Body condition and retaliation in the parental effort decisions of incubating great frigatebirds (Fregata minor)

Decisions about parental effort have the potential to be affected by an individual's body condition and, among species with biparental care, by the level of effort made by one's mate. Previous studies, primarily of short-lived species, have found that a reduction in the parental effort of one pair member typically leads to a compensatory increase by the mate. However, long lived species with short-term pair bonds might be expected to retaliate, rather than compensate, for a reduction in a mate's effort. I studied the factors affecting parental effort decisions during incubation by the great frigatebird, a long-lived seabird that forms new pair bonds for each breeding attempt. During incubation, males and females took turns incubating and for-aging. Individuals lost mass during an incubation shift and regained this mass during the subsequent foraging bout. If an individual was left on the nest for a long period of time while its mate was foraging, it subsequently went on a long foraging trip after being relieved by its mate, despite the fact that longer shifts were likely to lead to nest failure. This relationship between incubation shift length and duration of subsequent foraging excursion could be due to a need to regain body condition after a long fast, or it could reflect a retaliatory response to the mate's prolonged absence. To test these alternatives, I conducted a food supplementation experiment. Individuals engaged in a long incubation shift were assigned to a control group or to a treatment group that was fed until the end of that particular incubation shift. Overall, fed birds returned fi om the subsequent foraging trip sooner than control birds, demonstrating that the relationship between incubation shift duration and foraging trip duration is due primarily to a need to increase body mass, rather than being a retaliatory response to a mate's low level of parental effort. However, males and females differed in the extent of their responses to the experimental treatment, indicating that males may also exhibit some degree of retaliation.