From floater to breeder, starlings bide their time

Floaters are non-breeding individuals that do not hold a territory or breeding site of their own. In many species, floaters can often be observed visiting the territories of conspecifics, especially prior to eventually acquiring a breeding site. This behaviour, commonly referred to as prospecting, is thought to provide several potential benefits to floaters. These include the acquisition of valuable social and environmental information, increased familiarity with local breeding sites, and possibly gaining a competitive advantage over other floaters when breeding opportunities arise.

In this study, we examined prospecting behaviour by analysing detections of PIT-tagged male floaters in a population of spotless starlings (Sturnus unicolor). We explored how floater activity varied across different stages of the breeding season and assessed how these exploratory visits influenced the floaters' eventual choice of nest sites. Additionally, we investigated whether factors such as the distance from previously visited sites, reproductive success, and the phenotype and fate of the nest's previous owner played a role in final settlement decisions.

Our findings revealed that floater activity increased notably during the nestling-rearing period, with a peak corresponding to the peak of nestling growth. Interestingly, floaters were more likely to establish their own breeding sites in areas where they had been previously detected during the prior year. This pattern suggests that prospecting behaviour helps males to become familiar with specific areas, thereby increasing their chances of successfully settling there in the future.

Although we observed that prospecting visits were more frequent at nests with a larger number of nestlings, we found no evidence that breeding success, the physical characteristics of previous nest owners, or their parental investment (measured via provisioning rate) influenced the floaters’ eventual nest choice. This indicates that males may not be relying on public information such as reproductive success when deciding where to settle.

However, one notable pattern did emerge: floaters were more likely to breed in nest boxes where the previous occupant had disappeared from the colony. This suggests that floater visits may help males to detect upcoming vacancies, potentially allowing them to capitalize on newly available sites when the breeding season begins.

Overall, our results suggest that prospecting behaviour may serve several, potentially overlapping, functions in this species. These include not only assessing future breeding opportunities but also monitoring the social dynamics and changes in territory availability. Future studies conducted in non-saturated colonies could help disentangle the specific benefits and underlying mechanisms of prospecting behaviour in more detail.