Evening Grosbeak (Coccothraustes vespertinus)
Arguably, few birds are more tied to moths than the Evening Grosbeak. In the summer breeding season, Evening Grosbeaks feed primarily on the caterpillars of the spruce budworm, a small moth species that can be a pest of evergreens.
The grosbeaks have semi-nomadic populations that follow budworm outbreaks, with large concentrations of birds occurring in outbreak areas while the caterpillars are present, and moving on to other areas once the outbreak subsides.
Historically, Evening Grosbeaks were not found in eastern North America; their spread east is often attributed to increasing budworm outbreaks on that side of the continent. Their peak population levels in the east coincide with peak outbreak levels, in the 1970s and 1980s.
Since then, the forestry industry has worked to control the damaging spruce budworm outbreaks, and though no official studies have been completed to confirm cause-and-effect, grosbeak numbers have declined in lockstep.
Western populations of Eastern Grosbeak have also been on the decline, possibly due to budworm control by the forestry industry in the Rockies and western boreal forest. Both the spruce budworm and the Evening Grosbeak populations appear to have now stabilized, though at much lower levels than during the 70s.
photo by Daniel Arndt (Dan Arndt) on Flickr
(via: Peterson Field Guides)