An Illustrative Case for Muscular Fatigue Resistance Underlying Exaggerated Begging in Avian Brood Parasites

Food solicitation displays are critical for offspring survival and can become evolutionarily exaggerated because of sibling rivalry or parental conflict. Avian brood parasites represent extreme cases of these conflicts, using exaggerated begging displays to outcompete host offspring. Yet we know little about the physiology underlying such traits. Here, we investigated an interesting case of competitive begging asymmetry in brown-headed cowbirds (Molothrus ater) and host prothonotary warblers (Protonotaria citrea). Through behavioral assays, we found that cowbirds beg far longer than warblers, while each species’ nestlings initiate begging at similar speeds. Meanwhile, complementary in situ muscle stimulation assays suggested that these effects were due to greater fatigue resistance in cowbirds compared with their host. While cowbirds represent only one evolutionary origin of obligate brood parasitism, our findings present a compelling path for future comparative work to explore muscular fatigue resistance as a key physiological innovation that facilitates competitive begging success in parasitic chicks.