Plan ahead, or wing it? How storm-petrel parents adjust food delivery to young chicks

Parents must decide how to allocate energy gained from foraging between self and offspring. Storm-petrels (Procellariiformes: Hydrobatidae) are pelagic seabirds that travel hundreds of kilometers across multiple days before returning to the nesting burrow to feed a dependent chick. Parents return to the nest with food stored in the proventriculus, a portion of which is regurgitated to their offspring. As the chick grows, provisioning demands increase. However, it is unknown whether parents meet this increasing demand by (1) altering their foraging strategies to acquire more food or (2) allocating a greater proportion of their intake to the chick. We designed, validated, and implemented a new technology-the Burrow Scale Monitor-to measure Leach's storm-petrels (Hydrobates leucorhous) as they entered and exited the nesting burrow. We monitored breeding adults over the first 30 d of chick rearing to determine whether storm-petrel parents adjust their foraging intake to the age of the chick or simply adjust energy allocation at the nest. Food delivery increased with chick age, but this increase was driven to a much greater extent by parents delivering a greater proportion of their body mass as food (ie, a shift in parental allocation) rather than by adults adjusting their foraging strategy to match chick age. Only by measuring adult body mass on arrival and exit at the nesting burrow could we understand how parents adapt their provisioning strategy to the increasing demands of the growing chick.