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

Decisions about parental effort have the potential to be affectedby an individual's body condition and, among species with biparentalcare, by the level of effort made by one's mate. Previous studies,primarily of short-lived species, have found that a reductionin the parental effort of one pair member typically leads toa compensatory increase by the mate. However, long-lived specieswith short-term pair bonds might be expected to retaliate, ratherthan compensate, for a reduction in a mate's effort. I studiedthe factors affecting parental effort decisions during incubationby the great frigatebird, a long-lived seabird that forms newpair bonds for each breeding attempt. During incubation, malesand females took turns incubating and foraging. Individualslost mass during an incubation shift and regained this massduring the subsequent foraging bout. If an individual was lefton the nest for a long period of time while its mate was foraging,it subsequently went on a long foraging trip after being relievedby its mate, despite the fact that longer shifts were likelyto lead to nest failure. This relationship between incubationshift length and duration of subsequent foraging excursioncould be due to a need to regain body condition after a longfast, or it could reflect a retaliatory response to the mate'sprolonged absence. To test these alternatives, I conducteda food supplementation experiment. Individuals engaged in along incubation shift were assigned to a control group or toa treatment group that was fed until the end of that particularincubation shift. Overall, fed birds returned from the subsequentforaging trip sooner than control birds, demonstrating thatthe relationship between incubation shift duration and foragingtrip duration is due primarily to a need to increase body mass,rather than being a retaliatory response to a mate's low levelof parental effort. However, males and females differed in theextent of their responses to the experimental treatment, indicatingthat males may also exhibit some degree of retaliation.