On the lookout for danger: House Sparrow alert distance in three cities

The House Sparrow Passer domesticus, closely associated with human presence along urban–agricultural landscapes and widely distributed outside its native range, has shown great morphological and physiological variations, with its plasticity linked to its invasiveness. Yet, there is a dearth of knowledge on the escape behavior of this sparrow along its North American invasion range. We here assessed House Sparrows alert distances in Los Angeles and Mexico City, but also in Barcelona as a control within its native distribution, considering ‘city’, ‘sex/age’ (adult males vs. adult females and juveniles), and ‘flock size’ as alert distance predictors. Through a linear model and a classification and regression tree, we identified that House Sparrow alert distances in Barcelona were significantly larger when compared to both studied North American cities (Los Angeles, Mexico City). Given that alert distances were also significantly larger in Los Angeles when contrasted with those from Mexico City, where it has been recently shown to be hyper-abundant, our results also suggest that its boldness could be related not only to an origin (native/non-native) driver, but also to a density-dependent process.