Although primary cavity-nesting species are capable of excavating new cavities, they often reuse old ones. To determine potential factors driving such reuse, we studied nest-cavity reuse in the Acorn Woodpecker (Melanerpes formicivorus), a cooperatively breeding species that reuses old cavities for 57.2% of nests at Hastings Reservation in central coastal California, USA. We found no evidence for significant fitness costs or benefits of cavity reuse compared to using newly constructed cavities. In contrast, several lines of evidence supported a role for constraints on both cavity reuse and on new cavity construction. The main constraint on reuse was cavities failing to survive from one year to the next, usually because the limb fell apart, filled with water, or was usurped by another species. Evidence that constraints on new cavity construction may be important included more frequent cavity reuse when groups renested and use of artificial cavities when they were experimentally provided. Nest-cavity reuse in this population appears to be driven primarily by constraints, including the energetic costs and time required to excavate a new cavity, rather than fitness consequences, even though Acorn Woodpeckers regularly excavate small holes in trees for acorn storage and the energetic costs of new cavity construction are apparently insufficient to significantly depress reproductive success. Constraints play a significant role in cavity reuse and may affect both the intraspecific and interspecific frequency of cavity reuse among facultative excavating species.
LAY SUMMARY
We investigated the costs and benefits of cavity reuse in the cooperatively breeding Acorn Woodpecker (Melanerpes formicivorus), in which groups reuse old cavities for over half of nesting attempts.
We found no differences in the fledgling success of family groups that reused an old cavity compared to excavating a new one. Why, then, do birds reuse old cavities?
The answer appears to be constraints, both on cavity reuse and new cavity construction.
The main constraint on cavity reuse was that many cavities become unusable from one year to the next, usually because the tree limb disintegrates, fills with water, or is taken over by another species.
Birds frequently use artificial cavities when they are provided, suggesting that old cavities are reused because excavating a new one takes time and energy.
Constraints appear to play a significant role in cavity reuse and may affect both the frequency of cavity reuse among species and across populations of excavating species.
