Stopover of Neotropical Landbird Migrants on East Ship Island Following Trans-Gulf Migration
The ecophysiology of Neotropical landbird migrants was studied in relation to energetic condition following trans-Gulf migration in 1987, 1988, and 1989. Most birds captured on East Ship Island, a barrier island off the Mississippi coast, had exhausted fat reserves crossing the Gulf of Mexico and were near fat-free mass. Few birds were recaptured after the day of their first capture, suggesting that migrants departed the day of their arrival or that night. Lean migrants occurred more frequently in samples of recaptured migrants, although length of stay was not a clear function of arrival mass nor did change in mass simply increase with stopover length. Most species were characterized by loss of body mass or by rates of change that essentially maintained arrival mass during stopover. Rates of change in mass were positively correlated with energy demand for several species. Fat-depleted birds appeared to compensate for their condition, presumably by increasing their rate of energy acquisition. Estimates of prey abundance did not differ between years, which might explain why stopover length and rate of gain did not vary between years. We conclude that many migrants left ESI because they found few opportunities to satisfy energy demand, while others stayed because of constraints unrelated to their energy balance or because the benefits of rejecting the habitat were outweighed by the cost of finding a more suitable site.