Population expansion of the black-winged kite Elanus caeruleus has been a global trend in recent decades. Since first recorded in Taiwan in 2001, they rapidly occupied plains areas of this subtropical island which has led to interspecific competition with the wintering Eurasian kestrel Falco tinnunculus. In this study, we analyzed historical surveys to estimate the population trends of both species in Taiwan. We then used Pianka's index to quantify the degree of niche overlap based on diet composition and foraging peaks. Finally, we designed an experiment to test the aggressiveness of the two raptors when facing conspecific or heterospecific intruders. Airport avifauna survey (2002–2017) data from eight different airports and eBird data (2000–2019) from 21 hotspot grids both showed population turnover between the two raptors, with a decline in the wintering Eurasian kestrel and an expansion of the black-winged kite. Rodents were the largest share of prey for both species, indicating that the two raptors have a high degree of niche overlap (an overlap index of 0.74 by prey frequency, 0.97 by prey biomass and 0.89 by foraging time). The black-winged kite exhibited higher boldness and aggressiveness toward intruders, with its aggressiveness toward kestrel specimens (2.9 ± 2.4 strikes) significantly higher than that toward kite specimens (1.1 ± 1.8 strikes). Attacks by the black-winged kite may have prompted the Eurasian kestrel to choose other wintering grounds. As a result, the number of wintering kestrels decreased, leading to opposing population trends of the two species.
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