Determinants of Nestling Feeding Rates and Parental Investment in the Mountain Chickadee

The influence of parental sex, brood size, and nestling age in determining the rate of feeding of nestling Mountain Chickadees (Parus gambeli) is examined. Male Mountain Chickadees provide a majority of the food delivered to nestlings. For most of the nestling cycle, the male feeds each nestling a greater volume of food than does the female. His relative share of feeding visits increases with increasing brood size. Although the male's feeding contribution is large, the female predominates in other aspects of parental investment. Throughout the nestling cycle, female nest attendance times are at least twice as long, on average, as the male's. Her attendance time increases as brood size declines. Each nestling in a smaller brood receives a greater volume of food and more feeding visits than counterparts in larger nests. Prey volume per nestling, visits per nestling, size of individual prey, and prey volume per trip all exhibit a rising phase over the initial 5 to 10 days of the nestling cycle followed by a level phase till near fledging time. Load size, however, increases almost continuously throughout the cycle. Three solo measures (number of feeding visits, prey size, and number of prey) were evaluated as estimators of daily prey volume. Effectiveness of any one of these estimators varied between sexes and each of these variables generally explained less than 50% of the variation in daily prey volume returned by an adult. Therefore, care should be taken in equating such estimators to more complex measures of feeding effort like prey volume.