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Grooming behaviour, which maintains and cleans the integument, is a widespread behaviour of birds, mammals and arthropods. The extent to which grooming is influenced by large-scale environmental differences, such as geographical and seasonal variation, remains largely unexplored. We investigated spatial and temporal variation in the grooming behaviour of a common species of bird, the American kestrel, Falco sparverius. Specifically, we compared the self-grooming behaviour (preening and scratching) of kestrels in the subtropical Bahamas to kestrels in temperate northern Utah, U.S.A., in both summer and winter. We also compared differences in parasite 'pressure' by quantifying the prevalence and abundance of ectoparasitic lice (Insecta: Phthiraptera) on kestrels in both locations. Bahama kestrels had significantly more lice than Utah kestrels, which is typical of birds in humid regions, compared to arid regions. Bahama kestrels groomed significantly more than Utah kestrels, which may constrain the time–activity budgets of Bahama kestrels. Within each region, however, birds with more lice did not groom more. This pattern is consistent with geographical differences in programmed grooming, rather than stimulus-driven grooming. Kestrels in both locations groomed significantly more in summer than in winter, presumably because feather moult occurs in summer. To our knowledge, this study provides the first demonstration of geographical variation in the grooming behaviour of any animal. Experimental studies are needed to confirm the causes and consequences of this variation. Spatial and temporal comparisons of grooming in other birds, as well as mammals and arthropods, would be informative. • American kestrels varied in grooming time, both seasonally and geographically. • Birds in the subtropics groomed more than birds in the temperate region. • Parasitic feather lice were more abundant on subtropical than temperate birds. • Birds in both regions groomed more in summer (during moult) than in winter. [ABSTRACT FROM AUTHOR]

The beaks of Darwin's finches and other birds are among the best known examples of adaptive evolution. Beak morphology is usually interpreted in relation to its critical role in feeding. However, the beak also plays an important role in preening, which is the first line of defence against harmful ectoparasites such as feather lice, fleas, bugs, flies, ticks and feather mites. Here, we show a feature of the beak specifically adapted for ectoparasite control. Experimental trimming of the tiny (1-2 mm) maxillary overhang of rock pigeons (Columba livia) had no effect on feeding efficiency, yet triggered a dramatic increase in feather lice and the feather damage they cause. The overhang functions by generating a shearing force against the tip of the lower mandible, which moves forward remarkably quickly during preening, at up to 31 timesper second. This force damages parasite exoskeletons, significantly enhancing the efficiency of preening for parasite control. Overhangs longer than the natural mean of 1.6mm break significantly more often than short overhangs. Hence, stabilizing selection will favour overhangs of intermediate length. The adaptive radiation of beak morphology should be re-assessed with both feeding and preening in mind.