McCown's longspur (Calcarius mccownii) COSEWIC assessment and status report: chapter 6

Biology

Life cycle, reproduction, and diet

McCown's Longspurs breed annually from late April through July (With 1994a). Males arrive on the breeding grounds in small flocks as early as late March and establish territories. Territory size ranges from 0.6 -1.4 ha (With 1994a) and pairs often nest near one another (Felske 1971). Socially monogamous pairs form once the females have arrived approximately two weeks later and nesting commences in late April or early May. An open cup nest is constructed by the female by digging a shallow depression in the ground. Typically, the nest is constructed of coarse grass stems and blades and is lined with soft materials such as feathers or hair (With 1994a).

Clutch size is typically three to four eggs (With 1994a). The female alone incubates the clutch for 12 days (With 1994a). Both the female and male brood the young throughout the night until they are well-feathered by Day 6. Parents will also brood during inclement weather and shelter the young from the sun. Both the male and female McCown's Longspur feed the young and studies on the congeneric Chestnut-collared Longspur suggest that biparental care is essential for successful reproduction (Lynn and Wingfield 2003). Grasshoppers comprise 60 - 90% of the nestling diet (Felske 1971, Maher 1973, With 1994a). Moth and butterfly larvae and beetles may also be included. Young leave the nest at about Day 10 when they have attained 70 -80% of their adult mass and are dependent on their parents for another three weeks post-fledging. McCown's Longspurs will initiate a second brood following a successful nesting attempt and will renest following nest failure. Individuals may remain on the breeding grounds as late as early November, but breeding is generally completed by late July or early August.

McCown's Longspurs attain adult plumage by one year of age and are thus presumed to begin breeding at this age (With 1994a). However, this has not been confirmed because first year birds do not return to their natal territories.

Adult diet during the breeding season is comprised of 60 - 90% seeds of various grasses and forbs and the remainder includes arthropods, especially grasshoppers (With 1994a).

Predation and survival

Predation on eggs and nestlings is the primary cause of reproductive failure for McCown's Longspurs (Greer and Anderson 1989). Predation rates vary from 30 - 75% of nests (With 1994a) and are highest at the nestling stage (Maher 1973). Nests initiated in the second half of the breeding season (Felske 1971) and those located in more heavily grazed pastures or near shrubs (With 1994b) suffer higher predation rates than do other nests.

Reproductive success (no. fledglings/no. eggs) varies from 32 - 46%, resulting in an average of 1.1 - 2.0 offspring fledged per nesting attempt. Females make an average of 1.3 - 2.5 nesting attempts (second broods and renesting attempts combined) per breeding season. No information exists on post-fledging survival so it is difficult to determine annual productivity from these data (With 1994a).

There is no indication that space or food availability in the breeding season limits productivity (Greer and Anderson 1989). Greer and Anderson (1989) suggested that high nest predation rates prevent McCown's Longspur populations from reaching carrying capacity and, as such, they are not affected by density-dependent factors, such as food. No data exist on adult survivorship (With 1994a).

Other known and suspected causes of mortality include clutch and brood reductions due to inclement weather (With 1994a, 1994b), exposure to agricultural pesticides (McEwen and Ells 1975), and predation on the wintering grounds (Sovada et al. 2001).

Migration, dispersal, and site fidelity

McCown's Longspurs undergo an annual migration from the breeding grounds in southern Alberta and Saskatchewan and the northwestern Great Plains to the wintering grounds in the southwestern United States and northwestern Mexico.

Flocks begin forming on the breeding grounds in early August. Southern migration flocks include both immature birds and adults and are much larger than the flocks observed during the spring migration. Migration dates are variable and McCown's Longspurs have been recorded departing the breeding grounds from early August to late November (With 1994a). The earliest arrival dates on the wintering grounds are in late September.

On the wintering grounds McCown's Longspurs are most abundant on lightly grazed grasslands (Grzybowski 1982) and often occur in mixed flocks with Horned Larks (Eremophila alpestris) and other Calcarius species. Departures from the wintering grounds vary from late February to late April.

There are few data on site fidelity. Individuals apparently disperse from their natal breeding grounds because birds banded as nestlings have not been resighted in the same area in subsequent years (With 1994a). Little information exists on breeding site fidelity. With (1994a) found that two of two males banded as adults returned to the same pasture where they were initially captured, but neither of two banded adult females returned. McCown's Longspurs are known to abandon their nesting sites during years of high precipitation (Alsop 2001) and occupy a given breeding area unpredictably from year-to-year (With 1994a). No data exist on wintering site fidelity.

Interspecific interactions

McCown's Longspurs migrate and form winter flocks most often with Horned Larks. They also flock with other Calcarius species and Sprague's Pipits (Anthus spragueii; With 1994a, Rising 1996).

Known and suspected nest predators of McCown's Longspurs include thirteen-lined ground squirrels (Spermophilus tridecemlineatus), Richardson's ground squirrels (S. richardsonii) , black-tailed prairie dogs (Cynomys ludovicianus), badgers (Taxidea taxus), striped skunks (Mephitis mephitis), long-tailed weasels (Mustela frenata), coyotes (Canis latrans), red foxes (Vulpes vulpes), swift foxes (V. velox), deer mice (Peromyscus maniculatus), meadow voles (Microtus pennsylvanicus), western plains garter snakes (Thamnophis radix), red-sided garter snakes (T. sirtalis), and bull snakes (Pituophis melanoleucos; With 1994a, Sutter 1997). Nests are rarely parasitized by Brown-headed Cowbirds (Molothrus ater; With 1994a).

Adaptability

There is limited and conflicting evidence about the impact of human disturbance on McCown's Longspurs. Two studies reported that some adults deserted their nests apparently in response to human disturbance (Strong 1971 cited in With 1994a, Felske 1971). In contrast, two other studies found no evidence of nest desertion in response to disturbance (Dubois 1937, With 1994a).

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