Table Of ContentWilson Bulletin, 1 16(1), 2004, pp. 48-55
SPATIAL FORAGING DIFFERENCES IN AMERICAN REDSTARTS
ALONG THE SHORELINE OE NORTHERN LAKE HURON
DURING SPRING MIGRATION
ROBERT J. SMITH,’4“^ MICHAEL J. HAMAS,’ DAVID N. EWERT,^ AND
MATTHEW E. DALLMAN’-^
—
ABSTRACT. LowlandconiferousforestsadjacenttonorthernLakeHuron provideimportantstopoverhabitat
for landbirds during spring migration. Large numbers of aquatic insects emerging from nearshore waters of
northern Lake Huron appear to be an important food source. In this study we compared the foraging behavior
ofa long-distance landbird migrant, the American Redstart {Setophaga niticilla), in areas with high densities of
emergent aquatic insects to areas with few or no emergent aquatic insects to assess the significance of these
arthropods as an early spring food source. Redstarts foraged differently in shoreline habitats relative to inland
habitats of similar vegetation composition. Both males and females gleaned significantly more in shoreline
habitats as compared to inland areas of similar vegetation composition, and inland birds performed more sally
strikes than birds at the shoreline. Both sexes also varied the use oftree species in which they foraged. Redstarts
used northern white cedar {Thuja occidentalis) more at shoreline than inland, while inland redstarts foraged in
deciduous trees more than at the shoreline. We suggest that differences in foraging between shorelineand inland
locations were responses to differences in prey types and abundance, most notably the presence of emergent
aquatic insects (Diptera: Chironomidae) in shoreline habitat. Our results complement those of previous work,
suggestingthat midgesprovideacriticalearlyseason resourceforlandbirdsmigratingthroughMichigan’seastern
Upper Peninsula during spring. Received 30 September 2003, accepted 26 March 2004.
During spring and early summer in temper- nearshore, midge-abundant habitats than in-
ate North America, large numbers of aquatic land habitats with comparable vegetation
insects that have metamorphosed into sexually (DNE unpubl. data). Further, Black-throated
mature adults often amass in terrestrial habi- Green Warblers {Dendroica virens) forage in
tats adjacent to riparian and lacustrine systems and use habitat elements differently in midge-
(Armitage 1995, McCafferty 1998). These in- abundant habitats compared to inland, midge-
vertebrates are relatively weak fliers (e.g., Ko- depauperate habitats (Smith et al. 1998), and
vats et al. 1996) and tend to be restricted to American Redstarts {Setophaga ruticilla) and
nearshore terrestrial habitats. In Michigan’s Black-throated Green Warblers forage at high-
eastern Upper Peninsula, midges (Diptera: er rates in midge-abundant areas relative to
Chironomidae) are the predominant aquatic habitats with no midges (Seefeldt 1997).
arthropods when migratory landbirds stop Here we describe shoreline/inland differ-
during spring migration. These invertebrates ences in the foraging behavior of a long-dis-
swarm profusely in shoreline areas while be- tance landbird migrant, the American Red-
ing virtually nonexistent inland (DNE unpubl. start, during spring migration in Michigan’s
data). eastern Upper Peninsula. By focusing on
Recent evidence suggests that lowland co- spring migration, we were able to examine
niferous forests adjacent to northern Lake Hu- bird behavior when shoreline/inland differenc-
ron provide important stopover habitat for es in arthropod abundances were most dra-
spring migrants. More landbirds are found in matic (Smith et al. 1998; DNE unpubl. data).
We compared redstart foraging behavior in ar-
' Dept, of Biology, Central Michigan Univ., Mt. eas with high densities ofmidges to areas with
Pleasant, MI 48858, USA. few or no emergent midges to assess the sig-
-The Nature Conservancy, 101 E. Grand River,
Lansing, MI 48906, USA. nificance of these arthropods as a food source
^The Nature Conservancy, 707 Main St. W., Ash- during early spring.
land, WI 54806, USA.
Current address: Dept, of Biology, The Univ. of METHODS
Scranton, Scranton, PA 18510, USA. —
^Corresponding author; e-mail: Study area. The study area included ap-
[email protected] proximately 80 km of shoreline, extending
48
Smith et al. • FORAGING VARIATION IN AMERICAN REDSTARTS 49
from Horseshoe Bay to DeTour State Park in in a flock was used as a subject. We recorded
Mackinac and Chippewa counties, respective- additional data only after an observer had
ly (45° 51' to 46° 04' N, 84° 00' W to 84°44' traveled >30 m or >10 min had elapsed since
W). Limestone and dolomite peninsulas define the previous observation. To reduce discovery
much of the shoreline, with sandy coves and bias, we waited 5 sec after making initial vi-
marshes characterizing borders of intervening sual contact, and then recorded the subsequent
bays. Conifers, especially northern white ce- foraging maneuver performed by the bird
' dar {Thuja occidentalis), balsam fir {Abies (Morrison 1984, Hejl et al. 1990).
balsamea), white spruce {Picea glauca), east- Eoraging maneuvers were categorized as
ern white pine {Pimis strobus), and deciduous (1) glean, a maneuver directed at a prey item
species, including paper birch {Betida papyr- on a substrate while the bird was either
ifera), and quaking aspen {Populus trernuloi- perched or hopping; (2) sally hover, a maneu-
,
des), dominate the forest. Forest composition ver in which a stationary prey item on a sub-
was relatively uniform throughout the study strate is attacked by a hovering bird; and (3)
area, as determined by vegetation sampling sally strike, a maneuver i—n which both the in-
performed in association with a bird census sect and bird are in flight also known as fly-
j project that occurred in the same landscape catching or hawking behavior (Sabo 1980,
I (Smith et al. 1998; DNE unpubl. data). Using Petit et al. 1990a).
' the point-centered technique of Cottam and We described the location ofa foraging ma-
j Curtis (1956), DNE (unpubl. data) determined neuver both horizontally and vertically within
j forest composition at 45 permanent, 50-m a tree. To characterize horizontal position, we
1
fixed-radius bird census points positioned on arbitrarily divided a tree into inner, middle,
nine transects spaced uniformly across the and outer thirds (MacArthur 1958, Remsen
study area. Sample points were positioned at and Robinson 1990). We also recorded for-
0, 0.4, 0.8, 1.6, and 3.2 km from the lake- aging height for each bout by using a clinom-
shore. Canopy height ranged from 10 to 15 m. eter to measure the angle from observer to the
The understory was heavily browsed by bird and an optical rangefinder to measure dis-
white-tailed deer {Odocoileiis virginianus) tance; these data were converted to height (m)
and consisted mostly of balsam fir and white using the appropriate trigonometric equation.
spruce. At the end of each observation we also re-
—
Foraging observations. We collected data corded the species of tree in which the bird
j on foraging American Redstarts during spring was observed, time of day, distance from the
I
migration of 1993 and 1994. To avoid bias in shoreline, and weather conditions.
j our sampling effort, we spent equal amounts We pooled data across years to maximize
j of time in shoreline and inland habitats while sample sizes. Only the five most frequently
searching for foraging redstarts. Observations used tree species were considered when ana-
i
I of spring migrants commenced when the first lyzing tree species used during foraging:
1 birds arrived in early May and continued white spruce, balsam fir, northern white cedar,
through early June, the onset of breeding for quaking aspen, and paper birch. All data were
. birds in the area (Wood 1951; DNE and MJH analyzed using SPSS (2()()0). Treatment of
:
pers. obs.; R. J. Smith and E R. Moore un- variables depended upon whether data were
i publ. data). To compare foraging at shoreline continuous or categorical (Zar 1996). We used
and inland locations, observations made <0.4 a General Linear Model to resolve differences
km from Lake Huron were designated as in foraging height by sex and observation lo-
shoreline, and observations made 0.4-3.2 km cation (shoreline versus inland). Log-likeli-
from the lake were designated as inland. We hood ratio {(/) tests were used to analyze fre-
selected 0.4 km as the boundary between quency data such as spatial differences in use
shoreline and inland because midge abun- of tree species, maneuver used, and horizontal
I
ij dance drops precipitously beyond this point position while foraging. Log-likelihood ratio
j' (DNE unpubl. data). (f/) tests also were used to compare aelual to
We used the protocol of llejl et al. (1090) expected use of northern white cetlar by for-
to minimi/e the likelihood of |')scudoreplica- aging redstarts. If reilstarls preferred to forage
I tion. Only the first bird (of each sex) delected in (or a\oided) cedar, then they woukl exhibit
50 THE WILSON BULLETIN • Vol. 116, No. I, March 2004
Glean Sallyhover Sallystrike
Glean Sallyhover Sallystrike
FIG. 1. Comparison of American Redstart forag-
ing maneuvers at shoreline and inland habitats during FIG. 2. American Redstart tree use in shoreline
spring migration 1993-1994, northern Lake Huron, and inland habitats during spring migration 1993-
Michigan (female n — 67, male n = 379). 1994, northern Lake Huron, Michigan (female shore-
line n = 93, female inland n = 32; male shoreline n
= 590, male inland u = 87). The solid reference line
indicates relative density of northern white cedar in
a disproportionate use of cedar (Morrison et shoreline habitatsandthedashed line indicatesrelative
al. 1998).—We used relative density estimates density ofcedar inland.
of cedar calculated from vegetation sam-
—
pling to derive expected redstart use of ce-
dar. Statistical significance was accepted at P
< 0.05. There was no relationship between tree spe-
cies selected for foraging and age ofmales (G
RESULTS = 4.85, df = 4, P = 0.30, n = 229) or gender
Males did not differ by age in foraging ma- (G = 4.35, df = 4, P = 0.36, n = 363). Males
neuvers used (G = 4.89, df = 2, P = 0.087, used tree species for foraging differently in
II = 283), nor did males differ from females shoreline habitats as compared to inland (G =
in their use offoraging maneuvers (G = 3.32, 21.23, df = 4, P < 0.001, n = 677); the re-
df = 2, P = 0.19, u = 333). The type of lationship was not significant in females (G =
foraging maneuver used, however, was depen- 9.06, df = 4, P = 0.060, n = 125). In shore-
dent on habitat for both males (G = 6.24, df line habitats, both males and females foraged
= 2, P — 0.044, n — 379) and females (G = more in northern white cedarthan in othertree
6.22, df — 2, P — 0.045, n = 67). Both sexes species (males 55%, females 52%), but they
gleaned more at shoreline than inland loca- were observed more in aspen and birch inland
tions (males: 42 versus 18%; females: 56 ver- relative to the shoreline (Fig. 2). Redstarts for-
sus 29%) and used more sally strikes inland aging in shoreline habitats did not use north-
than at the shoreline (males: 64 versus 40%; ern white cedar disproportionately (male: G =
females: 63 versus 32%; Fig. 1). 0.16, df = 1, P = 0.66, n = 590; female: G
Smith et al. • FORAGING VARIATION IN AMERICAN REDSTARTS 51
= 0.033, df = 1, P = 0.81, n = 93), but inland tree species throughout our study area (Smith
DNE
they did use cedar significantly less than ex- et al. 1998; unpubl. data).
pected (male: G = 4.54, df = 1, = 0.038, It is possible that differences in migrant
n = 87; female: G = 4.33, df = 1, P = 0.042, densities between shoreline and inland (DNE
n = 32; Fig. 2). unpubl. data) may have caused birds to vary
Foraging height was not related to male age their foraging by creating a situation in which
(^1.374 ~ 2.60, P = 0.11), and male foraging shoreline birds experienced enhanced compe-
height did not differ between shoreline and tition. Consequently, migrants in shoreline lo-
inland habitats (Fj = 0.02, P = 0.89); there cations may have altered their foraging rela-
,74
was no male age by habitat interaction for for- tive to those inland as they sought to alleviate
aging height (F| = 0.14, P = 0.71). For- enhanced competition associated with higher
374
aging height of the sexes did not differ (F, numbers of competitors. While our data are
44,
= 2.27, P = 0.13), nor did foraging height insufficient to rule this out, we point out that
differ between shoreline and inland habitats the shoreline/inland differences observed in
(F, = 0.24, P = 0.63); there was no sex redstarts were very similar to foraging differ-
44,
by habitat interaction for foraging height (F, ences previously documented in Black-throat-
= 0.01, P = 0.99). We found no shoreline/ ed Green Warblers (Smith et al. 1998). As
441
inland differences in horizontal position of with redstarts, during spring migration Black-
males and females within trees (male: G = throated Green Warblers gleaned more at the
2.16, df = 2, F = 0.34, n = 378; female: G shoreline while exhibiting more sally hovers
= 0.36, df = 2, F = 0.84, n = 67). and sally strikes inland. Furthermore, Black-
throated Green Warblers foraging in shoreline
DISCUSSION locations used cedar more than other tree spe-
cies, while birds observed inland dramatically
Landbirds are known to vary their foraging reduced their use of cedar, instead foraging
in response to a number of factors, including more in deciduous species such as paper birch
structural differences in habitat (Collins 1983, and quaking aspen.
Holmes and Schultz 1988), inter- and intra- Differential distribution by age or sex also
specific competition (Rappole and Warner could have influenced foraging, if there were
1976, Laursen 1978, Morse 1980), including age or sex differences in how redstarts foraged
gender (Morse 1968, Holmes 1986, Ornat and at our site. Age- and/or sex-mediated habitat
Greenberg 1990, Petit et al. 1990b) and age segregation has been demonstrated among
(Woodrey 1995) interactions, weather (Pin- redstarts on the wintering grounds (Marra et
kowski 1977), ambient temperature (Ford et al. 1993, Parrish and Sherry 1994), during mi-
al. 1990), thermoregulatory requirements gration (Woodrey 1995), and during the
(Martin and Karr 1990), and distribution, di- breeding season (Sherry and Holmes 1997).
versity and abundance of prey species However, we found no age or gender effect in
(Holmes and Schultz 1988, Ford et al. 1990, how birds foraged or used tree species.
Szaro et al. 1990, Smith et al. 1998). While There were obvious microclimate differenc-
the observed shoreline/inland differences in es between shoreline and inland areas within
redstart foraging might be attributable to one our study site (DNE unpubl. data). Shoreline
or more of these factors, our results most like- habitats were, on average, cooler and re-
ly reflect shoreline/inland differences in the mained cooler longer. Redstarts may ha\e var-
abundance and activity of arthropods. ied foraging in response to these lemjK'rature
The observed variation in redstart foraging differences. Marlin and Karr 1990) found that
(
is likely not a consequence of dilTerences in a number of warbler species increased Hying
forest composition because our equal sam- maneuvers during the coklcr periods of early
pling effort, distributed across such a large spring and late fall migration, fhey suggested
landscape, should have eliminated any spuri- that use of aerial maneuvers tluring cokl pe-
ous effects between shoreline anti inland. riotls may be nu)ic criicient than nonllying
Moreover, vegetation sampling performetl at maneuvers because ol increased heat produc-
the same scale as behavior sampling indicaletl tion by night muscles. Houever. we fouiul
little shoreline/inlaiul tlilTerencc in density of that redstarts peilormcd more nonllying ma-
52 THE WILSON BULLETIN • Vol. JI6, No. 1, March 2004
neuvers (gleans) in cooler shoreline habitats vations and our results suggest that birds for-
and more aerial maneuvers (sally hovers and aged heavily on these insects. In temperate
sally strikes) in warmer, inland areas. These habitats, mating swarms of midges are tem-
results are contrary to what would be expected perature dependent (R. J. Smith and E R.
ifbirds increased aerial maneuvers to enhance Moore unpubl. data); depending on the spe-
thermoregulatory efficiency, but they corre- cies, they begin to swarm at a critical mini-
spond with expectations if birds were re- mum temperature of about 10°C (Armitage
sponding to increased activity of flying inver- 1995).
tebrates in warmer habitats (see below). Redstart foraging preferences corresponded
The Great Lakes’ influence on local cli- with the activity and presence/absence of
mate, vegetation, and animal communities is midges. Eor instance, redstart foraging activity
especially apparent in nearshore terrestrial was most intense during early morning when
habitats (Eichenlaub 1979). Temperature de- temperatures often were below 10° C. Lower
pression resulting from onshore winds delays shoreline temperatures may have delayed
the phenological development of vegetation swarming until later in the day, resulting in
(Albert et al. 1986; DNE unpubl. data) and high densities of substrate-bound (resting)
presumably the emergence and activity of ar- midges. Thus, resting midges may have elic-
thropods, as both are dependent upon ambient ited, at least in part, the more-frequent glean-
air temperature (Williams 1961). Shoreline ing observed in shoreline areas than inland ar-
microclimate differs from inland and likely re- eas. Eurthermore, the differential use ofnorth-
sults in unique abundance and distribution ern white cedar suggests that midges may be
patterns of arthropods. an important food resource; redstarts used ce-
Evidence for differences in arthropod abun- dar significantly more at the shoreline com-
dance between shoreline and inland areas is pared to inland, even though there was no sig-
suggested by differing foraging maneuvers in nificant shoreline/inland difference in cedar
the two areas. Inland, away from the cooling density across the study area (DNE unpubl.
influence of the lake, redstarts performed data). Moreover, shoreline redstarts did not
more sally strikes and fewer substrate-directed depart from random use of cedar while inland
gleans. Black-throated Green Warblers also birds foraged in cedar less than expected. This
increased their use of sally strikes at inland differential use of cedar may reflect differen-
sites in Michigan’s eastern Upper Peninsula tial midge abundance. Birds shifted foraging
(Smith et al. 1998). Increases in aerial maneu- away from cedar inland, where few midges
vers such as sally strikes suggest that either occurred. This reduction in use ofinland cedar
flying insects were more abundant inland, fly- suggests that suitable prey items, other than
ing insects were more active inland, or some midges, were not abundant in cedar.
combination of the two. Because of the influ- Similar results were documented in Black-
ence of temperature on invertebrate develop- throated Green Warblers foraging in cedar in
ment and activity, any temperature differential the same area (Smith et al. 1998). Birds used
between shoreline and inland may have re- cedar as expected at the shoreline during
sulted in disparities in activities and abun- spring migration and less than expected in-
dances of prey, thereby influencing foraging land. During the breeding season and fall mi-
behavior. Birds also may have altered foraging gration, when midges were not abundant in
behavior in response to shoreline/inland dif- shoreline habitats (DNE unpubl. data). Black-
ferences in the kinds of arthropods that were throated Green Warblers used cedar signifi-
available within a habitat. cantly less. The shoreline/inland differences in
The most obvious spatial disparity in prey cedar use by both redstarts and Black-throated
for migrants was the abundance ofadult midg- Green Warblers, and the decline in shoreline
es in shoreline habitats. These insects were use by Black-throated Green Warblers during
strongly associated with the lakeshore; beyond the breeding season and fall migration, sug-
0.40 km from shoreline, midge abundance gest that birds foraged in cedar in response to
dropped to near zero (DNE unpubl. data). the presence of midges and that prey items
Midges commonly rest on nearshore vegeta- other than midges were not abundant in cedar.
tion in high densities, and both field obser- Constructive replication in the behavioral
Smith et al. • FORAGING VARIATION IN AMERICAN REDSTARTS 53
sciences is rare (Martin and Bateson 1993). ing during both field seasons. We thank Hiawatha Na-
We have identified concordant results between tional Forest. Michigan Department of Natural Re-
two warbler species that are rather disparate sources, andM. Twining forpermissiontoconductthis
work on their properties. This manuscript benefited
foragers. Black-throated Green Warblers in- significantly from discussions with F. R. Moore and
clude a high percentage of gleans as part of members of The University of Southern Mississippi
their foraging repertoire (Morse 1993), where- Migratory Bird Group.
as American Redstarts use relatively fewer
LITERATURE CITED
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duce the likelihood that our results are spuri- mate choice and arrival date on breeding success
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