Avian sibling cannibalism: Hoopoe mothers regularly use their last hatched nestlings to feed older siblings Soler Cruz, Juan José Martín-Vivaldi Martínez, Manuel Lorenzo Brood reduction Clutch size Hatching asynchrony Ice-box hypothesis Infanticide Siblicide Sibling hierarchy Upupa epops DATA AVAILABILITY Data used in this paper can be found at https://figshare.com/s/55775bd4f5e31a464fd2 ACKNOWLEDGMENTS We thank Carmen Zamora, Manuel Soler Cruz, Juan Moreno Klemming, Anders Pape Møller, and Graham Tebb for discussion and comments on a preliminary version of the manuscript. Alberto Ruiz Moreno and Enrique Cortés Sánchez helped with the recording equipment. Natalia Juárez García Pelayo and Lola Barón helped with fieldwork. The Spanish research group was supported by the Spanish Ministerio de Ciencia, Innovación y Universidades and European (FEDER) funds (CGL2017-83 103-P), and from facilities, including an apartment, provided by the city authorities of Guadix, where a small lab to process samples quickly was installed. Finally, we would like to thank Renate Hengsberger for revising earlier drafts. 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. 2022-03-29T11:19:31Z 2022-03-29T11:19:31Z 2022-02-28 journal article Soler et al. Avian sibling cannibalism: Hoopoe mothers regularly use their last hatched nestlings to feed older siblings. Zool. Res. 2022, 43(2): 265−274 [https://doi.org/10.24272/j.issn.2095-8137.2021.434] http://hdl.handle.net/10481/73908 10.24272/j.issn.2095-8137.2021.434 eng http://creativecommons.org/licenses/by-nc/3.0/es/ open access Atribución-NoComercial 3.0 España Chinese Academy of Sciences