Parental investment in male and female offspring by the Peregrine Falcon, Falco peregrinus

Abstract

There is general theoretical agreement that parents should alter offspring sex ratios in response to the relative costs of producing and raising sexually dimorphic male and female offspring. Among raptors females are often much larger than males, yet there is little skew in nestling sex ratios at the population level. The food intake of male and female Peregrine Falcon, Falco peregrinus, nestlings was compared in order to investigate parental resource allocation to their highly size-dimorphic offspring. Intake of seven broods was measured by (i) counting the mouthfuls nestlings received during a feeding bout, and (ii) by monitoring their turnover of tritiated water, which was combined with the results of an earlier study of four broods. Despite significant differences in size, there was no significant difference in the daily food intake of male and female nestlings for about the first 21 days of the nestling period, after which time growth of males slowed. Females continued to grow and by the end of the nestling period their total weight and food intake exceeded those of males by 45% and 25%, respectively. We argue that many apparent discrepancies between studies on gender differences in food intake can be explained in part by the section of the nestling period that was monitored: any differences may not be apparent until the latter part of the nestling period. The crude growth efficiency of females was greater than that of males. It is concluded that female nestlings require a greater amount of food than do males, but because of greater growth efficiency, they need less than expected on the basis of body mass. A similar pattern was apparent in an interspecific comparison — the difference in food intake between the sexes of the more dimorphic species was less than expected on the basis of direct proportionality. In Peregrine Falcons, there was gender overlap in total food intake over the nestling period. Other studies show that individual nestlings in large broods consume less food than those in smaller broods. Together, these results indicate a wide range of variation in intake for successful growth. Lastly, resource allocation patterns in Peregrine Falcon nests seemed largely determined by sibling interactions. The closest nestling to the adult female during a feeding bout received the most food and no individual monopolised this position, regardless of gender or relative size or age. We propose that a regular supply of large prey to Peregrine Falcon nestlings makes dominance hierarchies unnecessary.