Competition and climate Great and Blue tit

We studied the relationship between temperature and the coexistence of
great tit Parus major and blue tit Cyanistes caeruleus, breeding in 75
study plots across Europe and North Africa. We expected an advance in
laying date and a reduction in clutch size during warmer springs as a
general response to climate warming and a delay in laying date and a
reduction in clutch size during warmer winters due to density-dependent
effects. As expected, as spring temperature increases laying date advances
and as winter temperature increases clutch size is reduced in both
species.  Density of great tit affected the relationship between
winter temperature and laying date in great and blue tit. Specifically, as
density of great tit increased and temperature in winter increased both
species started to reproduce later.  Density of blue tit affected
the relationship between spring temperature and blue and great tit laying
date. Thus, both species start to reproduce earlier with increasing spring
temperature as density of blue tit increases, which was not an expected
outcome, since we expected that increasing spring temperature should
advance laying date, while increasing density should delay it cancelling
each other out. Climate warming and its interaction with density affects
clutch size of great tits but not of blue tits. As predicted, great tit
clutch size is reduced more with density of blue tits as temperature in
winter increases. The relationship between spring temperature and density
on clutch size of great tits depends on whether the increase is in density
of great tit or blue tit. Therefore, an increase in temperature negatively
affected the coexistence of blue and great tits differently in both
species. Thus, blue tit clutch size was unaffected by the interaction
effect of density with temperature, while great tit clutch size was
affected in multiple ways by these interactions terms.