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Effects of Fat and Lean Body Mass on Migratory Landbird Stopover Duration

Abstract We used quantitative magnetic resonance body composition analysis and radiotelemetry to examine whether fat and lean body mass affected stopover durations of 11 birds captured during autumn migration in New York City, USA. Two Swainson's Thrushes (Catharus ustulatus), two Hermit Thrushes (C. guttatus), and seven Ovenbirds (Seiurus aurocapilla) were used in the study. Ovenbird stopover duration was significantly and negatively related to fat mass but unrelated to lean body mass. The same relationships were found when data from all three species were combined to increase sample sizes. Birds that departed within 1 day had fat stores upon capture that represented at least 11% of their total body mass whereas those with fat content <6% of total body mass remained for no fewer than 4 days. Arrival fat mass clearly influenced time birds spent at the site but lean body mass did not. Conditions for increasing or maintaining fat stores provided by urban stopover sites may affect the migration timing of birds.