Interspecific variation in the relationship between clutch size, laying date and intensity of urbanization in four species of hole-nesting birds

The increase in size of human populations in urban and agricultural areas hasresulted in considerable habitat conversion globally. Such anthropogenic areashave specific environmental characteristics, which influence the physiology, lifehistory, and population dynamics of plants and animals. For example, the dateof bud burst is advanced in urban compared to nearby natural areas. In somebirds, breeding success is determined by synchrony between timing of breedingand peak food abundance. Pertinently, caterpillars are an important food sourcefor the nestlings of many bird species, and their abundance is influenced byenvironmental factors such as temperature and date of bud burst. Higher temperaturesand advanced date of bud burst in urban areas could advance peakcaterpillar abundance and thus affect breeding phenology of birds. In order totest whether laying date advance and clutch sizes decrease with the intensity ofurbanization, we analyzed the timing of breeding and clutch size in relation tointensity of urbanization as a measure of human impact in 199 nest box plotsacross Europe, North Africa, and the Middle East (i.e., the Western Palearctic)for four species of hole-nesters: blue tits (Cyanistes caeruleus), great tits (Parusmajor), collared flycatchers (Ficedula albicollis), and pied flycatchers (Ficedulahypoleuca). Meanwhile, we estimated the intensity of urbanization as the densityof buildings surrounding study plots measured on orthophotographs. For thefour study species, the intensity of urbanization was not correlated with layingdate. Clutch size in blue and great tits does not seem affected by the intensityof urbanization, while in collared and pied flycatchers it decreased with increasingintensity of urbanization. This is the first large-scale study showing a species-specific major correlation between intensity of urbanization and theecology of breeding. The underlying mechanisms for the relationships betweenlife history and urbanization remain to be determined. We propose that effectsof food abundance or quality, temperature, noise, pollution, or disturbance byhumans may on their own or in combination affect laying date and/or clutchsize.