Suomala et al • MIGRANT STOPOVER ECOLOGY ON ISLANDS 
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remained longer and gained mass. However, there 
was wide variation between species, seasons, and 
individuals, similar to that found by Morris et al. 
(1996) over 10 years on Appledorc Island. 
Caution should be used in interpreting the mass 
change results of the ANCOVA analysis. This 
analysis measures differences in the rate of mass 
gain that might indicate differences in food 
supplies and quality, but is based on the assumption 
of equal foraging lime for all birds (i.e.. arrive at or 
before dawn). Thus, mass measurements represent 
changes that occurred at the stopover site (Dunn 
2000). Birds that stray to the east of the Isles of 
Shoals over the Gulf of Maine may have to 
continue flying past sunrise (Baird and Nisbet 
I960), arriving well after sunrise in a fat-depleted 
condition. Their condition would be unrelated to 
stopover habitat quality and represents a confound¬ 
ing factor in the analysis. This phenomenon may 
contribute to the small number of species with 
significant mass gains estimated by regression 
analysis on either island, as suggested by Morris et 
al. (1996). However, the results of our regression 
and recapture analyses were consistent. This 
suggests the ANCOVA analysis was not affected 
by large numbers of fat-depleted birds arriving 
later in the day or that Star and Appledore islands 
were similarly affected and a between-island 
comparison is valid, although the slope of the 
regression may be flattened. Small sample size 
may also be a factor in the lack of significant trends 
in the mass-change regression analysis due to high 
individual variation in condition (Winker et al. 
1992. Winker 1995a, Dunn 2001). 
The general lack of significant differences 
between the islands in percent mass change 
among recaptures, percent of recaptures gaining 
mass, and diurnal mass gains of all species 
provides a strong indication that factors other 
than food availability, as measured by the ability 
to increase mass, influence stopover at the Isles of 
Shoals. Winker (1995b) also found that capture 
rate did not correspond with mass gain and 
abundance did not correlate with use of food 
resources as measured by fat deposition (Winker 
et al. 1992). suggesting that food availability is 
not the only factor in site selection. We 
hypothesize that migrants may select stopover 
sites based on habitat structure that is similar to 
familiar breeding habitat, regardless of the actual 
food availability at that site (Suomala et al. 2010). 
The islands may also function ns an important 
stop for rest . reorientation, muscle repair, or water- 
