The Wilson Journal of Ornithology 124 ( 2 ): 217 - 229 , 2012 
COMPARISON OF MIGRANT SONGBIRD STOPOVER ECOLOGY ON 
TWO ISLANDS IN THE GULF OF MAINE 
REBECCA W. SUOMALA, 1 M SARA R. MORRIS.' AND KIMBERLY J. BABBITT 1 
ABSTRACT.—We compared migrant Bird recapture rate, stopover time, mass gain, and fat class between Star Island. 
New Hampshire and Appledore Eland. Maine during spring and fall migration in relation to differences in relative species 
abundance between the islands: and examined potential movement of migrants between the islands. The average recapture 
rate in spring was 5.7% on Star Island and 3.6% on Appledore Island. Five species were recaptured more frequently on Star 
Island and one species more frequently on Appledore Island. There was no difference in mean minimum stopover time 
during spring <2.4 days on Star Island; 2.5 days on Appledore Island! and fall (2,‘> days on Star Island: 3.2 days on 
Appledore Island). Three species had a longer mean stopover time on Appledore Island than Star Island. The island w ith the 
greater percentage of recaptures and longer stopover had more captures lor a given species. Mass gains were significant for 
six species during spring (27.3%) and 10 during fall (38.5% l on Star Island, and live 122.7%) during spring and L3 (50.0%) 
during fall on Appledore Island. Five species had a difference in rate of mass gain between the islands. The between-island 
difference in species abundance was not reflected in between-island differences in mass gam. except lor Red-eyed Vireo 
[Vireo olivuceus1 during fall. There was no clear pattern in species differences in fat levels and differences in captures, 
stopover length, or mass gain between the islands. The fat-class and mass-gain results suggest habitat use. as measured by 
relative abundance, is not based on the ability to gain mass at the time of stopover. The notable tall mass gains for Red-eyed 
Vireo illustrate the availability of food resources for some species on both islands. Only 42 of 1U.437 migrant birds banded 
moved from one island to the other. There was little evidence of movement between islands in a seasonally-appropriate 
direction for continued migration, or evidence indicating a shift between islands after initial capture. Received 21 April 
2010. Accepted II November 2011. 
Stopover sites provide migrants with an oppor¬ 
tunity to replenish critical energy supplies (Moore 
et al. 1995), reorient (Baird and Nisbet I960), 
escape adverse winds and weather, recover from 
muscle fatigue or injury, evade predators (Moore 
et al. 1992. Cimprich el al. 2005), and avoid the 
dehydration ol daytime flight (Moore el al. 1992). 
Lack of suitable stopover habitat may increase the 
risk of mortality during migration (Bairlein 1992, 
Moore et al. 1992, Hutto 1^98) contributing lo 
population declines (Moore 2000, Petit 2000). 
Measuring the importance of a given stopover 
site and site variables (i.e., habitat type, vegetation 
structure, food, etc.) to migrant songbirds is 
challenging. Variability in weather, wind direc¬ 
tion. and an individual’s energetic condition result 
in variability of stopover site use in any given 
year, making it easy to view stopover sites as 
interchangeable and dismiss any given site as 
unimportant to passerine populations (Moore el al. 
1992. Hutto 1998). Migrants may choose specific 
1 Department of Natural Resources and the Environment. 
James Hall, University of New Hampshire. Durham. NH 
03824. USA. 
Canisius College. Department of Biology. 2001 Main 
Street. Buffalo. NY 14208. USA. 
Current address: New Hampshire Audubon Society; 84 
Silk Fanil Road. Concord, NH 03301. USA. 
'Corresponding author: e-mail: 
bsuomala@nhaudubon.org 
stopover sites or may use stopover sites different¬ 
ly based on available resources, environmental 
conditions, and priorities. 
Star and Appledore islands in the Isles of Shoals 
on the New England coast, USA. provide a unique 
opportunity to examine migrant stopover patterns. 
Large numbers of songbirds stopover during spring 
and fall migration (Mortis et al. 1996) and the 
islands arc <1 km apart, eliminating weather as a 
factor in between-site comparisons. They are on the 
edge of the Gulf of Maine, and most migrants 
travel along the coast, south or southwest during 
fall (Drury and Keith 1962. Drury and Nisbet 1964, 
Richardson 1978) and northeastward during spring 
(Dairy and Keith 1962). Thus, the islands are not 
necessarily the first or last stop for a migrant before 
a long flight; and stopover due to extreme 
physiological stress is less frequent than at many 
other ecological barriers. 
We previously (Suomala el al. 2010) found 
differences in ihe relative abundance of migrant 
species between Appledore and Star islands. 
These differences were most closely related to 
dilferences in habitat structure and area between 
the islands. Interspecific differences in island use 
during migratory stopover were most closely 
related to the species' breeding habitat. Star 
Island has lower scrub-shrub vegetation and more 
closely resembles a typical scrub-shrub habitat. 
Species that breed in scrub-shrub or open habitat 
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