Cool Spring Causes Collapse in Bird Breeding

Some songbirds breeding off the east coast of Louise Island hatched fewer young in the cool spring of 1999.

The May 1999 air temperatures in this part of British Columbia's Queen Charlotte Islands were the lowest in two decades, 20 percent lower than average.

The frigid spell was instigated by impacts of the previous year's La Niña on sea surface temperatures, and corresponded to the Southern Oscillation Index.

Most affected were birds feeding off insects from plant foliage. Orange-crowned warblers suffered greatest, with their young hatching nearly four weeks later than the previous year. They also produced relatively few nestlings in 1999. Hatchlings typically comprised over half the birds studied during breeding, but amounted to only 8 percent of the orange-crowned warbler population in 1999.

Townsend's warbler and golden-crowned kinglets also delayed breeding and produced substantially fewer young in the chilly 1999 weather. Although hermit thrush and fox sparrows delayed breeding as well that year, they still managed to hatch as many nestlings as in warmer years.

During the four-year study, the timing of breeding followed May air temperatures for both migrant and resident birds. Cooler air temperatures probably affected the insectivorous birds most by reducing their food supply.