Brood‐switching in Eagle Owl Bubo bubo fledglings

Abstract

Nest-switching, the (permanent or temporary) intrusion of one or several foreign young in another brood (which gener ally occurs when fledglings are capable of flight but are not yet completely independent of parents) and their sub sequent adoption by a foster family (parents + native young), has been recorded in many bird species (Riedman 1982). These include several birds of prey (Ospreys Pandion haliaetus , Poole 1982, Gilson & Marzluff 2000; Eurasian Sparrowhawks Accipiter nisus , Wyllie 1985; American Kestrels Falco sparverius , Lett & Bird 1987; Egyptian Vultures Neophron percnopterus , Donazar & Ceballos 1990; Black Kites Milvus migrans and Red Kites Milvus milvus , Bustamante & Hiraldo 1990; Northern Goshawks Accipiter gentilis , Kenward et al. 1993; Spanish Imperial Eagles Aquila adalberti , Ferrer 1993; Lesser Kestrels Falco naumanni , Tella et al. 1997; Montagu’s Harrier Circus pygargus , Arroyo & Garcia 2002) and owls (e.g. Barn Owl Tyto alba , Roulin 1999). One of the most intriguing aspects of this form of alloparental care (Riedman 1982) is the investment of resources by birds into non-genetic offspring, instead of allocating breeding effort exclusively to their own genetic contribution to future generations. Such behaviour seems incompatible with classic evolutionary theory, because it apparently violates the Darwinian principle by which selection does not act to benefit competing genotypes. To explain adoptions in birds, several hypotheses have been invoked and tested. The potential explanations for nest-switching and alloparental care form two main groups (see reviews in Redondo et al. 1995, Bize et al. 2003). First, there may be adaptive explanations such as (1) benefit to fledglings (e.g. better care than in the natal nest, acquisition of a dominant rank within a younger brood, reduction of the ectoparasite load), and (2) kin-selected benefits from a parental perspective when young switch to nests of related adults, diluting predation risk, or increasing breeding experience of related individuals. Secondly, there may be non-adaptive explanations such as reproductive errors or adoptions with negligible costs which do not affect reproductive success or survival of the foster parent. The different intensities of selection pressure for intruder chicks (surviving vs. dying) and foster parents (cost of investment in unrelated fledglings) can also be conceived of as an arms race (Pierotti & Murphy 1987). Under such a scenario, chicks are likely to ‘win’ (and consequently gain adoption) because the selection pressure is stronger than on foster parents to discriminate and reject intruders. Here, we report detailed observations of two cases of nest-switching in radiotagged Eagle Owl Bubo bubo fledglings and discuss them in an evolutionary context. Because nest-switching occurred during the post-fledging dependence period (i.e. owlets were no longer in the nest because they abandon it completely), hereafter we consider it more appropriate to replace the term nest-switching with brood-switching.