In their recent article, Fernandez-Duque et al. (2023) examined instances where fledglings, able to move freely, were found in the nests of conspecifics with chicks too young to fly. In these nests, foster parents fed the intruder fledglings. While Fernandez-Duque et al. (2023) deemed this behavior a new form of food acquisition, it is important to note that this behavior in birds has been documented for quite some time. At least three terms have been proposed to describe this behavior (Table 1), with variation in their use in different contexts. Here, we propose a unified terminology that can encompass the behavior referred to by Fernandez-Duque et al. (2023) as “nest integration”.

Table 1 Some bird species in which nest switching by young has been reported. Shown is where intruder chicks were fed by foster parents and the term used to describe the observed behavior
Full size table

The act of fledglings actively departing from their natal nest and relocating to a different nest with younger chicks to secure food is not a new phenomenon (Table 1). This has most commonly ben referred to as “nest switching” (Kenward et al. 1993; Redondo et al. 1995) and as part of the broader category of “parental-care parasitism” (Roldán and Soler 2011). However, it is worth noting that “nest switching” has also been applied to cases in which adults change nests for subsequent breeding attempts (Donahue et al. 2018; Turjeman et al. 2021). Parental-care parasitism also encompasses behaviors beyond chick rearing, such as nest usurpation. To specifically address the presence of conspecific young among nestlings from another family, we propose the term “nest switching by young”. For instances in which adults switch nests for subsequent breeding attempts, we suggest using “nest switching by adults”. Importantly, our definition does not require the young to be fledglings and can include young that do not have yet attained the ability to fly but move to nearby nests, as seen in colonial species, both altricial and semi-precocial (Quinn et al. 1994; Jouventin et al. 1995; Tella et al. 1997; Brown 1998). Beyond nest switching by young, there are instances where post-fledgling individuals do not enter the nests of other conspecifics but instead occupy their territories, where they receive nourishment from foster parents (Arroyo and García 2002). This behavior is functionally similar to nest switching by young and has been sometimes termed “brood switching” (Kouba et al. 2017; Penteriani and Delgado 2008).

Fernandez-Duque et al. (2023) emphasized the importance of understanding whether this behavior is advantageous for fledglings but detrimental for the provisioning adults to gain insights into its evolution. They also posited that nest switching by young should occur more frequently in non-colonial birds, as colonial birds have more refined mechanisms for recognizing their offspring. By unifying terminology, it is apparent that there is existing literature that is relevant to some of the questions that are raised by Fernandez-Duque et al. (2023), such as cost-benefit analyses involving the parties involved (Redondo et al. 1995; Brown 1998; Bize and Roulin 2006) and the impact of breeding density (Arroyo and García 2002; Bustamante and Hiraldo 1995) as functional explanations for young birds switching nests.

The provision of food to unrelated offspring can be viewed as a form of kleptoparasitism (Redondo et al. 1995; Roldán and Soler 2011). Kleptoparasitic birds acquire food through various means, including piracy, pilfering, and peculation. Vollrath (1984) defined piracy as obtaining food through force, pilfering as acquiring it stealthily, and peculation as obtaining it through disguise. While some young birds that enter the territories of foster parents may attempt food acquisition through piracy (e.g., Arroyo and García 2002), in most reported nest-switching cases, young birds obtain food from foster parents through peculation. This kleptoparasitic behavior may be facilitated by the indiscriminate response of parents caring for young offspring to begging behavior (Jamieson and Craig 1987) and because the costs associated with errors in kin discrimination (Sherman et al. 1997; Brown 1998) may outweigh the benefits.