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Williamson's Sapsucker ( Sphyrapicus thyroideus (Cassin, 1852)) breeds primarily in managed forests in Canada. Its status as endangered will require reliable information to develop habitat management guidelines. Adults provision nestlings with ants gleaned from tree trunks, so we estimated the preferred nestling diet by comparing nestling fecal sac composition with the ant fauna collected in tree trunk traps. For ants, we determined nest densities and substrates by counts along transects within sapsucker nest areas. Ants made up 98% of >10 000 arthropods identified in Williamson's Sapsucker fecal sacs. Of 11 ant taxa available, Williamson's Sapsucker consumed six, of which two large-bodied ant taxa, carpenter ants of Camponotus subgenus Camponotus and ants of the Formica rufa species group, were preferred. Together these two taxa averaged 80% of the nestling diet by biomass and formed >48% of the nestling diet at every nest examined ( n = 89). Preference varied regionally: west of the Okanagan Valley, only the two large-bodied ant taxa already mentioned were preferred, in the Okanagan-Boundary and East Kootenay regions, smaller bodied ants of the genus Lasius were also preferred, although these comprised ≤5% of the nestling diet by biomass. Camponotus ( Camponotus) spp. nests were 3.3 times more common in wood than soil, and the F. rufa species group were entirely dependent on dead or decaying wood for nest substrate. Given the dependence of Williamson's Sapsucker on these ants, retention and recruitment of dead and decaying wood should be considered a management priority for Williamson's Sapsucker breeding habitat. [ABSTRACT FROM AUTHOR]