Vesper sparrow (Pooecetes gramineus affinis) COSEWIC assessment and status report: chapter 6

Biology

General

Vesper Sparrows are ground-nesting birds of sparsely vegetated, grassland habitats. Very little is known about the breeding ecology of the Coastal Vesper Sparrow in the Georgia Depression; therefore most of the following information is inferred from data available for other regions or other subspecies of Vesper Sparrow.

Reproduction

Vesper Sparrows are seasonally monogamous (Jones and Cornely 2002). Males typically arrive on the breeding grounds first with females following shortly after (Best and Rodenhouse 1984). The female alone builds the nest. Nests are usually on level ground or in a slight depression. Nests are usually placed beside a tuft of vegetation to help conceal the location from potential predators (Jones and Cornely 2002) as well as to help maintain optimal microclimate in the nest (Nelson and Martin 1999).

Clutches contain three to six eggs and are incubated for 11 to 14 days, primarily by the female. Both sexes feed the nestlings. Nestlings fledge after approximately 10 days and are dependent on the adults for another 20 to 30 days (Baicich and Harrison 1997). Vesper Sparrows may raise a second brood in a single breeding season. If the first brood has successfully fledged, the male typically cares for those young while the female tends the second nest (Jones and Cornely 2002).

In coastal British Columbia, the breeding season is estimated to extend from late April to mid-July (Beauchesne 2002a, 2003, 2004). If a second brood is reared, the breeding season could extend into early August (Campbell et al. 2001). No information exists on the timing of dispersal from breeding sites (Campbell et al. 2001; Jones and Cornely 2002).

In the interior, the Vesper Sparrow is a fairly common host of the Brown-headed Cowbird, Molothrus ater (Friedman 1963; Dechant et al. 2001). There are no records of parasitism for the Coastal Vesper Sparrow in Canada (Campbell et al. 2001).

Survival

In the Georgia Depression, potential predators of eggs, young birds and adults include birds of prey (e.g., Cooper’s Hawk, Accipiter cooperii and Merlin, Falco columbariu), corvids (e.g., Northwestern Crow, Corvus caurinus and Common Raven, C. corax), small and medium-sized mammals (e.g., coyotes, Canis latrans, foxes, Vulpes vulpes, raccoons, Procyon lotor, skunks Mephitis mephitis, domestic dogs, Canis familiaris), and snakes (e.g., Common Garter Snake, Thamnophis sirtalis and Western Terrestrial Garter Snake T. elegans). Domestic cats, Felis catus, however, probably represent the greatest predation threat on Vancouver Island. Cats are known to be competent predators of small to medium-sized birds (George 1974; Cooper 1993; Coleman and Temple 1993; Coleman et al. undated; N. Dawe pers. comm.). On southeastern Vancouver Island domestic cats were frequently observed on the Nanaimo Airport grounds and at all other sites with suitable habitat, a result of close proximity to human habitation (Beauchesne 2002a).

Nests may also be destroyed by trampling, particularly in areas that are heavily grazed by livestock, or in high-traffic areas for people and domestic dogs (Rogers 2000). Mowing and other mechanical agricultural practices, however, probably represent the greatest hazard for this ground-nesting species (Rodenhouse et al. 1993). Mowing or mechanical harvesting of fields during the incubation and nestling period destroys most nests (Jones and Cornely 2002). Kershner and Bolinger (1996) evaluated the productivity of airport grassland habitats in the eastern United States and found many to be population sinks for grassland bird species, a situation they attributed to mowing practices.

Movements/dispersal

Vesper Sparrows are partial migrants. The northernmost breeding populations move south to winter in areas occupied by other populations during the summer (Jones and Cornely 2002). Birds from Vancouver Island probably winter in California. Breeding birds begin to arrive on Vancouver Island in early April and most depart in the fall by mid-October (Fraser et al. 1999; Campbell et al. 2001).

Migration is probably opportunistic, timed with changes in vegetation. Vesper Sparrows primarily migrate at night and move in small groups (Jones and Cornely 2002).

Banding studies have shown that breeding site fidelity is strong for adults, with an average return rate of approximately 50% (Best and Rodenhouse 1984). The repeated use of a single site on Vancouver Island also suggests that birds are returning to the same breeding territories (Beauchesne 2002b.).

Nutrition and interspecific interactions

The Vesper Sparrow’s diet consists of insects and the seeds of native and introduced grasses and forbs. During the breeding season, insects, particularly grasshoppers, were found to form the bulk of the diet in research conducted in south eastern Washington, Montana, and North Dakota (Adams et al. 1994; Jones and Cornely 2002).

This ground-dwelling sparrow primarily forages in low vegetation while walking or hopping. It will also hop and hover to glean insects from higher vegetation (Jones and Cornely 2002). On Vancouver Island, Vesper Sparrows were observed gleaning insects from low forbs and eating dandelion (Taraxacum officinale) seeds. Adults were observed carrying insects, presumably to feed to nestlings (Beauchesne 2002a).

Behaviour/adaptability

In British Columbia, Vesper Sparrow have adapted to sites that have been modified by humans. Other researchers, however, have suggested that Vesper Sparrows avoid areas with high human population concentrations (Bock et al. 1999). They also avoid areas with intensive agricultural practices (e.g., hayfields: Campbell et al. 2001; Beauchesne 2002b).

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