DNA metabarcoding reveals broad woodpecker diets in fire-maintained forests

Ecological disturbance is a key agent shaping the spatial and temporal
landscape of food availability. In forests of western North America,
disturbance from fire can lead to resource pulses of deadwood-associated
arthropods that provide important prey for woodpeckers. Although the
foraging strategies among woodpecker species often demonstrate pronounced
differences, little is known about the ways in which woodpeckers exploit
and partition prey in disturbed areas. In this study, we employed DNA
metabarcoding to characterize and compare the arthropod diets of four
woodpecker species in Washington and California, USA – Black-backed
Woodpecker (Picoides arcticus), Hairy Woodpecker (Dryobates villosus),
Northern Flicker (Colaptes auratus), and White-headed Woodpecker (D.
albolarvatus) – primarily using nestling fecal samples from burned forests
1–13 years post-fire. Successful sequencing from 78 samples revealed the
presence of over 600 operational taxonomic units (OTUs) spanning 32
arthropod orders. The nestling diets of two species in particular –
Northern Flicker and Black-backed Woodpecker – proved to be much broader
than previous observational studies suggest. Northern Flicker nestlings
demonstrated significantly higher diet diversity compared to other focal
species, all of which displayed considerable overlap in diversity.
Wood-boring beetles, which colonize dead and dying trees after fire, were
particularly important diet items for Black-backed, Hairy, and
White-headed woodpeckers. Diet composition differed among species, and
diets showed limited differences between newer (≤5 yr) and older
(>5 yr) post-fire forests. Our results show mixed evidence for
dietary resource partitioning, with three of the four focal species
exhibiting relatively high diet overlap, perhaps due to the pulsed subsidy
of deadwood-associated arthropods in burned forests. Woodpeckers are
frequently used as management indicator species for forest health, and our
study provides one of the first applications of DNA metabarcoding to build
a more complete picture of woodpecker diets.