Traits and ecological space availability predict avian densities at the country scale of the Czech Republic

Species geographical distributions and abundances are a central focus of
current ecological research. Although multiple studies have been conducted
on their elucidation, some important information are still missing. One of
them is the knowledge of ecological traits of species responsible for the
population density variations across geographical (i.e. total physical
area) and ecological spaces (i.e. suitable habitat area). This is crucial
for understanding how ecological specialisation shapes the geographical
distribution of species, and provides key knowledge about the sensitivity
of species to current environmental challenges. Here, we precisely
describe habitat availability for individual species using fine-scale
field data collected across the entire Czech Republic. In the next step,
we used this information to test the relationships between bird traits and
country-scale estimates of population densities assessed in both
geographical and ecological space. We did not find any effect of habitat
specialisation on avian density in geographical space. But when we
recalculated densities for ecological space available, we found a positive
correlation with habitat specialization. Specialists occur at higher
densities in suitable habitats. Moreover, birds with arboreal and
hole-nesting strategies showed higher densities in both geographical and
ecological spaces. However, we found no significant effects of
morphological (body mass, structural body size) and reproductive (position
along the slow-fast life-history continuum) traits on avian densities in
either geographical or ecological space. Our findings suggest that
ecological space availability is a strong determinant of avian abundance
and highlight the importance of precise knowledge of species-specific
habitat requirements. Revival of this classical but challenging ecological
topic of habitat-specific densities is needed for both proper
understanding of pure ecological issues and practical steps in the
conservation of nature.