Measures of the spatial scale of population synchrony provide
valuable information on the role of dispersal and environmental variables in
population dynamics. In addition,
spatial patterns of population synchrony may also reveal patterns of resource tracking
and large-scale habitat specificity among sub-populations that influence
movement patterns and subpopulation structure. Numerous bird species of coniferous forests in
North America rely on the masting production
of conifer seeds that fluctuate over large spatial areas. Red crossbills rely on continent-scale
movements to locate new areas each year with abundant conifer seeds and are unique
in that they are characterized by several different morphological forms. Foraging trials in captivity have suggested
that these different types may specialize on specific conifer species and this may
influence the scale and frequency of movements.
However, the degree to which red crossbills are food specialists in the wild
and how this might influence population dynamics is unknown. Using North American Breeding Bird Survey and
Christmas Bird Count data from over 30 years, I measured habitat-specific continent-scale
patterns of population synchrony in red crossbill abundance to examine their
large-scale movements and determine whether population synchrony among regions is
influenced by habitat type. Population
synchrony is greater among sites and regions that are dominated by the same
conifer used by red crossbills. In
addition, the spatial extent and pattern of synchrony among habitat types
varies, possibly reflecting species differences in spatio-temporal cone
production among conifers that influence population movements of crossbills.