eriikäisillä

Eriikäisillä refers to a phenomenon observed in certain species of birds, particularly in the family Fringillidae, which includes finches and buntings. The term originates from the Finnish language, where "eri" means different and "ikäisillä" refers to the age of the birds. This behavior is characterized by the presence of two distinct age classes within a single breeding season: adults and immatures.

In species exhibiting eriikäisyys, the adults are typically older birds that have previously bred and are more experienced in raising offspring. These adults are often more aggressive and territorial, and they often mate with immatures, which are younger birds that have not yet bred. The immatures, on the other hand, are less aggressive and more likely to accept the adults' territories.

The phenomenon of eriikäisyys is thought to be an adaptation that allows for the efficient use of resources. By allowing immatures to breed in the adults' territories, the species can increase its reproductive success without the need for additional resources to establish new territories. This behavior is also thought to be a way for the species to maintain genetic diversity, as the immatures are often related to the adults.

Eriikäisyys is not observed in all species of birds, and its occurrence is thought to be influenced by a variety of factors, including the availability of resources, the density of the population, and the social structure of the species. The phenomenon is of particular interest to ornithologists and evolutionary biologists, as it provides insights into the complex social dynamics and reproductive strategies of bird species.