Reciprocal territorial responses of parapatric African sunbirds: species-level asymmetry and intraspecific geographic variation

Territorial competition regularly occurs between ecologically similar species with substantial divergence in territorial signals. Strong responses to heterospecific signals can result either from the conservation of broad response functions or from species discrimination with competitor recognition. The African sunbird sibling species Nectarinia moreaui and Nectarinia fuelleborni share similar niches and an extremely narrow parapatric boundary. These species sing strikingly different songs, yet have only subtly different morphologies. Familiarity with sibling species is only possible in the small contact zone where their ranges abut. A series of simulated territorial intrusion experiments reveal asymmetry in response frequency to heterospecific songs: N. moreaui respond less frequently to N. fuelleborni than to conspecific songs, whereas N. fuelleborni respond with an approximately equal frequency to both N. moreaui and conspecific songs. However, there is strong geographic variation in heterospecific song response among N. fuelleborni populations, with one allopatric population exhibiting stronger responses to conspecifics than to N. moreaui. These results indicate that N. fuelleborni populations can retain broad response functions absent contact with N. moreaui, such that competitor recognition is not necessary to explain strong territorial responses to N. moreaui. Strong geographic variation within N. fuelleborni, however, implies that spatial replication of allopatric/sympatric treatments should benefit future territorial response experiments. Lastly, a multimodal experiment in the contact zone area reveals that, following initial approaches to song signals, response differences are not greater to conspecific signals. This result suggests that associative learning may result in high heterospecific aggression via competitor recognition, even in N. moreaui, where syntopy occurs.