Ecological and social factors affecting the occurrence of kleptoparasitism in two recently established sympatric breeding falcons

Co-occurrence of ecologically similar species can lead to direct agonistic interactions, including kleptoparasitism, where one individual consumes trophic resources acquired by another. We documented facultative kleptoparasitism in two similarly-sized raptors, the lesser kestrel (Falco naumanni) and the red-footed falcon (Falco vespertinus). These two species currently co-occur in Northern Italy due to recent range shifts influenced by climate and land-use changes. Multi-year focal observations revealed that single or multiple red-footed falcons were associated with 72% of foraging groups of lesser kestrels. Red-footed falcons initiated kleptoparasitic attacks on lesser kestrels in 46% of foraging group observations, with a success rate of 34%. Attacks were more likely when the prey capture rate (i.e. a proxy of foraging efficiency) of lesser kestrels was high. Red-footed falcons were more successful in stealing prey when the food items carried by lesser kestrels were larger, and kleptoparasitic attacks by groups of red-footed falcons had a higher success rate than attacks by singletons. Overall, we propose that such frequent kleptoparasitic events, which have never been previously documented in these two species, may have emerged as a consequence of their recently established co-occurrence. Kleptoparasitism could reduce the foraging efficiency and fitness of lesser kestrels, potentially leading to broader ecological consequences, such as population declines or range shifts. These findings highlight how species redistributions associated with global changes may lead to novel interspecific interactions with unforeseen ecological implications.