A predation risk-and-avoidance model of nestling responses to parental vocalizations

Question: How does the nest type of a bird species - cup versus cavity - affect the evolution of its nestlings' responses to parental vocalizations? Background: Nestling birds show distinct behavioural responses either to parental beggingsolicitation calls (switch-on begging, lower-frequency sounds) or to parental alarm calls (switch-off begging, high-frequency sounds), but rarely to both. Previous theory suggested that differences in the vibrational conductance of cup versus cavity nests drive the evolution of divergent nestling responses to the different types of parental vocalizations. Many predators eavesdrop on begging displays of nestling birds, then locate the nest and prey on the brood. The risk of predation on broods in cup nests is consistently greater than on broods in cavity nests. New theory: Whether nestlings respond to parental begging-solicitation calls (switch-on) or to parental alarm calls (switch-off) is dependent on a trade-off influenced by predation risks. Although switch-on calls are more efficient at protecting the nestlings that perceive them, they are less efficient at delivering their signal to younger nestlings with poorer sensory and discriminatory abilities (compared with switch-off calls). Thus, where predation risk is high, 'off' calls are favoured; where predation risk is lower, 'on' calls are favoured. Hypotheses: Cup-nesting species, which have higher rates of predation, use 'off' signals, which their young are able to sense early in life before they can sense 'on' signals. Cavity-nesting species are the opposite in all particulars. In addition, cavity nestlings develop the ability to sense low-frequency 'on' sounds later than cup nestlings develop the ability to sense high-frequency 'off' signals. Method: We examined existing comparative data on predation rates at different nesting locations. Using auditory brainstem response studies, we also examined data on the development (ontogeny) of acoustic sensitivity in altricial avian species. Results: Nestlings of cup-nesting species suffer relatively higher predation rates than nestlings of cavity-nesting species. At first, nestlings can hear only a narrow range of sound frequencies. Nestlings gradually develop sensitivity to the full spectrum of vocalizations only after several weeks. The sparse data available on avian auditory ontogeny indicate that cup-nesting species develop early sensitivity to high-frequency sounds, and cavity nesters do so to lower-frequency sounds. Conclusion: The higher predation rates of cup nesters compared with cavity nesters, coupled with the post-hatching delay of all nestlings in achieving full sensitivity to the different types and frequency ranges of parental calls, provide a reasonable explanation for the evolutionary advantage of alarm calls to cup nesters and begging-solicitation calls to cavity nesters.