Predation risk and reproductive effort: impacts of moonlight on food provisioning and chick growth in Manx shearwaters

According to life history theory, it is particularly important for long-lived iteroparous species to balance effort invested in reproduction against risks to future reproductive potential, leading to a conflict between investment in current versus potential future offspring. If providing care for offspring entails a risk of predation, parents must compromise between ensuring offspring growth and survival and ensuring their own survival. We addressed three hypotheses for the outcome of this conflict, which focus on different costs and benefits, and relate variation in parental effort to: (1) the parent's own risk of predation; (2) the reproductive value of the offspring; and (3) the harm that offspring would suffer in the absence of such care. We tested these hypotheses using Manx shearwaters, Puffinus puffinus, which, in common with many burrow-nesting seabirds, are active above ground only at night as a means of reducing attacks by diurnal avian predators. We found that parents were far less likely to return to the colony on moonlit nights, when they were vulnerable to such predators, and tended to delay their arrival until after moonset, which strongly supported hypothesis (1). Also, parents of chicks in good condition were more likely to attend the burrow on moonlit nights than those of chicks in poor condition, which supported hypothesis (2) but not hypothesis (3). Growth of chicks was suppressed on clear nights around the full moon, such that they experienced well-below average growth during one week in four. This could help explain the accumulation of fat stores by the chick that has evolved in some nocturnal seabirds.