Causes and consequences of calcium limitation in breeding passerine birds

This work investigated in mechanistic detail both the causes and the consequences of low Ca availability for temperate passerines as well as how passerines are adapted, behaviorally and physiologically, to breed in a Ca-poor environment. In the first of three studies, experiments showed that Brown-headed Cowbirds on a Ca-poor diet, with access to Ca-rich supplements only in the morning, could lay eggs with normal shells using mostly Ca from endogenous stores. Compared to cowbirds with evening access to Ca supplements, however, these birds tended to lay fewer eggs and to lay more eggs with defective shells or even no shells. The difference between treatments was ascribed to an unwillingness to consume Ca supplements in the morning and reasons for this behavior are discussed.; The second study experimentally tested differences in reproductive performance between groups of wild Tree Swallows breeding in a Ca-poor area. Groups that were provided ad libitum Ca supplements exhibited improvements relative to controls in many variables in each reproductive lifestage: egg, nestling and adult. It was further found that parents worked harder when Ca-availability was low, and allocated resources to reproduction in a manner that prevented failure due to Ca limitation, but at a cost to other components of eggs and nestling diets. Calcium use by supplemented birds was monitored to investigate patterns of Ca consumption relative to weather, reproductive phenology and time of day. Finally, it was found that Tree Swallows are generalists with respect to Ca foraging, and vertebrate sources such as bone are taken opportunistically when invertebrate sources are scarce. The consequences of this generalist diet for Ca limitation are discussed.; The final study investigated environmental correlates of the availability of four groups of Ca-rich invertebrate prey for forest birds. Overall Ca-rich prey availability was strongly correlated with soil and litter Ca levels, but not as strongly with forest stand characteristics such as tree composition and basal area. As Ca availability changed, the relative importance of different invertebrate groups changed for the overall availability of invertebrate Ca. The importance of different controls on the abundance of different Ca-rich prey for birds is discussed.