present.
The park’s perimeter is lined with 27 mature Platanus x acerifolia Aiton
(London plane trees) that range in height from approximately 15–20 m. An
additional 28 London plane trees of similar size are distributed throughout
the park interior. The interior tree density creates a closed canopy that conceals
most of the park’s understory conditions when viewed from above. All
but nine of the interior and perimeter trees stand in tree-pits, as the majority
of the park’s ground is impermeable surface. Aside from the tree-pits, the
annual beds around the office building and monument represent the only
permeable and vegetated areas in the park. The beds are planted with two
non-native species, Itea virginica L . (Virginia sweetspire “Henry’s garnet”)
and Liriope muscari Decne. (big blue lilyturf).
Throughout spring and autumn 2005 and 2006, dead birds were salvaged
daily from beneath the large, highly reflective windows of a 6-story building
(341 Ninth Avenue) that abuts the northern boundary of Chelsea Park
(see Gelb and Delacretaz 2006 for details). The windows reflect the Park’s
greenery, giving a false appearance of additional habitat. As a result, many
migrants are killed by daytime collisions with these windows during stopovers
in Chelsea Park.
Most of the birds used in this study were collected (and presumed to
have died) during daylight hours (Gelb and Delacretaz 2006). The exact
time between death and collection was unknown, but was estimated to be no
longer than six hours (Y. Gelb, New York City Audubon Society, New York,
NY, pers. comm.). Collected birds were bagged and frozen until the time of
processing. The specimens used here represented 29 Nearctic-Neotropical
migratory species (Appendix A).
2008 C.L. Seewagen 89
Specimens were thawed at room temperature until flexible enough to
manipulate. Visible subcutaneous fat in the furcular hollow was scored on a 6-
point scale (Moore and Kerlinger 1987): (0) no visible fat; (1) trace of fat, but
not completely lined; (2) completely lined with thin fat layer; (3) filled with
fat but still concave; (4) filled with fat even with pectoralis or slightly bulging;
and (5) filled to bulging and at least partially covering keel. Fat was scored
by the same individual throughout the study to avoid inter-observer variation
(Krementz and Pendleton 1990). Specimens were then weighed to the nearest
0.001 g (Denver 410-g digital balance). Fall birds were aged as hatching-year
(HY) or after-hatching-year (AHY) by plumage characteristics and extent
of skull ossification (Pyle 1997). A ventral midline incision was made from
the furcula to the cloaca to expose the thoracic and abdominal cavities, and
carcasses were oven-dried to a constant mass at 75 °C. Dry carcasses were
re-weighed and homogenized (including feathers) with an electric blender.
Duplicate 1-g (± 0.100 g) samples of the homogenate of each bird were placed
in cellulose thimbles, and soluble fat was extracted with petroleum ether in
a Soxtec apparatus (FOSS Inc., Laurel, MD). Following extraction, samples
were oven-dried overnight and weighed the following day. The percentages of
mass lost from both samples were averaged to yield total body fat %.
Because birds were collected only when fatal window collisions occurred,
the total sample of study specimens was composed of a wide array of species
with most species represented by no more than a few individuals. These small
samples prohibited analysis at the species level. Instead, birds were grouped
by family into Parulidae (wood warblers), Turdidae (thrushes), and Vireonidae
(vireos). Wood warblers were further separated by foraging guild because
of the considerable difference in canopy and understory habitat availability in
Chelsea Park. Species that primarily forage among the foliage of trees and tall
shrubs were placed in the group “arboreal warblers” and species that are more
restricted to foraging on or near the ground were placed in the group “ground/
understory warblers” (Appendix A).
I used a lipid index (g fat / g lean dry mass, where fat mass equals total body
fat percentage multiplied by total dry body mass, and lean dry mass equals total
dry body mass minus fat mass) to control for body-size variation when comparing
chemically-derived fat content (Johnson et al. 1985, Rogers 1991), because
multiple species were grouped together.
I used Mann-Whitney U-tests to investigate differences in fat scores (Benson
and Winker 2005, Hailman 1965) among seasons, families, foraging guilds,
and autumn age classes. Unpaired, two-tailed t-tests were used to examine differences
in lipid indices within these groups, with lipid index as the dependent
variable and season, family, foraging guild, or age as the independent variable.
In some cases, one of the two lipid-index groups being compared was not normally
distributed. T-tests were still used in these situations because the tests
remain robust if assumptions are not met, especially when sample sizes do not
differ markedly and two-tailed tests are performed (Zar 1999).
90 Northeastern Naturalist Vol. 15, No. 1
Data from both years were pooled in each of the above analyses after no
significant annual differences in lipid indices (Spring: t = 0.76, df = 49, P = 0.45;
Autumn: t = 0.17, df = 76, P = 0.87) or fat scores (Spring: Z = 1.84 , P = 0.07;
Autumn: Z = 1.11 , P = 0.27) were found among all birds. Statistical tests were
performed with SYSTAT, version 10.0. Results were considered significant
when P ≤ 0.05. When tests of lipid indices yielded insignificant results retrospective
power analyses (GPOWER; Faul and Erdfelder 1992) were conducted
to determine if Type II errors may have occurred (α = 0.05, effect size = 0.5).
Average total body fat percentage was 29.4% in spring and 24.1% in autumn.
In spring, 80.4% of all birds were >20% fat and only 2.0% were <10% fat.
In autumn, birds >20% fat accounted for 58.8% of all birds, while 3.8% of birds
were <10% fat. Lipid indices and fat scores were significantly higher in spring
than autumn when all birds were grouped together (Table 1). Comparisons of
individual groups among seasons showed that warblers (both guilds combined)
were significantly fatter in spring than autumn (Table 1). Thrushes and ground/
understory warblers showed no significant seasonal differences in lipid indices
Table 1. Seasonal differences in body fat percentage, lipid index (t-test), and fat scores (Mann-
Whitney U - test) of migratory passerines killed during stopovers in Chelsea Park, New York
City. Values presented are mean ± SD. Body fat percentage calculated as subsample lipid mass/
dry mass and lipid index calculated as total lipid mass/total lean dry mass; see Methods. * = P
< 0.1, ** = P < 0.05, *** = P < 0.005.
Group Season n % fat % range
All wood warblers (Parulidae) Spring 37 28.92 ± 10.04 7.89–46.00
Fall 53 22.74 ± 11.96 6.60–54.87
Arboreal Spring 15 26.88 ± 10.69 13.04–46.00
Fall 39 19.85 ± 10.92 6.60–48.13
Ground/understory Spring 18 31.98 ± 8.40 18.48–45.66
Fall 14 28.55 ± 12.33 13.91–54.87
Thrushes (Turdidae) Spring 12 32.31 ± 7.29 20.09–43.26
Fall 12 26.68 ± 10.64 12.87–50.83
Vireos (Vireonidae) Spring 2 21.33 ± 6.29 16.88–25.78
Fall 8 22.97 ± 7.51 12.82–37.54
All birds Spring 51 29.42 ± 9.48 7.89–46.00
Fall 80 24.13 ± 11.26 6.60–54.87
Group Lipid index t df Fat score Z
All wood warblers 0.435 ± 0.209 2.11** 88 2.56 ± 1.30 0.24***
0.322 ± 0.245 - - 1.46 ± 1.48 -
Arboreal 0.404 ± 0.238 1.71* 52 2.33 ± 1.35 3.00***
0.290 ± 0.214 - - 1.08 ± 1.22 -
Ground/understory 0.492 ± 0.185 0.54 30 2.94 ± 1.21 0.23
0.446 ± 0.295 - - 2.62 ± 1.61 -
Thrushes (Turdidae) 0.493 ± 0.158 0.63 20 3.00 ± 1.41 1.50
0.439 ± 0.244 2.08 ± 1.38
Vireos (Vireonidae) 0.275 ± 0.102 - - 3.50 ± 0.71 -
0.310 ± 0.138 - - 1.63 ± 0.92 -
All birds 0.443 ± 0.198 2.20** 127 2.67 ± 1.32 4.04***
0.356 ± 0.233 - - 1.58 ± 1.34 -
2008 C.L. Seewagen 91
(power = 0.22 and 0.27, respectively) or fat scores, whereas arboreal warblers
had significantly higher fat scores and marginally higher lipid indices in spring
than fall (Table 1). Among groups in spring, there were no significant differences
in lipid indices or fat scores (all P > 0.1), but statistical power of the lipid
index t-tests was low (all warblers v. thrushes: power = 0.31; ground/understory
v. arboreal warblers: power = 0.28). In autumn, ground/understory warblers had
significantly higher lipid indices (t = 2.12, df = 51, P = 0.04) and fat scores (Z =
3.23, P < 0.01) than arboreal warblers, while no significant differences were
observed between thrushes and all warblers (t-test: t = 1.28, df = 61, P = 0.21,
power = 0.34; Mann-Whitney: Z = 1.38, P = 0.17). Differences in lipid indices
and fat scores between all HY (n = 21) and AHY (n = 29) warblers in autumn
were not statistically significant (t-test: t = 1.62, df = 48, P = 0.11, power = 0.40;
Mann-Whitney: Z = 1.58, P = 0.11).
Season, age, and family differences
The seasonal differences in lipid indices and fat scores found here do not
necessarily reflect a greater availability of food in Chelsea Park in spring, as
the fattening strategies of migrants may vary between spring and autumn. The
“spring fatter hypothesis” (Sandberg and Moore 1996) suggests that migrants
should carry more fat in spring than in fall as insurance against potentially
unfavorable environmental conditions encountered upon arrival on breeding
grounds. Other hypothesized benefits of arriving on breeding grounds
with ample fat stores include greater reproductive output by females and the
allowance of males to devote less time to foraging, and more time to mate solicitation
and territory defense (Sandberg and Moore 1996, Smith and Moore
2003). In a study of high-latitude migrants in Alaska, Benson and Winker
(2005) did not find evidence to support the spring fatter hypothesis. Conversely,
a recent study in Bronx Park, New York City found migrants were
heavier and fatter in spring than fall (Seewagen 2005; C.L. Seewagen and E.J.
Slayton, Wildlife Conservation Society, Bronx, NY, unpubl. data). The signifi-
cantly higher spring fat loads of birds in Chelsea Park are consistent with these
other findings in New York City and the spring fatter hypothesis.
Many studies of age-related differences in stopover ecology have found
adults to be in greater energetic condition than immature migrants during
autumn stopovers (e.g., Morris et al. 1996, Wang et al. 1998, Woodrey and
Moore 1997, but see Jones et al. 2002). Banding data collected in New York
City, however, did not fit this trend, with HY and AHY birds found to be in
comparable energetic condition (Seewagen 2005; C.L. Seewagen and E.J.
Slayton, unpubl. data). The autumn fat loads of adult and immature birds in
this study also did not differ significantly, although statistical power was low.
Ground/understory warblers had higher lipid indices and fat scores than
arboreal warblers in autumn despite a very limited amount of herbaceous
plants and woody shrubs, and consequent lack of a true understory in which
to forage. If migrants are attempting to deposit fat during stopovers in Chelsea
Park, the relatively abundant canopy habitat does not appear to favor
those species that are primarily arboreal foragers.
92 Northeastern Naturalist Vol. 15, No. 1
Twelve of the 14 individuals in the autumn ground/understory warbler
sample were represented by only two species: Geothlypis trichas L. (Common
Yellowthroat) and Seiurus aurocapillus L. (Ovenbird). If migrants
typically arrive in Chelsea Park with fat stores remaining from previous
stopovers, the higher fat content of the ground/understory warbler group
may then simply reflect a tendency for these two species to carry greater fat
loads than those species in the arboreal warbler group.
Stopover site quality
The refueling rate of birds is the standard indicator of stopover habitat
quality (Dunn 2000, 2001). Estimating refueling rate, however, requires data
acquired by capturing live birds. Inappropriate habitat conditions (e.g., lack of
a sufficient understory for mist-netting) and the presence of park-users make
live capture in Chelsea Park unfeasible. Examining the lipid stores of window
casualties was the only practical means of obtaining energetic-condition data.
A limitation of considering fat content indicative of stopover site quality
is the necessary assumption that birds did not arrive with equal or greater fat
stores deposited at previous stopovers. Without accompanying mass-change
data, it cannot be certain whether the fat loads of birds used in this study were
deposited during stopovers in Chelsea Park or were deposited elsewhere prior
to arrival. As such, I offer two interpretations of the results reported here.
The first interpretation assumes that the majority of fat was acquired by
the birds during stopovers in Chelsea Park. Under this scenario, the data
suggest the park is indeed a stopover site in which migrants can adequately
replenish energy stores, as the fat content found in this study is similar to
that typically observed in migrating songbirds (e.g., Caldwell et al. 1963;
Child 1969; Rogers and Odum 1964, 1966). Seewagen and Slayton (In press)
concluded that an urban park in another section of New York City is also
a high-quality stopover site; although, the characteristics of that site (e.g.,
larger size, primarily native tree and understory plant communities, permanent
natural water source) are drastically different than those of Chelsea
Park. If the fat content of birds examined in this study was in fact deposited
in Chelsea Park, it then suggests that the behavioral plasticity of birds during
migration (see Petit 2000) allows them to exploit even the most atypical and
unfamiliar habitats to replenish energy stores.
The second interpretation assumes that the birds arrived in Chelsea Park
already possessing considerable quantities of stored fat. The low proportion
of lean birds (less than 10% fat) suggests this interpretation (assuming there are no
hidden biases in the tendencies of fat and lean birds to collide with windows).
But barring an unfavorable change in weather or the encounter of a significant
ecological barrier, why would birds terminate a night flight when they still have
plenty of energy to continue? Perhaps large metropolitan areas act as artificial
geographical barriers (Moore et al. 1993), and birds encountering cities behave
in the same way as they often do when approaching natural obstacles—land and
further increase existing energy stores before attempting to overcome the barrier.
Perhaps birds land with fat stores remaining as a margin of safety against
potentially poor feeding conditions (Woodrey and Moore 1997) in an unknown
2008 C.L. Seewagen 93
city environment. Possibly, urban light pollution causes navigational disorientation
and birds seek places to land until they can re-orient and continue. Direct
observations of nocturnal migrants from the top of New York City’s Empire
State Building (<1 km NE of Chelsea Park), however, recently provided little
evidence to suggest that this illuminated skyscraper (and presumably other
similar buildings) disorients migrants and that migrants cannot adequately
negotiate the City’s matrix (DeCandido 2007; DeCandido and Allen 2006;
R. DeCandido, Hawk Mountain Sanctuary, Orwigsburg, PA, pers. comm.).
It is unclear whether birds are stopping over in Chelsea Park and other
New York City parks as a reaction to an unfamiliar and inhospitable landscape,
to replenish energy stores, or a certain degree of both. A study of
live migrants in Bronx Park, New York City found few birds were lean
(i.e., fat scores ≤ 1) upon initial capture, but nonetheless continued to gain
substantial mass during stopover (Seewagen 2005; Seewagen and Slayton,
in press), providing support for each scenario. Under either scenario, urban
parks should represent a valuable resource to songbirds that need to cross
metropolitan areas during migration. If migrants land in urban parks because
their energy stores are depleted, then the value of urban parks is the provision
of a place in which migrants can potentially refuel and continue further
than may be possible if such stopover sites were completely absent. If the
light pollution and composition of urban areas cause birds with ample fat
stores remaining to stop over, the value of urban habitats is their provision
of a place in which migrants can maintain their energy balance while reorienting.
The potential importance of urban habitats to migrants warrants
insurance of their conservation and proper management. This conclusion is
particularly pertinent in the Northeast where Atlantic migration routes cross
the most urbanized region of the United States.
This research was supported by the Species Survival Fund of the Wildlife Conservation
Society. I thank the staff of The Bronx Zoo’s Wildlife Health Center for their
kindness and accommodations while using their laboratory facilities. This manuscript
benefited from comments on earlier drafts by Robert DeCandido and Eric Slayton. I am
appreciative of the invaluable laboratory assistance provided by Nancy Clum, Tanya
Johnston, Taralynn Reynolds, and Eric Slayton. Lastly, I am indebted to the New York
City Audubon Society, notably Yigal Gelb, Nicole Delecretaz, and the Project Safe
Flight volunteers for providing me with the specimens used in this study.
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96 Northeastern Naturalist Vol. 15, No. 1
Appendix A. Number of individuals by species and foraging guild salvaged during
spring and autumn 2005 and 2006 near Chelsea Park, New York City. Arboreal
warbler species are indicated by “A” and ground/understory warbler species are
indicated by “G/U.”
Species guild* Spring Autumn
Setophaga ruticilla L. (American Redstart) A1,2 1 1
Mniotilta varia L. (Black-and-White Warbler) A3 3 6
Dendroica striata Forster (Blackpoll Warbler) A1,4 1 2
Dendroica caerulescens Gmelin (Black-throated Blue A3,5,6 1 4
Dendroica virens Gmelin (Black-throated Green Warbler) A1,3 0 1
Vireo solitarius Wilson (Blue-headed Vireo) - 1 2
Vermivora pinus L. (Blue-winged Warbler) A3,7 1 0
Wilsonia canadensis L. (Canada Warbler) - 4 0
Dendroica pensylvanica L. (Chestnut-sided Warbler) A1,8 0 3
Geothlypis trichas L. (Common Yellowthroat) G/U1 3 6
Catharus minimus Lafresnaye (Gray-cheeked Thrush) - 0 4
Dendroica magnolia Wilson (Magnolia Warbler) A1,9 0 5
Oporornis philadelphia Wilson (Mourning Warbler) G/U10,11 0 1
Vermivora ruficapilla Wilson (Nashville Warbler) A1,3 0 2
Parula americana L. (Northern Parula) A1,3 6 10
Seiurus noveboracensis Gmelin (Northern Waterthrush) G /U1 4 1
Seiurus aurocapillus L. (Ovenbird) G/U1 10 6
Dendroica pinus Wilson (Pine Warbler) A1 0 2
Vireo olivaceus L. (Red-eyed Vireo) - 1 6
Pheucticus ludovicianus L. (Rose-breasted Grossbeak) - 0 4
Piranga olivacea Gmelin (Scarlet Tanager) - 0 3
Catharus ustulatus Nuttall (Swainson’s Thrush) - 5 3
Vermivora peregrina Wilson (Tennessee Warbler) A1,3 0 1
Catharus fuscescens Ridgway (Veery) - 2 2
Wilsonia pusilla Wilson (Wilson’s Warbler) A1,12 1 1
Hylocichla mustelina Gmelin (Wood Thrush) - 5 3
Helmitheros vermivorus Gmelin (Worm-eating Warbler) G/U1,3 1 0
Dendroica petechia L. (Yellow Warbler) A1 1 0
Dendroica coronata L. (Yellow-rumped Warbler) A1,3 0 1
*Foraging guild was only determined for species of wood warbler (Parulidae).
Foraging guild was determined from these authorities: 1Todd (1940), 2Sherry and
Holmes (1997), 3Bent (1963), 4Hunt and Eliason (1999), 5Holmes (1994), 6Holmes
(1986), 7Gill et al. (2001), 8Richardson and Brauning (1995), 9Hall (1994), 10Pitocchelli
(1993), 11Cox (1960), and 12Ammon and Gilbert (1999). Canada Warbler was
not included because it forages in trees, the understory, and on the ground (Conway
1999, Forbush and May 1955, Todd 1940).