Background
The authors write that "the timing of annual breeding is a crucial determinant of reproductive success, individual fitness, and population performance, particularly in insectivorous passerine birds," because "by synchronizing hatching with the narrow time window of maximal food abundance, parents can enhance their reproductive success through an increase in offspring growth rate and body condition, survival to fledging, and subsequent recruitment into the breeding population." And many people worry, in this regard, that global warming may upset such biological synchronizations, leading to downward trends in the populations of many species of birds and other animals.

What was done
Thomas et al. state that they used "confirmatory path analysis and data on laying date" for two populations of Blue Tits in northern Corsica (Muro and Pirio) in order to determine "how laying date is related to spring temperatures and vegetation phenology" -- which two factors both figure highly in determining the peak period of Blue Tit food abundance (in this case caterpillars) -- in order to provide "critical information on how passerine birds may adjust breeding in the face of directional climate change [such as regional warming] by identifying the causal paths that link laying date and environmental cues."

What was learned
The French and Canadian researchers discovered that "Blue Tits use a cue system that is context specific to fine-tune laying dates to match local conditions both on a spatial (habitat) scale and on a temporal (interannual) scale," and that their "reliance on both temperature and phenology when breeding late in the season, as occurs in most populations where tits have been intensively studied north of the Mediterranean region, satisfactorily explains how these populations can advance breeding in response to rising spring temperatures while maintaining a relatively large variation in the onset of breeding on a local spatial scale."

What it means
Thomas et al. acknowledge that "if a single environmental feature [such as temperature] were responsible for the timing of breeding, climate change could cause a severe decline in breeding success, with negative demographic consequences." However, they say they "have not detected any consistent mismatch between Blue Tit breeding dates and caterpillar peak [abundance] dates over the 14 and 21 years for which they have data for Muro and Pirio [two sites in northern Corsica], respectively." Hence, they conclude that their analyses "offer some hope that breeding populations will respond well to global warming."