Effects of interspecific coexistence on laying date and clutch size in two closely related species of hole‐nesting birds
2018
Co-existence 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
variation in co-existence and its consequences for competition remain poorly
understood. 2.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. 3.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. 4.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.
5.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. This article is protected by copyright.
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