Effect of Food Supply on the Breeding Biology of Western Kingbirds

We studied the breeding ecology and behavior of Western Kingbirds in relation to abundance of flying insects in an area of sympatry with Cassin's Kingbirds in southeastern Arizona. Day—to—day variation in abundance of flying insects was related to date and temperature, while annual differences appeared to reflect precipitation during the previous year. Western Kingbirds nesting in study sites with relatively high insect biomass had earlier clutch initiation dates, larger clutch sizes, higher nestling growth rates, shorter time between foraging flights, and shorter time between nestling feedings, compared to birds nesting where food supply was lower. Annual differences in insect biomass were also reflected in kingbird clutch initiation dates, clutch sizes, and nestling growth rats. Fledgling survival was related to rate of mass gain as a nestling. Indications were that food was not superabundant, yet no detectable effects of kingbird density on clutch size or nestling growth rate existed. Kingbirds did not appear to reduce significantly the local abundance of flying insects, which calls in question the importance of competition for food in determining local habitat separation.