Long-term consequences of fostering: Single egg fostering leads to decreased survival in zebra finch females, but not in males

Cross-Fostering, i.e., the exchange of eggs or hatchlings, is a widely used technique, to disentangle genetic from environmental effects or to manipulate the clutch size. In most bird species, this manipulation is easily accepted by the social parents, leading to the conclusion that fostering has no detrimental effect. Using a dataset of four cohorts (N=298) of zebra finches (Taeniopygia castanotis), in which we fostered routinely a single egg into another nest of zebra finches, we explored potential short- and long-time effects of fostering. Noteworthy, these experiments were not designed to test this hypothesis. The objective of the egg fostering experiments was to test for parental recognition (Caspers et al. 2017) and mate choice decisions (Goluke 2018). Consequently, the aim of the present study is purely explorative. Our study confirmed previous findings that fostering has no short-term effects on the morphology and growth rates of the chicks, neither in males nor in females. However, we found that fostering has a sex-specific long-term effect. Females originating from fostered eggs had a significantly reduced lifespan compared to those from non-fostered eggs. Conversely, the lifespan of fostered males was similar to that of non-fostered males. All birds were housed in large groups, experiencing the same conditions after nutritional independence (day 35). Therefore, we can only speculate that fostering might result in early developmental stress, which may affect the individual fitness of females later in life, ultimately leading to shorter lifespans.