JOURNAL OF AVIAN BIOLOGY 33: 167–174, 2002 Diet preferences of warblers for specific fatty acids in relation to nutritional requirements and digestive capabilities Scott R. McWilliams, Shannon B. Kearney and William H. Karasov McWilliams, S. R., Kearney, S. B. and Karasov, W. H. 2002. Diet preferences of warblers for specific fatty acids in relation to nutritional requirements and digestive capabilities. – J. Avian Biol. 33: 167–174. During energy-demanding periods of the annual cycle such as migration or during cold days in winter, birds store fat comprised mostly of 16- or 18-carbon unsaturated fatty acids. In such situations, birds may feed selectively on foods with specific fatty acids that enable efficient fat deposition. We offered wild-caught yellow-rumped warblers Dendroica coronata paired choices between semi-synthetic diets that differed only in their fatty acid composition. Warblers strongly preferred diets containing long-chain (18:1; carbon atoms:double bonds) unsaturated, unesterified fatty acids to diets containing long-chain saturated, unesterified fatty acids (18:0) and they pre- ferred diets containing mono-unsaturated fats (18:1) to diets containing poly-unsatu- rated fats (18:2). The preference for diets containing long-chain unsaturated fatty acids to diets containing long-chain saturated fatty acids was consistent in birds tested one week after capture at 21°C, one month after capture when cold-acclimated (1°C), and six weeks after capture at 21°C. Birds acclimated to a diet with 50% of the fat comprised of unesterified stearic acid (18:0) lost mass and reduced their food intake when we reduced ambient temperature from 21°C to 11°C over three days. We conclude that especially in energy-demanding situations there are limits to the yellow-rumped warblers’ ability to assimilate some long-chain saturated fatty acids and that this digestive constraint can explain in part why yellow-rumped warblers prefer diets containing long-chain unsaturated fatty acids to diets containing long- chain saturated fatty acids. S. R. McWilliams (correspondence), Department of Natural Resources Science, 105 Coastal Institute in Kingston, Uniersity of Rhode Island, Kingston, RI 02881, USA. E-mail: srmcwilliams@uri.edu. S. B. Kearney and W. H. Karaso, Department of Wildlife Ecology, Uniersity of Wisconsin -Madison, Madison, WI 53706, USA. During migration birds undergo significant changes in body composition (Blem 1976, 1980, 1990, Ramenof- sky 1990) and diet (reviewed by Berthold 1996). Dur- ing periods of fat storage (e.g., during migration or in the cold), birds store large amounts of fats comprised mostly of 16- or 18-carbon unsaturated fatty acids (Blem 1976, Dawson et al. 1983, Blem 1990, Eggler and Williams 2000). Migratory birds deposit these fat stores as a result of eating more food (Blem 1990, Bairlein and Gwinner 1994), and by feeding more se- lectively on energy profitable prey (Moore and Simm 1985, Wheelright 1988, Whelan and Willson 1994) that may have higher proportions of long-chain unsatu- rated fatty acids (reviewed by Bairlein and Gwinner 1994, Bairlein and Simons 1995, Berthold 1996). Selec- tively feeding on long-chain unsaturated fatty acids can be advantageous because such fatty acids may be absorbed and/or metabolized more efficiently than sat- urated fats into a bird’s fat depots (Johnston 1973, Blem 1976, Heitmeyer and Fredrickson 1990, Bairlein and Gwinner 1994, Zurovchak 1997). However, only Bairlein (1991) has directly tested whether birds prefer diets with specific fatty acids. In these pilot studies (sensu Bairlein 1991), he found garden warblers Sylia borin tended to prefer diets with long-chain unsatu- rated fatty acids compared to diets with either long- chain saturated fatty acids or shorter chain-length unsaturated fatty acids. © JOURNAL OF AVIAN BIOLOGY JOURNAL OF AVIAN BIOLOGY 33:2 (2002) 167