Temporal and spatial patterns in daily mass gain of Magnolia Warblers during migratory stopover

Whether or not migrants gain mass at a stopover site is an index of site qu ality. Previous studies have examined mass gain of recaptured birds, and of short-term stop-overs by regressing mass at first capture on hour of day. I developed an extension of the latter method using multiple regression to examine the effects on mass gain of hour of day, date, and year. I then use d the method to compare the quality of three stopover sites at Long Point, Ontario, for Magnolia Warblers (Dendroica magnolia). At the peak of fall mi gration, warblers at all three sites gained sufficient mass for a net gain over 24 h, but they gained mass at only two of three sites during spring. M ass gain varied significantly over the course of the day, by date in the se ason, and among years. The earliest spring migrants lost mass at ail sites, but rate of mass gain increased as the season progressed. Similar informat ion fur many more species and stopover sites might aid in habitat conservat ion for migrants.