Egg rejection responses of two sympatric Acrocephalus warblers under contrasting cuckoo parasitism rates

Abstract

The recognition and rejection of foreign parasitic eggs by host birds serves as effective anti-parasitism strategies, with sympatric hosts exhibiting different rates of egg rejection depending on the level of parasitic pressure. However, it remains unclear whether sympatric breeding hosts with different actual parasitism rates differ in their egg rejection responses to foreign eggs of varying colors, and whether egg color itself influences rejection behavior in these species. To address this, we conducted egg rejection experiments on the oriental reed warbler (Acrocephalus orientalis) and the black-browed reed warbler (Acrocephalus bistrigiceps), using different colored eggs (red, blue, and red eggs together with blue eggs), with the aim of determining whether there are significant differences between the two host species with respect to the recognition and rejection of eggs, to verify the hypothesis that the actual risk of common cuckoo (Cuculus canorus) parasitism will affect the egg rejection rate of sympatric breeding hosts. Results revealed that the actual cuckoo parasitism rate differed between the two sympatric breeding hosts, however, oriental reed warblers and black-browed reed warblers are characterized by relatively high rates of egg rejection. Comparisons between the two species with respect to the rejection of eggs in the three different color groups revealed no significant differences, suggesting that both oriental reed warblers and black-browed reed warblers could recognize and reject foreign eggs at high rates, irrespective of egg color. These findings may thus indicate that although the two sympatric breeding hosts are exposed to different actual rates of cuckoo parasitism, they are both exposed to the same perceived risk of parasitism and hence are likely to exhibit similar level of anti-parasitism behavior.