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

Abstract

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.

Food supply and provisioning behavior of parents: Are small hoopoe nestlings condemned to die?

“…Which is the functionality of such nestlings or of last laid eggs? The high rates of hatching failures in hoopoe nests, which in our study area are close to two eggs on average ( Martín-Vivaldi et al 1999 ; Soler et al 2008 , Soler, Martín-Vivaldi, et al 2022 ) might suggest an insurance function of the last laid eggs ( Forbes 1990 , 1991 ; Hardy 1992 ). This is likely the case because the intensity of brood reduction, that we detected up to 2 days after the end of hatching, associated negatively with hatching failures (β(SE) = −0.365 (0.143), F = 6.50, df = 1, 44, P = 0.014), even after controlling for the effect of hatching date (β(SE) = 0.287 (0.143), F = 4.02, df = 1, 44, P = 0.051).…”

“…Preferential feeding of females for larger nestlings might seem contradictory to previous results published by Ryser et al (2016) , where they demonstrated that females are more selective than male hoopoes when it comes to food allocation to the smallest chick. However, those authors studied food allocation by adult hoopoes after most of the brood reduction had occurred (i.e., when females leave the nests and together with males bring food to feed leftover nestlings) and, then, it is possible that food allocation behavior of females changes at that nestling stage (see also Soler, Martín-Vivaldi, et al 2022a ).…”

“…In these situations, females preferred to use extra-prey to feed themselves, which strongly suggest that smaller nestlings have almost nil probability of survival. Hoopoes produce asynchronous broods with a pronounced size hierarchy with nestlings of intermediate size that, together with smallest nestlings, might also starve due to females ignoring their signals of need ( Martín-Vivaldi et al 2014 ; Soler, Martín-Vivaldi, et al 2022 ). Consequently, it might be possible that the tested hypothesis applies to hoopoe nestlings of intermediate size and, thus, the expected effects of food availability were only detected in that kind of nestlings.…”

“…The hoopoe is a species with an extreme hatching asynchrony and nestling size hierarchy, in which females stay at the nests during incubation and hatching, and feed nestlings with prey delivered to the nest by the male ( Martín-Vivaldi et al 1999 ; Díaz-Lora et al 2020 ; Diaz-Lora et al 2021 ). In addition, it is worth to mention that females prefer to feed larger offspring ( Martín-Vivaldi et al 1999 ), that more than one nestling die soon after hatching in more than 70% of the nests ( Ryser et al 2016 ; Soler, Martín-Vivaldi, et al 2022 ), and that brood reduction more often occurs in poor environments ( Hildebrandt and Schaub 2018 ). Thus, following the hypothesis that food availability should affect feeding behavior of females and begging success of nestlings, we predicted that experimental food supply would increase begging success of small nestlings and feeding preference of females for them.…”

Sibling Cannibalism in Group-Living Larvae of the Solitary Wasp,Isodontia harmandi(Hymenoptera: Sphecidae): Effects of Prey Availability and Size Discrepancy

“…Siblicidal brood reduction has long been studied in birds, in which cannibalistic events occur occasionally and are considered of limited importance Environmental Entomology, 52(2), 2023, 157-168 https://doi.org/10.1093/ee/nvad002 Advance Access Publication Date: 28 January 2023 Research nutritionally (Bortolotti et al 1991, Stanback and Koenig 1992, Mock and Parker 1997. Thus, the dichotomization of the sibling rivalry context may be affected by a taxonomic bias in studies of sibling rivalry (see Morandini and Ferrer 2015), although sibling cannibalism in birds may occur much more frequently than previously thought, as shown by recent studies (Anderson et al 2015, Allen et al 2020, Soler et al 2022. Siblicide and sibling cannibalism share costs (risk of injury and loss of inclusive fitness) and benefits (reduction in resource competition), but cannibalism also has other costs (risk of infection) and benefits (gains in nutrition) (Fox 1975, Polis 1981, Edgar and Crespi 1992, Mock and Parker 1997, Forbes 2005.…”