Effects of interspecific coexistence on laying date and clutch size in two closely related species of hole-nesting birds

Coexistence between great tits Parus major and blue tits Cyanistes caeruleus, butalso other hole-nesting taxa, constitutes a classic example of species co-occurrenceresulting in potential interference and exploitation competition for food andfor breeding and roosting sites. However, the spatial and temporal variations incoexistence and its consequences for competition remain poorly understood.We used an extensive database on reproduction in nest boxes by great and bluetits based on 87 study plots across Europe and Northern Africa during 1957–2012for a total of 19,075 great tit and 16,729 blue tit clutches to assess correlative evidence for a relationship between laying date and clutch size, respectively, anddensity consistent with effects of intraspecific and interspecific competition.In an initial set of analyses, we statistically controlled for a suite of site-specificvariables. We found evidence for an effect of intraspecific competition on blue titlaying date (later laying at higher density) and clutch size (smaller clutch size athigher density), but no evidence of significant effects of intraspecific competitionin great tits, nor effects of interspecific competition for either species.To further control for site-specific variation caused by a range of potentially confoundingvariables, we compared means and variances in laying date and clutchsize of great and blue tits among three categories of difference in density betweenthe two species. We exploited the fact that means and variances are generallypositively correlated. If interspecific competition occurs, we predicted a reductionin mean and an increase in variance in clutch size in great tit and blue tit whendensity of heterospecifics is higher than the density of conspecifics, and for intraspecificcompetition, this reduction would occur when density of conspecificsis higher than the density of heterospecifics. Such comparisons of temporal patternsof means and variances revealed evidence, for both species, consistent withintraspecific competition and to a smaller extent with interspecific competition.These findings suggest that competition associated with reproductive behaviourbetween blue and great tits is widespread, but also varies across large spatial andtemporal scales.