Urban systems are known to have a number of effects on avian richness, density, and morphological and behavioral traits. However, no study to date has simultaneously examined the wide range of urban variables in relation to the avian dawn chorus, a complex behavioral phenomenon. Previous studies investigating adjustments of the dawn chorus onset in urban settings have mainly been confined to relationships with noise and light levels. In addition to noise and light levels, in this study we included other potentially related environmental characteristics describing vegetation structure, urban infrastructure, and human activity, all of which have been shown to be drivers of bird diversity in urban areas. We conducted dawn chorus surveys at 38 Los Angeles urban greenspaces and used a classification and regression tree analysis to identify specific urban scenarios that best explained timing differences in the dawn chorus onset. Our results show that light level was the most important variable related to the dawn chorus onset time, in which, counter-intuitively, bird communities in greenspaces with higher light levels had later onsets. In addition, noise was an important factor for the chorus onset in greenspaces with higher light levels. Although our results differ from those of previous studies, these findings highlight the importance of noise and light levels in explaining dawn chorus onset variation, indicating the need for further research in untangling this complex and ecologically important phenomenon.

The House Sparrow Passer domesticus, closely associated with human presence along urban-agricultural landscapes and widely distributed species 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 immatures), 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 2 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.