Prudent mothers?: paternal investment, female reproductive strategies and offspring development in the barn owl (Tyto alba)

Megan L. Seifert

doi:10.7273/000005772

Parental investment directly affects offspring survival. If this investment is not equal across offspring, unequal or differential survival will occur among progeny. In birds, mothers affect the success of the nest and of individual offspring by clutch/brood size, egg size, brood sex ratio (especially in size dimorphic species), maternally derived yolk hormones, and nestling size hierarchies (caused by laying interval and onset of incubation). In the barn owl (Tyto alba), males provide their partners with food throughout egg laying and halfway through the nestling phase. By measuring male food delivery, we were able to quantify food resources available to females during reproduction and relate food to reproductive investment. We measured clutch size, brood size, egg mass, yolk hormones, and sex ratio differences within and among broods to examine the differing ways in which females invest in their offspring. Brood sex ratios were not related to current food resources, but more resources allowed parents to produce more offspring and fledge more offspring successfully. Population sex ratios did not follow Fisher’s theory of equal allocation; parents fledged equal numbers of male and female offspring in spite of sexual size dimorphism and a significantly male biased hatching sex ratio. Barn owl mothers differentially allocated yolk hormones to eggs. The first 3-4 eggs (depending on clutch size) laid had increasing androgen levels with each successive egg and then androgen levels decreased in later laid eggs. Eggs laid later in the laying order were more likely to be male, partially supporting the sharing-out hypothesis. We found higher survival in early-hatched than in late-hatched nestlings. Male and female offspring had similar circulating plasma androgens and corticosterone concentrations. At an age when tarsus growth had reached its maximum, nestling testosterone levels were positively related to body condition. Circulating corticosterone levels increased with nestling age. Additionally, later-hatched nestlings had higher corticosterone levels; corticosterone was negatively related to body condition at two different points of the nestling phase. These results support the hypothesis that corticosterone plays a role in facilitating honest signaling of nestling need to parents. Thus parental decisions influence offspring fitness in a variety of ways.

Prudent mothers?: paternal investment, female reproductive strategies and offspring development in the barn owl (Tyto alba)

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