More than 10% of bird species breed colonially, in assemblages. This strategy has several advantages. Birds in colonies can detect predators more efficiently, defend communally or perform communal feeding trips.
One particular form of communal breeding is the nesting of small-bodied species, such as passerines within nests of larger birds, like raptors. Passerines that inhabit nests of other, non-passerine birds may have an advantage over others that breed in a less safe location in terms of predation. Usually, they choose nests of more aggressive species, which may help to increase breeding success.
An example of this phenomenon is the nesting of passerines in white stork nests. In Europe, house and tree sparrows and starling are the most common residents of a white stork nest. Storks build large, thick nests, that are reused and rebuilt for several breeding seasons, providing good shelter from harsh weather conditions, like low temperatures and strong wind or rain, and they are available year-round.
Although this phenomenon is well-known thanks to many papers, the particular factors driving the occurrence of passerines within stork nests have not been widely studied.
It was worthwhile to address this issue especially because populations of sparrows are currently declining in many European countries, including Poland. Availability of nesting niches has become lower because many settlements have been restored and covered by thermal insolation. This is the most important factor, besides food availability, causing the decline of sparrows. In this context, stork nests may be a key element in farmland to provide favorable breeding sites for passerines.
In our study, we aimed to find out which factors determine the presence or absence, the number, and their abundance of species within stork nests. We conducted our research in an extensively used, traditional farmland in northeast Poland. There, the white stork breeds in the highest densities. We surveyed over 200 stork nests to record the presence of breeding passerines, and which species, in what number breed there. More than a half (57%) of stork nests were inhabited by passerines. These were as follows: house sparrows, 68%; tree sparrows, 65%; and starlings, 30%.
During the survey, we also recorded factors such as stork nest thickness, whether the nest was currently occupied by the white stork, the type of nest, location, and several environmental variables like the surface of arable fields, meadows, and distance to human settlements. We checked which factors drove the number of passerine inhabitants of the nest of the stork.
We also checked if the number of residents was increased by the interaction between different species. Sparrows usually forage in open areas like crop fields, meadows, or set-asides. We predicted a possible inter-species competition between them when sharing the same foraging and nesting niches. Research from urban populations led us to assume so. We found more breeding passerines in thicker stork nests and in those currently occupied by a pair of white storks. The probability of breeding passerines was higher in stork nests located on electricity poles and in those which were closer to buildings. We found more passerines in stork nests with a higher number of crop fields in the surrounding areas.
White Stork nests are favorable nesting sites for passerines because they are well-insulated and probably safer from predators (mammal and avian). They may offer specially protected sites for small passerine birds. By protecting stork nest sites, we can also protect several other declining birds.
These findings are described in the article entitled Factors determining presence of passerines breeding within White Stork Ciconia ciconia nests, recently published in the journal The Science of Nature. This work was conducted by Adam Zbyryt from The Polish Society for Birds Protection, Dariusz Jakubas from the University of Gdańsk, and Marcin Tobółka from Poznań University of Life Sciences.