Background
Many vertebrates develop in the company of their siblings, a social context that profoundly influences their life histories and phenotypes. While avian begging behaviour in parent–offspring conflict and sibling competition/cooperation have been largely studied in nestlings, the fledglings’ begging remains under-researched, despite the continued dependency of young birds throughout the post-fledging dependence period (PFDP). Siblings typically maintain close proximity during the PFDP, a pattern that might be explained by the following four mutually non-exclusive hypotheses: competition over the prey delivered by parents (competition, H1), mutual affection established during the nestling period (cooperation-friendship, H2), various antipredation behaviours (cooperation-antipredation, H3) or begging negotiation about whose need is greater (cooperation-negotiation, H4).

Results
We radio-tracked 123 boreal owl (Aegolius funereus) fledglings from 34 nests over 6 years in Czechia and Finland and recorded daily inter-sibling distances throughout the PFDP. Distances were longer in Czechia than in Finland during both night (82 ± 81 vs 56 ± 65 m; mean ± SD) and day (75 ± 109 vs 64 ± 86 m). In both areas, distances increased with an augmenting time gap in tracking, siblings’ age and the number of present siblings at night but decreased during the day. These consistent patterns across different environments indicate that the life history of the target species prevails over the environmental effects. Consistent with the predictions of the cooperation-antipredation hypothesis (H3), the mean inter-sibling distances were shorter in Finland compared to Czechia during both day and night, and inter-sibling distances were shorter within larger sibling flocks during the day. The cooperation-negotiation hypothesis (H4) is supported by the finding that vocally begging siblings were closer to each other than their non-begging counterparts.

Conclusions
Our results demonstrate, for the first time, that sibling cooperation can occur during the PFDP, when offspring are vulnerable to predators, both during diurnal roosting and at night while begging for food. We conclude that the inter-sibling distances in boreal owls during the PFDP are maintained because siblings cooperate through collective antipredation behaviour during diurnal roosting and through collective negotiation over access to the indivisible prey items delivered by their parent during nocturnal activity.