We used a
information-theoretic approach to investigate how habitat patterns
across three spatial extents influenced habitat selection decisions
and demographic patterns for black-throated blue warblers (Dendroica
caerulescens) at 20 study sites in west-central Vermont, USA
from 2002-2005. These
sites represented gradients of habitat patterns at different spatial
extents, including: 1) territory-level understory shrub density, 2)
patch-level understory shrub density occurring within 25 ha of
territories, and 3) landscape-level habitat patterns occurring
within 5 km radius extents of territories.
We considered multiple vital population parameters including
abundance, age ratios, pairing success, and annual fecundity.
We found that territory-level shrub cover was most important
for determining which individuals occupied which habitats but that
landscape-level habitat structure strongly influenced reproductive
output. Consistent with
long-term studies of this species in New Hampshire, sites with
higher territory-level shrub density had higher abundance, were more
likely to be occupied by older, more experienced individuals, and
males that were paired compared to sites with lower shrub density.
However, annual fecundity was higher on sites located in
contiguously forested landscape where shrub cover was low.
In addition, we found evidence that the effect of habitat
pattern at one spatial level depended on habitat conditions at
different levels. The
interaction between territory-level and landscape-level habitat
structure influenced both abundance and annual fecundity.
Abundance was highest at sites located in more fragmented
landscapes with the highest shrub densities, but females occupying
these same sites fledged fewer offspring per year.
Our results suggest that the proximate cue of territory-level
shrub density used for breeding territory selection by this species
may be decoupled from realized fitness, where individuals are unable
to recognize and occupy habitats best suited for reproduction.
We considered ecological factors associated with this
disconnect including predation, parasitism, and food limitation, and
suggest that cowbird parasitism is a leading factor contributing to
disconnect between selection cues and fitness in this system.
