Avian sibling cannibalism: Hoopoe mothers regularly use their last hatched nestlings to feed older siblings

Sibling cannibalism is relatively common in nature,but its evolution in birds and certain other vertebrates with extended parental care had been discarded.Here,however,we demonstrate its regular occurrence in two European populations of the Eurasian hoopoe(Upupa epops)and explore possible adaptive and non-adaptive explanations.Results showed that sibling cannibalism was more frequently detected in Spain(51.7%)than in Austria(5.9%).In these two populations,the hoopoes laid similar clutch sizes,resulting in similar fledging production,but hatching failures were more frequent in the northern population.Consequently,having more nestlings condemned to die in the southern population may explain the higher incidence of sibling cannibalism.In accordance with this interpretation,hatching span and failure,but not breeding date,explained the probability of sibling cannibalism in the Spanish hoopoes,while all three variables predicted brood reduction intensity.Furthermore,experimental food supply reduced the probability of sibling cannibalism,but not the intensity of brood reduction.Finally,females allocated fewer resources to the smallest nestlings when they were going to starve,but not necessarily when they were going to be used as food for their siblings.These results suggest that hoopoes produce extra eggs that,in the case of reduced hatching failure and food scarcity,produce nestlings that are used to feed older siblings.These findings provide the first evidence that sibling cannibalism occurs regularly in a bird species,thus expanding our evolutionary understanding of clutch size,hatching asynchrony,parent-offspring conflict,infanticide,and sibling cannibalism in the animal kingdom.

Functional explanation of extreme hatching asynchrony: Male Manipulation Hypothesis

Hatching asynchrony in birds is considered an adaptation to facilitate brood reduction because under conditions of food scarcity, the smallest nestling usually dies soon after hatching, thereby minimizing parental effort. However, in species with extreme hatching asynchrony, the last hatchlings paradoxically experience a very low probability of survival and death can take so long that it can hardly be considered an adaptation. Here, we propose and experimentally tested a new adaptive hypothesis explaining the brood reduction paradox, namely the“Male Manipulation Hypothesis”. Our hypothesis suggests that by inducing asynchronous hatching,females increase the feeding requirements of the brood, which will induce males to increase provisioning effort. In addition, females may extend the period of male manipulation by feeding the smallest nestling just enough to sustain life. Our study showed that male common blackbirds(Turdus merula) increased their effort(i.e., number of food items per hour) in experimental asynchronous broods compared to synchronous broods, while females reduced their contribution, as predicted by the hypothesis.