Suomala et al. • MIGRANT STOPOVER ECOLOGY ON ISLANDS 
225 
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balance (e.g.. Kuenzi et al. 1991, Moore et al. 1995, 
Ahorn and Moore 2004). Competition for resources 
occurs among migrants (Moore and Wang 1991) 
and the ability to gain mass may be related to the 
ability to obtain a transient feeding territory 
(Rappole ami Warner 1976). Predation can be a 
significant risk during migration (Moore et al. 1990) 
and migrants may prefer locations with fewer 
predators or dense vegetation to better escape 
raptors. Merlins (Fcilco lolumburitts), the most 
common raptor at the Isles of Shoals, are seen more 
frequently on Appledore Island than Star Island 
(SRM and RWS. unpubl. data). Raptors are also 
known to perch on isolated trees and watch for prey 
during migration (Moore et al. 1990). a behavior 
seen for Merlins on Appledore Island but Star Island 
lacks suitable perches (RWS and D. W. Holmes, 
pers. obs.). 
There w as little evidence of movement between 
islands in a seasonally-appropriate direction for 
continued migration. Nor was there evidence 
indicating a shift between islands after initial 
capture, as would be expected if birds were 
exploring habitat prior to settling for the day. If 
birds were searching available habitat, we would 
expect those that landed in low quality habitat on 
one island would move to the island with more 
suitable habitat and be recaptured there. Our data 
suggest that decisions regarding stopover habitat 
at the Isles of Shoals were made without apparent 
exploration and were unrelated to the ability to 
gain weight. This is consistent with Winker’s 
1 1995b) observations and suggestion that birds are 
able to rapidly select habitat with no observable 
exploration. 
Cedar Wax wing and Purple Finch crossover 
results were consistent with both species roving 
among the Isles of Shoals for several days to follow 
food supplies and crossing to Appledore Island 
where there was a better chance of mass gain. Both 
were more numerous on Star Island but gained 
significantly more weight on Appledore Island in the 
fall. These species are highly frugivorous in fall and 
have irregular migration patterns that vary with local 
food sources ( Wootton 1996. Winner el al. 1997). 
The two Red-eyed Vireos that crossed during fall 
from Star Island to Appledore Island (the island with 
more initial captures and greater mass gain) may also 
have moved to gain more mass. However, so few 
birds crossed over that it is difficult to gain any 
insight into the relationship of crossovers to species 
distribution at initial capture or mass gain differences 
between the islands. 
