Geothlypis tolmiei

Breeding habitat consists of dense shrubby areas, such as coniferous forest undergrowth and edge, brushy hillsides, riparian thickets, and chaparral (AOU 1998), often in cut-over or burned areas. Nests are generally low, 0.6-1.5 meters above ground, in bushes, saplings, clump of ferns, etc. (Terres 1980).

In Centennial Mountains, Idaho, this species is associated with xeric willow (Salix sp.) communities with mixed grasses and forbs, willow-conifer habitats, and willow riparian communities (Douglas et al. 1992). In montane habitats of Colorado, it is associated with riparian aspen-willow and spruce aspen communities (Winternitz 1976). In Montana, it bred in a cottonwood (Populus trichocarpa)/Ponderosa Pine (Pinus ponderosa) riparian area with 58 percent ground cover, 61 percent bush cover, 31 percent mid-canopy cover, 36 percent bush volume, 23 percent canopy volume, 4 trees less than 10 centimeters dbh per 10 meter radius plot, and 6 shrubs per 10 meter radius plot (Mosconi and Hutto 1982). On National Forests in northern Idaho and western Montana, it was most abundant in clearcuts and riparian shrubland; followed by seed tree and shelterwood harvest units; marsh/wetland, cottonwood/aspen and spruce-fir habitats; and group-select harvest units (Hutto 1995).

In southeastern Wyoming, this warbler is narrowly associated with mid-elevation (2,290 - 2,530 meters) shrub willow and thin-leaf alder (Alnus tenuifolia) riparian communities. It prefers habitats with few trees (less than 10 trees per 100 square meters); dense shrub cover (mean distance to nearest shrub less than 2.0 meters); high percentage of willow species (more than 70 percent of shrubs); high foliage density of small shrubs (0.3-1 meter height interval); moist sites (more than 5.5 percent water cover); and dense grass cover (more than 65 percent cover; Finch 1989a, 1989b).

In California, breeding habitat includes seedling valley-foothill riparian, seedling and dense canopy (60-100 percent) sapling montane riparian, and seedling and dense canopy (60-100 percent) sapling aspen. Shrubs are essential for breeding, and presence of streams, bogs, wetlands, or other aquatic habitats is preferred (Timossi 1990, cited in USDA Forest Service 1994).

In migration and winter, MacGillivray's warbler occurs in open woodland undergrowth, scrubby areas, and thickets (AOU 1998). It is moderately common in undergrowth of humid to semi-arid forest and edge, river-edge, woodland edge, dense scrubby woodland and shrublands, second growth, and gardens and parks (Edwards 1972, Howell and Webb 1995). Insectivorous passerines migrating through Arizona use riparian habitats in higher densities than adjacent non-riparian habitats; they were observed in both xeric mesquite shrubland and in riparian forest with Populus- and Salix-dominated overstory (Stevens et al. 1977).

Densities vary with habitat quality, elevation, season, and location (Pitocchelli 1995). Breeding densities recorded in western Oregon ranged from 0.56 birds per hectare (Morrison and Meslow 1983) to 0.73 males per hectare (Morrison 1981); in Utah, from 0.83 to 1.21 males per hectare, increasing with vegetation cover (Blakesley and Reese 1988); in Washington 0.26 birds per hectare (Miller et al. 1972, cited in Pitocchelli 1995); in Wyoming 0.16 to 0.33 territorial pairs per hectare (Finch 1989a). On wintering grounds in western Mexico, Hutto (1981) recorded 8.5 birds per hectare in evergreen forest edge, 5.6 birds per hectare in thornscrub, 5.0 birds per hectare in plantation, 2.8 birds per hectare in second growth, and 1 bird per hectare in mangrove. Territory size not reported. Site fidelity observed on breeding site in Oregon (Klimkiewicz and Futcher 1989, cited in Pitocchelli 1995) and on wintering sites in El Salvador (n = 4) and Mexico (n = 1; Loftin 1977).

This warbler is preyed on by accipiters, small mammals and snakes (Zeiner et al. 1990, cited in USDA Forest Service 1994). Females respond to intruders with distraction displays and feigning injury when near nest or fledglings (Jewett et al. 1953, cited in Pitocchelli 1995). On the Mogollon Rim, Arizona, a nest predation rate of 49 percent reported in snowmelt drainages of mixed pine-oak woodland (Martin 1993).

Courtship occurs in late April and early May (Bent 1953). Clutch size is three to six (usually four). Incubation, by female, lasts 11 days. Young are tended by both adults, leave nest when 8-9 days old.