Many annual monitoring programs assume that availability—the probability that an animal will be visible, make an audible sound, or leave other evidence of its presence—is not changing systematically from year to year. Until recently, this assumption of unchanging availability seemed reasonable, but recent studies report changes in breeding phenology that are presumed to be linked to climate change. Because the rate of bird song is often correlated with stage of breeding, earlier breeding could shift timing of peak bird availability, changing the number of birds counted during annual surveys on set dates. Such changes could be erroneously interpreted as population trends. To better understand how changes in phenology might affect the probability of detecting birds, we modeled availabilities of 31 species in southern Alabama through the breeding season, documenting strong seasonal variation in availability. Then, using our availability estimates, we investigated whether changes in detection probabilities could underlie observed changes in the abundances of some species. We calculated the expected change in the number of times a species would be recorded during surveys conducted within fixed dates by assuming a 1-week shift in breeding activity. We found that summer residents were more likely to show changes in availability but that such changes in availability did not account for trends in Breeding Bird Survey (BBS) data for Alabama. Our results suggest that while summer residents' availability may be declining during BBS dates, population declines observed in BBS data for Alabama cannot be dismissed as due to shifting phenology.
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