Woodpeckers and Their Habitat Use in the Columbia River Basin Region:
A Literature Review
Lisa J. Bate, Research Wildlife Biologist
389 LaBrant Road, Kalispell, MT 59901
This document was produced through a contract with the Interior Columbia Basin Ecosystem Management Science Team. The document was never published as part of the GTR series, but the information and literature sources may be helpful to land managers who are assessing habitat for woodpecker species.
This document was produced through a contract with the Interior Columbia Basin Ecosystem Management Science Team. The document was never published as part of the GTR series, but the information and literature sources may be helpful to land managers who are assessing habitat for woodpecker species. 1
SECTION I 5
Nesting and Roosting Requirements 5
Foraging Requirements 11
Long-Term And Year-Round Studies 14
SECTION II 16
Black-backed Woodpecker 16
Downy Woodpecker 21
Hairy Woodpecker 24
Northern Flicker 27
Lewis’ Woodpecker 30
Pileated Woodpecker 33
Red-naped Sapsucker 36
General Information 36
Three-toed Woodpecker 39
White-headed Woodpecker 44
Williamson’s Sapsucker 49
REFERENCES CITED 53
Considerable resources have been used in an effort to describe woodpecker habitat selection and use within the Columbia River Basin (CRB). The primary objective of this document was to summarize the existing literature on woodpeckers and their habitat needs. Secondary objectives include: 1) identifying the gaps in current scientific knowledge about cavity-nesters and their habitat needs in the CRB region; and 2) make recommendations for future research.
I summarized the existing information on cavity-nesters and their habitat needs in the CRB, focusing primarily on woodpeckers. Initially, the focus of this document was only on studies conducted within the CRB. This then expanded geographically to include any studies that could be found on some of the lesser-known species or that had possible implications for cavity-nesters within the CRB.
In addition to this formal literature review, I contacted numerous wildlife biologists (n 100) from Forest Service district and Bureau of Land Management offices throughout the CRB to determine whether any unpublished or graduate studies had been conducted in their area concerning woodpeckers and their habitat. Finally, I used CAVNET, an Internet resource, in an attempt to collect additional information on some of the lesser-known species from biologists throughout North America.
Woodpeckers are an integral part of any forest. As insectivores, woodpeckers have been instrumental in maintaining endemic levels of forest insect pests or by controlling insect outbreaks once they occur (Koplin 1969, Beebe 1974, Otvos 1979, Thomas et al. 1979, Machmer and Steeger 1993, Steeger and Machmer 1997). As primary cavity-nesters, woodpeckers are considered “indicator” species because they excavate nest and roost sites for themselves and a multitude of other species dependent on, or associated with, these cavities for nesting. It had been assumed that if woodpeckers were present and abundant in a forest, then this indicated that the habitat was also of sufficient quality to support most secondary cavity-nesting species (Thomas et al. 1979, Nietro et al 1985). Indeed, in the absence of viable woodpecker populations the future of secondary cavity-nesters, or non-excavators, becomes uncertain.
Aside from the ecological importance of maintaining adequate cavity-nesting bird populations on Forest Service lands, managers are required by law to maintain viable populations of all native species on federal lands (Forest and Rangeland Renewable Resources Planning Act 1974). In 1976, the National Forest Management Act was passed as an amendment to the RPA. This second law requires the preparation of forest plans that include periodic evaluation and monitoring of management objectives, standards, and guidelines for National Forest Service lands and its resources (U.S. Laws, National Forest Management Act 1976). Consequently, woodpecker-habitat models were devised (Thomas et al. 1979, Nietro et al. 1985) to predict woodpecker densities in both unmanaged and managed forests to provide an information source for land managers charged to meet this objective. These models were based on the best information available at the time.
We now know, however, that these habitat models do not accurately reflect suitable habitat for all cavity-nesters; their simplicity does not account for the array of habitat requirements of all cavity-nesting species. Land managers have begun to move away from using these models and are relying more on historic conditions, and new information on the nesting, roosting, and foraging needs of cavity-nesters, to help manage these species.
Not all snags or trees are suitable as nesting or roosting substrates for cavity-nesters. Therefore, the condition or quality of nest and roost trees should always be considered. The quality of a tree or snag for nesting or roosting is a function of its species, size, structural class, and location within a stand and forest. As a result, many biologists suggest that both quantitative and qualitative factors should be considered in any cavity-nester management program (Conner 1979, Brawn et al. 1982, Bull et al. 1997).
For example in northeastern Oregon, Bull et al. (1992) observed that “nest” trees differed significantly from “roost” trees for Pileated Woodpeckers (Dryocopus pileatus); whereas Pileated Woodpecker nest trees were solid, their roost trees were hollow. Live, hollow trees on our landscapes are uncommon and their retention is critical because of the high wildlife value they provide for a number of species (Bull et al. 1997). Vaux’s Swifts (Chaetura vauxi), deemed secondary cavity-nesters, are considered hollow tree obligates. They require hollow trees for both their nesting and roosting needs. For these two species, hollow trees should be a factor in their habitat modeling.
Recent studies suggest that current snag density guidelines may be too low to adequately sustain woodpecker populations (Bull et al. 1997). Studies have found that foraging substrates often differ from nesting and roosting substrates (Franzeb 1977, Bull and Holthausen 1993, Bate 1995, Dixon 1995) with many species exhibiting seasonal shifts in their foraging behaviors (Kilham 1965, Stallcup 1969, Ligon 1973, Morrison et al. 1985, Morrison and With 1987). Caton (1996) observed that cavity-nesting bird communities in her burned study areas were determined more by foraging than nesting opportunities. The foraging needs of species, therefore, must be taken into account to address land management for these species.
Black-backed Woodpeckers (Picoides arcticus) forage primarily on wood-boring beetles (Dixon and Saab 2000), which only invade dying or recently dead trees. Therefore, providing habitats that have high densities of dying or recently dead trees—burns, floods, wind throws, old-growth forests—is critical for the survival of this species. Simply providing for the nesting needs of this species is not enough (Caton 1996, Goggans et al. 1988).
We also now realize that we cannot assume that the needs of secondary cavity-nesters are met by providing only snags of certain sizes for primary cavity-nesters. For example, Li and Martin (1991) found secondary cavity-nesters had lower rates of nesting success than primary cavity-nesters in the same area. Northern Flickers (Colaptes auratus), a primary cavity-nester, will nest in trees as small as 30 cm dbh. Some of the secondary cavity-nesters that rely on flicker holes, however, are more restricted selecting larger (> 50 cm dbh) trees for their nest sites (Scott et al. 1980, Hitchcox 1996). Finally, certain secondary cavity-nesters, such as the Brown Creepers (Certhia americana), require snags or trees large enough to have sloughing bark, because they nest under the loose bark, rather than nesting in cavities (Davis 1978).
Most woodpeckers have large home ranges and the structure of the forests they occupy is also important (McClelland 1979, Harris 1982, Saab et al. 2000). Their habitat, therefore, should be evaluated on both a local (microhabitat) and landscape (macrohabitat) scale (Saab et al. 2000). Most forests in the CRB have undergone some amount of timber harvesting resulting in forest fragmentation and loss of snags and large trees (Dickson et al. 1983, Ffolliot 1983, Styskel 1983, Titus 1983, Hope and McComb 1994, Hann et al. 1997). How many woodpecker pairs a landscape can support is partially determined by the degree of forest fragmentation. For example, Dixon (1995) suggested resource specialists consider 140 ha as an average home range size for each pair of White-headed Woodpeckers in contiguous forests comprised of >69% old-growth ponderosa pine. This home range size increases to 288 ha, however, in fragmented ponderosa pine forests containing a minimum of 26% old-growth.
It is also important to understand that while some woodpeckers exhibit site tenacity throughout a year (Bull and Holthausen 1993, Dixon 1995), other woodpeckers tend to wander in search of better foraging resources in the nonbreeding seasons (Jackman 1974, Dixon and Saab 2000). For example, black-backed and Three-toed Woodpeckers are known to migrate great distances (>100 km) in search of optimal foraging resources (West and Spiers 1959, Yunick 1985). Kreisel (1998) found Downy Woodpeckers foraging in burned areas (> 0.8 km away from any deciduous forests) in northeastern Washington during four consecutive winters. The Downy Woodpeckers did not, however, remain within the burned stands throughout the breeding season. This information emphasizes the need to assess woodpecker habitat requirements both spatially and temporally.
Mikusinski and Angelstam (1994) found the greatest diversity of boreal and hemiboreal woodpeckers in areas where woodpeckers can find an abundance of dead wood and large trees to provide for their nesting, roosting, and foraging requirements; essentially, in areas where little or no timber harvests have occurred. The difficulty facing resource specialists, however, is that these habitat elements, dead wood and large trees, are not compatible with intensive forest management. “Timber stand improvement”, or thinning by removal of diseased and dying trees (Stribling 1990), often results in the removal of the very habitat components required by cavity-nesting birds.
The remainder of this paper is organized in two sections. The first section discusses the nesting, roosting, and foraging habitat requirements of cavity-nesters as a group. Also included are short discussions on the importance of both year-round and long-term studies. Burned habitats are treated separately because of the unique habitat that they provide in regards to cavity-nesters (Bate 2001). The second section includes a literature review for each woodpecker and sapsucker species found within the CRB: Black-backed, Downy, Hairy, Lewis’, Pileated, Three-toed, and White-headed Woodpeckers, the Northern Flicker, and the Red-naped and Williamson’s Sapsuckers.