RAMENOFSKY, M.**; MOFFAT, J.; COVERDILL, A.J.; BENTLEY, G.E.; University of Washington, Seattle; University of Washington, Seattle; University of Washington, Seattle; University of California, Berkeley: How do migrants respond to prolonged migratory restlessness?

Migratory flight is energetically costly yet birds fly for great distances crossing diverse barriers. Long distance, short-bout migrants like white-crowned sparrows (Zonotrichia leucophrys gambelii) (WCS) cover approximately 4000km and do so in nightly flights followed by periods of refueling and resting throughout the daylight hours. Baseline levels of corticosterone (Cort) parallel these daily fluctuations in energy expenditure but little is known of what transpires during flight or if flight is extended beyond one night. Studying cycles of flight in relation to fuels and Cort are difficult under natural conditions but captive migratory WCS display daytime and nocturnal activity (migratory restlessness or MR) that correspond well with free-living birds. We have shown that MR is extended by lengthening the dark phase of a 18L:6D photocycle and use this to measure the impact of prolonged nocturnal activity on morphological features and Cort. In light of previous studies, we hypothesize that Cort increases with heightened energy demand of prolonged MR. As a test, we extended the dark phase of a18L:6D photocycle to 8, 24 and 48h. Experimental birds exposed to dim light (<1 Lux) expressed active and persistent MR while controls held in complete darkness showed little movement. At 48h, MR decreased appreciably. All birds reduced body weight and fat depots with prolonged darkness but the MR active birds lost significantly more observable fat (over 41%). Contrary to our prediction, Cort did not continue to rise with prolonged MR and fuel use suggesting that short-bout migrants are able to extend MR and may be minimizing energy expenditure by some means of conservation. We propose that migrants exercise a stringency model of energy utilization.