The population converges to a state where ability to recognize and reject unlike eggs is neutral to selection, and a small cost ignored in this model will suffice to prevent the evolution of egg rejection. If variability of egg appearance is small enough, conspecific parasitism selects for lower inter-clutch variation, and all individuals have nearly identical eggs. Simulation analysis suggests that egg rejection can logically evolve due to conspecific brood parasitism and that variability of egg appearance plays a key role to determine the evolutionary trajectory of the three adaptive traits. These findings provide impetus to further study apparently unsuitable hosts and perhaps even reconsider traditional classifications of host suitability in the context of brood parasite-host coevolution.Īn individual-based simulation model is constructed to explore evolutionary dynamics of the three adaptive traits: (1) proportion of eggs allocated as conspecific parasitism, (2) rejection ability to reject unlike eggs in own nest, and (3) egg appearance. Although Cuckoo chick fledging mass was highly host species-specific (i.e., showed high statistical repeatability across various host species), we did not find any evidence for the hypothesis that host body size (mass) positively affects parasite chick growth (fledging mass or age). Cuckoo chicks grew similarly in nests of all four species of non-hosts, similarly to chicks in host Redstart nests, and generally better than in nests of the most numerous Cuckoo host, the Reed Warbler Acrocephalus scirpaceus. We did not find any evidence for chick discrimination in non-hosts, i.e., no chicks were rejected, attacked, or neglected. Natural nests of non-hosts, as opposed to artificial nest boxes with small entrance holes, are often placed in cavities that show both entrance and inner cavity sizes large enough for female Cuckoos to lay and Cuckoo chicks to fledge. Therefore we examined Cuckoo chick performance in five cavity nesting host species, including one regular Cuckoo host - the Common Redstart Phoenicurus phoenicurus and four non-hosts: the Pied Flycatcher Ficedula hypoleuca, Spotted Flycatcher Muscicapa striata, Great Tit Parus major, and Coal Tit Periparus ater.

In general, any species can become an unsuitable host for a parasite at laying, incubation, or nestling stages with the last one being much less studied than the others. Cavity nesting passerines always represented a textbook example of unsuitable hosts but recent evidence casts multiple doubts on this traditional view.

Why other sym-patric hosts are not used, or actively avoided, remains one of the main gaps in our understanding of parasite-host coevolution. Generalist brood parasites, like Common Cuckoos Cuculus canorus, target many host species. Our studies shed light on the puzzle why some species are not utilized by cuckoo parasites as hosts.

They may use visual cues in chick recognition, although they accept sister species Tree Sparrow (Passer montanus).Ĭonclusions: By rejecting nestlings of foreign species, Russet Sparrows have succeeded to escape from the brood parasitism by cuckoos and other parasites. Results: The present study showed that Russet Sparrows have no egg recognition ability, but recognize their own nestlings and eject alien chicks or starve them to death. To solve such puzzle, we conducted artificial parasitism and cross-fostering experiments in Russet Sparrow (Passer cinnamomeus). Methods: Sparrows Ploceidae are widespread throughout the Old World, and they have a suitable diet for rearing cuckoos, but still they are rarely exploited by brood parasites. Egg rejection is an efficient and common defense against parasitism, although some apparently suitable hosts do not reject cuckoo eggs. Background: In coevolutionary interactions between brood parasites and their hosts, host parents are under strong selection to evolve defenses against parasitism.

Sparrow eggs