Trophic niche partitioning between two Rock Nuthatches (Sitta tephronota&Sitta neumayer) in a contact zone in Iran

AbstractTwo sympatric nuthatch species,Sitta tephronotaandS. neumayer, provided the exemplar case of character displacement when the term was coined. The two species are quite similar morphologically but have diverged from one another in sympatry, presumably because of competition‐driven selection favoring the partitioning of food resources. However, actual diet partitioning has never been shown. Here, we report the dietary composition of the two species by analyzing items recovered from the gastrointestinal tract. We found thatS. tephronatafed predominantly on Coleopterans, Hemipterans and Orthopterans with the most important seed plants taken beingAmygdalus lycioides, Ficus caricaand Fabaceae, whereas,S. neumayerfed mainly on Coleopterans, Hemipterans and Lepidopteran, withA. lycioides, Fabaceae andTritium aestivumas the most important plants taken. To determine whether the two species partition prey into different sizes, we examined two mostly abundant insect preys (Hemipterans and Coleopterans) and measured head length for Hemipterans and mandible size for Coleopterans as proxies for prey body size. We found a significant relationship between bird body size and prey size, asS. tephronata, which is larger, captured larger bugs and beetles. We also measured the ability of each species to crack seeds of different hardness, finding thatS. tephronatatakes larger and harder seeds thanS. neumayer. Analyses of trophic niche breadth and overlap showed (1) that trophic niche breadth forS. neumayerwas wider than that forS. tephronata;(2) overall trophic niche overlap was considerably high but (3) overlap in the size of the most frequently consumed insects was low, suggesting that the species may be partitioning resources by prey size. We conclude that the relative body size of consumed prey is an important axis of niche partitioning and may have driven character displacement in associated trophic traits, allowing the stable coexistence of the two species.