Incubating females signal their needs during intrapair vocal communication at the nest: a feeding experiment in great tits

Parental care is a key factor of breeding success and parents face evolutionary trade-offs between investment in current offspring and future reproduction and survival. Incubation in birds is energetically costly and the balance between parental and offspring's energetic needs is especially challenging when only one sex incubates, generally the female. In that case, males can contribute indirectly to incubation effort by feeding their mate and females may use begging behaviours to signal their needs to their visiting partner. The great tit, Parus major, is a good model species to test whether females use acoustic communication from the nest to signal their needs to their mate outside because females interact vocally with their mate during incubation. To test whether females use these vocal exchanges to communicate how hungry they are, great tit pairs were recorded on 2 days during incubation: 1 day with a feeder of mealworms in the nestbox and 1 day with an empty feeder. First, food supplementation increased females' nest attentiveness, revealing a decrease in foraging activity and consequently in females' needs. Second, females signalled their need for food both during their male's visits inside the nest but also during the vocal exchange immediately preceding the male's entrance by calling more and by modifying the frequency spectrum of their calls. So females' calling behaviour is an honest signal of need not only during their male's visits, but also before his entrance in the nest.