Data from: 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, but also other hole‐nesting taxa, constitutes a classic example
of species co‐occurrence resulting in potential interference and
exploitation competition for food and for breeding and roosting sites.
However, the spatial and temporal variations in coexistence and its
consequences for competition remain poorly understood. We used an
extensive database on reproduction in nest boxes by great and blue tits
based on 87 study plots across Europe and Northern Africa during 1957–2012
for 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, and density consistent with effects of intraspecific
and interspecific competition. In an initial set of analyses, we
statistically controlled for a suite of site‐specific variables. We found
evidence for an effect of intraspecific competition on blue tit laying
date (later laying at higher density) and clutch size (smaller clutch size
at higher density), but no evidence of significant effects of
intraspecific competition in great tits, nor effects of interspecific
competition for either species. To further control for site‐specific
variation caused by a range of potentially confounding variables, we
compared means and variances in laying date and clutch size of great and
blue tits among three categories of difference in density between the two
species. We exploited the fact that means and variances are generally
positively correlated. If interspecific competition occurs, we predicted a
reduction in mean and an increase in variance in clutch size in great tit
and blue tit when density of heterospecifics is higher than the density of
conspecifics, and for intraspecific competition, this reduction would
occur when density of conspecifics is higher than the density of
heterospecifics. Such comparisons of temporal patterns of means and
variances revealed evidence, for both species, consistent with
intraspecific competition and to a smaller extent with interspecific
competition. These findings suggest that competition associated with
reproductive behaviour between blue and great tits is widespread, but also
varies across large spatial and temporal scales.