Insect prey population changes in habitats with declining vs. stable Three-toed Woodpecker Picoides tridactylus populations

Abstrakti

The effect of food supply on the Three-toed Woodpecker Picoides tridactylus breeding density and productivity was investigated by comparing the pattern of annual changes in prey abundance in habitats with declining vs. stable woodpecker populations. In a bumt area of eastern Finland, where the woodpecker breeding population has been continuously diminishing, following a major increase the first year after fire, only the abundance of spruce bark beetles (Col., Scolytidae) (the woodpecker autumnspring main insect prey) decreased through the years. Conversely, the abundance of longhorn beetles (Col., Cerambycidae) (the nestling main food supply) increased dramatically. On the other hand, in neighbouring old-growth patches where one pair was breeding annually, abundance of spruce bark and longhorn beetles did not change significantly between years, although the abundance level ofthe bark beetles differed among patches. Hence, my results suggest the woodpecker breeding population to be limited by food supply available outside the breeding season, as shown for other forest bird species. The finding, however, that woodpeckers breeding 6-7 years after the fire reared larger broods than earlier pairs, while brood size did not change annually under old-growth conditions, suggests longhorn beetle availability to be critical to reproductive success.