Forest Investment Account (FIA) - Forest Science Program
FIA Project Y091172

    Dietary dependence of Williamsonís Sapsucker on coarse woody debris-associated ants
Project lead: Meggs, Jeff (Streamline Environmental Consulting Ltd.)
Contributing Authors: Meggs, Jeffrey M.; Higgins, R.; Todd, Melissa A.; Gyug, Les W.; Lindgren, B. Staffan; Fort, K.
Subject: Forest Investment Account (FIA), British Columbia
Series: Forest Investment Account (FIA) - Forest Science Program
Williamson's Sapsucker (WISA), Sphyrapicus thyroideus, is a medium-sized cavity-nesting woodpecker that has been assessed as Endangered by the Committee on the Status of Endangered Wildlife in Canada and has been placed on Schedule 1 of the Species at Risk Act. Recent research to inform recovery planning has focused on characterizing the primary breeding requirements of WISA within its breeding range in southern British Columbia: trees suitable for nest cavity excavation, sap trees, and invertebrate prey.

Of all North American woodpeckers, WISA is considered to have the highest dependence on ants during the breeding season (Beal 1911). Ants are the primary food fed to the nestlings, and are a major adult breeding season food source with carpenter ants (Camponotus spp.) believed to form a large component of the diet (Beal 1911, Crockett 1975, Stanford & Knowlton 1952, Otvos & Stark 1985). These ants are gleaned primarily from the trunks of live trees (Stallcup 1968, Crockett 1975). Gyug et al. (unpubl. manuscript) in a study of the relationship of the abundance of foraging resources to WISA nest productivity, used Bayesian Belief Network (BBN) modeling to relate ant and sap tree availability in the 16 ha area around active nests to WISA breeding productivity. They found that nest productivity was little affected by sap tree availability but was affected by total ant availability (measured as ant-nest density). In turn they found that ant-nest density was correlated with relatively high coarse woody debris (CWD) volumes. On this basis they concluded that CWD volumes could be used as an indicator of WISA breeding habitat quality and recommended specific CWD retention targets for the maintenance of high quality habitat. These recommendations are under review by the WISA Action Plan team as part of a process to develop stand management objectives for the species.

This indirect linkage of WISA nest productivity and CWD volumes is held together by the untested assumption of a high-level of dietary dependence of WISA on CWD-associated ants. Until this assumption is tested, the effectiveness of stand-level CWD retention targets for the maintenance of high quality WISA breeding habitat is open to question.

WISA clean out fecal sacs that contain invertebrate fragments from the nest, and usually drop them from the entrance of the nest hole. This allows for fairly simple collection of samples reflective of what the young have been fed. Fecal sac analysis for invertebrate composition has been used to determine the diet of Dippers (Ormerod 1985), and scat analysis has been used to determine the relative diet composition of Pileated Woodpeckers (Beckwith & Bull 1985; Raley & Aubry 2006). When coupled with data on relative prey availability, fecal sac analysis can be used to examine patterns in dietary selection. For example, Omerod (1985) was able to develop prey electivity indices for Dippers by comparing the invertebrate composition of nestling fecal sac and adult feces to the macro-invertebrates present in foraging streams. As part of their study of WISA foraging habitat quality, Gyug et al. (unpubl. manuscript) collected 54 nestling fecal sac samples from below 31 nests in 2006, and a further 85 fecal sacs from 62 nests in 2007, including some repeat samples of nests from the previous year (L. Gyug unpubl. data). A cursory examination of the fecal sacs collected indicated that they are mainly comprised of ant fragments, including intact heads (L. Gyug pers. obs.). The elytra of metallic wood-boring beetles (Buprestidae) were also observed in some fecal sacs.

We propose to examine the extent of dietary dependence of WISA on CWD-associated ants through a study of ant availability and use within WISA nest areas. A stratified sampling design will be used that stratifies WISA nest areas by WISA population (Western, Okanagan-Boundary or East Kootenay) and WISA nest productivity (high or low). Ant abundance and availability will be assessed using two methods: trunk trapping to directly sample ants on live trees; and ant-nest surveys as per Gyug et al. (unpubl. manuscript), but expanded to include soil-and rock-associated ant nests. WISA prey-selection will be examined by an analysis of ant species abundance and diversity in WISA nestling fecal sacs collected in 2006 and 2007 (L. Gyug unpubl. data). An ant reference collection to support fecal sac analysis will be compiled from specimens collected in the ant-nest surveys and trunk traps.
Related projects:  FSP_Y102172


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Updated August 16, 2010 

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