Variation in Parental Care with Offspring Age in the Greater Flamingo

Abstract

Several studies have predicted how parental investment should change in relation to the age of offspring (Williams 1966, Winkler 1987). The Reproductive Value Hypothesis (RVH) states that parents should be prepared to invest more in older juveniles because they have a higher probability of surviving to breeding age. This may occur because older juveniles are closer to maturation and because the instantaneous rate of juvenile mortality generally decreases with increasing age (Clutton-Brock 1991). Increase in feeding effort with chick age has been documented for some species of colonial waterbirds. In Pigeon Guillemots (Cepphus columba) provisioning rates increased with chick age, only during the first part of the nestling period (Emms and Verbeek 1991). Feeding rates were not observed to vary with chick age in the closely related Black Guillemot (Cepphus grylle), but size of fish delivered to the nest increased with chick age (Cairns 1987). As chicks grow older they also have greater food requirements, and increased parental care might simply correspond to the higher energetics and nutritional demands of the offspring. Further evidence for increased parental care with increasing age of offspring comes from studies of brood defense. Brood defense has been reported to increase with nestling age in several passerine species (Andersson et al. 1980, Redondo and Carranza 1989, but see Knight and Temple 1986, Westmoreland 1989). However, the observed increase in parental care with offspring age can also be influenced by confounding variables such as parental age and or quality. Breeding success is known to increase with parental age in several bird species (Saether 1990) and this can be the result of increased experience with age. In addition, as parents grow older, their potential for future reproduction decreases. Thus the cost of reduced future reproductive success should decline with age and older parents should be selected to invest more in offspring compared to younger parents (Pugesek 1981). Therefore, it is important to control for parental age when considering variation in parental care with offspring age. Here, we analyze data from two years on the duration of feeding bouts by Greater Flamingo (Phoenicopterus ruber roseus) parents of known age and sex to their offspring. We show that only male parental care increases with offspring age. We discuss our results in relation to lifetime reproductive success and costs of reproduction.