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Horned lark strigata subspecies (Eremophila alpestris strigata) recovery strategy: chapter 2


1. Background

This recovery strategy outlines a multi-species approach for the Horned Lark strigata and Vesper Sparrow affinis. It was recognized that there is considerable overlap between these species with respect to current and historical distribution in Canada, general ecological requirements, principal threats to the species and their habitat, and biological expertise. Adopting a multi-species approach to recovery planning also represents an opportunity to make efficient use of conservation resources.

1.1 Species Assessment Information from COSEWIC

Horned Lark strigata

Date of Assessment: November 2003

Common Name: Horned Lark strigata

Scientific Name: Eremophila alpestris strigata

COSEWIC Status: Endangered

Reason for Designation: Although this subspecies has always been rare in Canada, it has declined steadily throughout its range over the last 50 years and is now nearly extirpated from Canada.

Canadian Occurrence: British Columbia

COSEWIC Status History: Designated Endangered in November 2003. Assessment based on a new status report.

Vesper Sparrow affinis

Date of Assessment: April 2006

Common Name: Vesper Sparrow affinis

Scientific Name:Pooecetes gramineus affinis

COSEWIC Status: Endangered

Reason for Designation: This songbird, a subspecies of the Vesper Sparrow, is found in Canada only in coastal grasslands in the extreme southwestern corner of British Columbia, where it now breeds only at one site with a population of about 5 pairs. The taxon is declining in the United States as well, where it has a restricted distribution in western Washington and Oregon. Habitat loss is the greatest threat, both through direct destruction of habitat for urban development and through invasion by alien plant species.

Canadian Occurrence: British Columbia

COSEWIC Status History: Designated Endangered in April 2006. Assessment based on a new status report.

1.2 Description

Horned Lark strigata

The Horned Lark (Eremophila alpestris) is the only true North American member of the lark family (Alaudidae). It is a slender ground-dwelling passerine of open country. The adult male is distinctively marked with a dark facial mask and breast band that contrasts with the pale face and throat. The “horns” for which the species is named are tiny, black feather tufts that are apparent only at close range. The tail is square and blackish with pale central feathers and whitish edges that are evident in flight. The Horned Lark strigata is smaller than the other subspecies. The upperparts are dark brown, and the nape is walnut brown. The bird has a yellow throat and eye stripe and yellowish underparts (Beason 1995). There is heavy brown streaking on the sides of the breast that should allow separation of males to subspecies in the field (Sibley 2000). In the hand, the longest uppertail covert is distinctly streaked (Pyle 1997). Adult females are similar to males, but they are duller and smaller and lack “horns.” Females cannot be separated to subspecies in the field. Juveniles of both sexes are generally duller than adult females (Sibley 2000).

Vesper Sparrow affinis

The Vesper Sparrow (Pooecetes gramineus) is a medium- to large-sized sparrow (length approximately 16 cm) with a chestnut shoulder patch (lesser coverts), white outer tail feathers, and a whitish eye ring (Sibley 2000). Sexes are similar in appearance. Juveniles are similar in appearance to adults, but duller, and they usually lack chestnut lesser coverts (Pyle 1997).

The three Canadian subspecies of Vesper Sparrow (P. g. confinis, P. g. gramineus,and P. g. affinis) are similar in appearance and cannot be reliably separated in the field, varying only in shading and measurements. The Vesper Sparrow affinis has medium greyish brown upperparts and white underparts with a buff tinge. Vesper Sparrow confinis, the common B.C. interior subspecies, has pale greyish brown upperparts and creamy underparts. Vesper Sparrow affinis is slightly smaller overall than confinis and has a shorter tail (Pyle 1997).

1.3 Populations and Distribution

Current and historical populations of both taxa in Canada, together with those in the United States, likely comprise a single metapopulation.

Horned Lark strigata

Horned Larks are found across much of North America and Eurasia. The breeding distribution of the Horned Lark strigata, however, is restricted to the Georgia Basin/Puget Trough, the coast of Washington, and islands in the lower Columbia River (Beason 1995; Rogers 2000; Pearson and Altman 2005; Stinson 2005) (see Figure 1). The centre of its breeding distribution is the glacial outwash prairies of the south Puget Sound area of western Washington (Rogers 2000). The Canadian population is extremely small, and may be extirpated: the last observation of a Horned Lark in Canada was made in 2002 (COSEWIC 2003). U.S. researchers have estimated that the Washington and Oregon population is likely fewer than 1000 individuals (an estimated 774 individuals, based on recent breeding and winter surveys; Pearson and Altman 2005).

Figure 1. Current and historic Horned Lark strigata breeding localities and possible historic nesting or uncertain breeding season locations (information from Altman 1999; Rogers 2000; Pearson and Hopey 2005; Stinson 2005). All current breeding sites are in Washington and Oregon states.

Figure 1. Current and historic Horned Lark strigata breeding localities and possible historic nesting or uncertain breeding season locations


The distribution of the Horned Lark strigata in Canada is restricted to southwestern British Columbia, where it historically occurred only on southeastern Vancouver Island and in the lower Fraser River valley from Chilliwack west to the mouth of the Fraser River (Campbell et al. 1997). Breeding has not been documented on Vancouver Island or the southern Gulf Islands. However, it is possible that this subspecies did breed at these locations in the past, as some suitable habitat would have been available and birds were known to occur in the area (Munro and Cowan 1947; Beauchesne 2003; COSEWIC 2003). Central eastern Vancouver Island probably represents the northern limit of suitable habitat and range of this subspecies. Although most historical records for the Horned Lark strigata on Vancouver Island are not from the peak breeding season, it seems likely that these sightings were of breeding birds, occurring as they did in presumed nesting habitat. The Horned Lark strigata is known to have bred in the lower Fraser River valley. Breeding records are concentrated near the mouth of the Fraser River on Sea Island, Iona Island, and Lulu Island. Other confirmed historical breeding localities include the University of British Columbia at Point Grey and near Chilliwack (Campbell et al. 1997). Historically, the Vancouver Island and lower Fraser River valley populations were likely very small and locally distributed. Given that these areas represent the northern extent of the geographic range of this taxon, it is also likely that not all apparently suitable habitats were utilized.

NatureServe (2006) ranks the Horned Lark strigata subspecies globally as an imperilled subspecies of a globally common species (G5T2); the subspecies is also ranked as nationally imperilled (N2) in the United States and as presumed extirpated (NX) in Canada. The NatureServe ranks for Washington and Oregon are critically imperilled (S1B) and imperilled (S2B), respectively (B refers to a breeding population). The British Columbia rank is presumed extirpated (SX). The B.C. Conservation Data Centre (2006) lists the Horned Lark strigata on the Red List.

Vesper Sparrow affinis

Vesper Sparrows are widespread in appropriate habitat across North America. The Vesper Sparrow affinis subspecies occurs in a disjunct population in the Pacific Northwest, separated from the interior populations (i.e., Vesper Sparrow gramineus, confinis, and altus) by the Cascade Mountain Range. This subspecies' breeding range extends from southeastern Vancouver Island and the lower Fraser River valley south through western Washington and Oregon to extreme northwestern California (Figure 2). The Vesper Sparrow affinis is the only subspecies found west of the Cascades (AOU 1957; Pyle 1997; Cannings 1998; Rogers 2000; Campbell et al. 2001; Jones and Cornely 2002; Altman 2003). In Canada, the population on southeastern Vancouver Island is estimated to number between 6 and 10 pairs (Beauchesne 2006).

Figure 2. Breeding range of the Vesper Sparrow affinis, shown in light green; Nanaimo airport site shown as red dot. The interior subspecies of the Vesper Sparrow (P. g. confinis) breeds in grassland areas shown in the satellite image as a buff colour, separated from the coastal grasslands by high mountains (dark green and white).

Figure 2. Breeding range of the Vesper Sparrow affinis

In Canada, the Vesper Sparrow affinis is currently known to breed only on Vancouver Island at a single location: the Nanaimo airport, near Cassidy (Beauchesne 2002a, 2003, 2004a). Historically, it has been reported during the breeding season on Vancouver Island from the Englishman River estuary in the north to Cobble Meadows and Mill Bay to the south. It was also formerly a local breeder in the Fraser Lowland on British Columbia's southwest mainland coast. The last confirmed breeding record for that area is from 1968 (Campbell et al. 2001). The historical population size is unknown, but it is likely that the subspecies was never common in Canada, as it was never recorded in large numbers or from more than a few localities.

NatureServe (2006) ranks the Vesper Sparrow affinis subspecies globally as a vulnerable subspecies of a globally common species (G5T3). The subspecies has not yet been ranked by NatureServe in the United States or Canada (NNR). The NatureServe ranks for Washington and Oregon are critically imperilled (S1B) and imperilled (S2B), respectively (B refers to a breeding population). The British Columbia rank is critically imperilled (S1B). The B.C. Conservation Data Centre (2006) lists the Vesper Sparrow affinis on the Red List.

1.4 Habitat and Biological Needs of the Horned Lark strigata and Vesper Sparrow affinis

Horned Lark strigata

The Horned Lark strigata is a ground-nesting passerine. Across their range, Horned Larks are birds of open areas with short, low-density vegetation (Beason 1995). Horned Lark strigata habitat requirements are similar to those of other subspecies. In British Columbia, larks have used agricultural fields, airports, beaches, sand dunes, short-grass playing fields, roadsides, and other areas with a high percentage of bare ground. Documented breeding habitat is restricted to short-grass fields in agricultural areas, airports, and estuaries and to sparsely vegetated, sandy beaches along the lower Fraser River (Butler and Campbell 1987; Campbell et al. 1997). Based on habitat requirements, the Horned Lark strigata may have used Garry oak ecosystems, especially those recently burned by First Nations peoples, but this has not been documented.

In Washington State, the Horned Lark strigata breeds primarily in the glacial outwash prairies of south Puget Sound. These prairies are remnant grasslands that likely developed shortly after the last ice age. Subsequent regional climate change, beginning about 6000 years ago, resulted in a shift to moister conditions that typically produced a succession to forest ecosystems. However, prairie conditions were maintained in some areas as a result of a high frequency of low-intensity fires, most set by First Nations peoples (Crawford and Hall 1997). The soils of these prairies are deep, have low nutrient levels, and drain rapidly. These characteristics, along with the frequent anthropogenic burning, also helped maintain the prairie grassland condition.

Other breeding sites in Washington and Oregon include airfields, dredge spoil islands in the Columbia River, sandy coastal beaches, and disturbed areas on military training bases (Rogers 2000; Pearson and Altman 2005). In the Willamette Valley of Oregon, the Horned Lark strigata breeds in agricultural fields, generally selecting sparsely vegetated fallow fields (Pearson and Altman 2005). The majority of the global population of the Horned Lark strigata is believed to winter along the lower Columbia River and in the Willamette Valley (Pearson and Altman 2005).

The common characteristics of all breeding sites are short sparse vegetation dominated by grasses and forbs with very few or no trees or shrubs and a relatively high percentage of bare (i.e. non-vegetated) ground (Pearson and Altman 2005). The size of the habitat patch is also likely important. Grassland sites may need to be very large to be effective. In the United States, habitat patches in the Puget Lowlands grassland areas used by the Horned Lark strigata ranged in size from 131 to 390 ha. In coastal areas, the Horned Lark strigata breeds adjacent to expanses of open water; under these conditions, it uses habitat patches as small as 10 ha (S. Pearson, pers. comm.).

Availability of large patches of suitable breeding habitat with low levels of disturbance during the nesting period appears to limit the extant distribution of this species in Canada. In many areas with otherwise suitable habitat, activity of machinery, people, livestock, or domestic pets can destroy or depredate nests directly. Additionally, anthropogenic disturbance can lead to frequent flushing of nesting birds, resulting in nest abandonment and increased rates of nest predation (Pearson and Altman 2005).

Vesper Sparrow affinis

The Vesper Sparrow affinis is also a ground-nesting passerine. Like the Horned Lark strigata, the principal limiting factor for this species is believed to be the availability of suitable habitat with low disturbance levels during the nesting season. Vesper Sparrows are grassland birds, preferring dry, open areas with short, sparse grass or herbaceous cover (Campbell et al. 2001; Dechant et al. 2001; Jones and Cornely 2002). Structural diversity of vegetation is important, however: taller vegetation, such as scattered or edge shrubs or trees, is used for cover and for singing perches, whereas areas with shorter vegetation are used for foraging and nesting (Davis and Duncan 1999; Beauchesne 2002a, 2003, 2004). In foraging areas of western Washington, the Vesper Sparrow affinis used sites with a mean cover of 32% bare ground, with the remaining cover consisting of grasses and forbs (Rogers 2000). Fence posts, wire fences, and other human-made structures may also be used as singing perches (Beauchesne 2002b). A combination of short-grass and herbaceous ground cover, bare dirt, and scattered taller vegetation seems to be preferred (see numerous citations in Dechant et al. 2001).

Several studies indicate that Vesper Sparrows avoid permanent pasture and hayfields (see Campbell et al. 2001; Jones and Cornely 2002) or areas where shrubs were completely removed and planted to grass (Castrale 1982). In Saskatchewan, Vesper Sparrows did occur in hayfields (McMaster et al. 2005). However, sparrows initiated nesting in habitat that had clumps of short vegetation with considerable amounts of bare dirt until mid-June, after which the vegetation grew in height and density. On southeastern Vancouver Island, hayfields tend to support dense, tall vegetation early in the breeding season, precluding nesting by the Vesper Sparrow affinis. Breeding territories at the Nanaimo airport occur in areas adjacent to, but not within, areas used for hay production (Beauchesne 2002a, 2003, 2004a).

Habitat patch size may also be important (Kershner and Bollinger 1996; Rogers 2000). For example, in Washington, the Vesper Sparrow affinis is currently found primarily in large prairie areas, but not in small patches of similar habitat (S. Pearson, pers. comm.). In eastern Oregon, however, Vesper Sparrows have been recorded breeding in areas smaller than 4 ha (Jones and Cornely 2002), and on Vancouver Island, the extant population occurs in an area of suitable habitat of approximately 10 ha (Beauchesne 2002a). Populations of Vesper Sparrow affinis may therefore be able to persist in smaller habitat patches than those required for the Horned Lark strigata. However, the minimum habitat patch size requirement for Vesper Sparrow affinis is currently unclear.

On Vancouver Island, the plant community at the known breeding site includes both native and non-native flora. Birds frequently use clumps of introduced Scotch broom (Cytisus scoparius) for singing perches and escape cover. They forage on the ground in adjacent open areas with gravelly soil and sparse forb and grass cover (Beauchesne 2002a, 2003, 2004a). Two of three nests found at the Nanaimo airport were in a mixed forb/grass area, concealed within a clump of non-native English plantain (Plantago lanceolata). The third nest was in a rocky area, placed against the side of a tuft of mown broom (Beauchesne 2006).

Multi-species Habitat Management

Given their similar habitat requirements, management of habitat at a single site for the Vesper Sparrow affinis and the Horned Lark strigata is possible. However, for management purposes, it is important to recognize that Vesper Sparrows require some shrub cover in open areas, whereas larks avoid shrubby areas. In many open habitats, the lark may nest in the middle of larger open areas and the sparrow may use the edges where both short and tall vegetation are available. This has been demonstrated at study sites in Washington State, where both species occur (Rogers 2000). In this recovery strategy, it is not recommended that any sites be managed specifically for both species, as the currently most suitable candidate sites for each species are not likely to overlap geographically.

1.5 Threats to the Horned Lark strigata and Vesper Sparrow affinis

1.5.1 Habitat Loss and Degradation

Habitat loss

Loss of high-quality breeding habitat is considered the primary limiting factor for both species.

Although the extent of breeding habitat in the Georgia Basin for the Horned Lark strigata has always been limited, suitable open habitats, including 95% of the Garry oak and associated ecosystems (Fuchs 2001), have been lost as a result of industrial, commercial, and residential developments and dyking. It is assumed that prior to European settlement, Garry oak ecosystems, prairies associated with Garry oak ecosystems, and other sparsely vegetated or burnt areas would have been the key open habitats used by the breeding Vesper Sparrow affinis. Urban development, modern agricultural practices, and infilling of most remaining open spaces by invasive exotic shrubs have resulted in loss of this habitat.

A lack of suitable winter habitat in known wintering areas along the lower Columbia River and in the Willamette Valley is also of concern for the Horned Lark strigata. Very few of the sites used by the Horned Lark strigata for breeding or wintering are protected or are managed primarily for larks (Pearson and Altman 2005).

The Vesper Sparrow affinis is believed to winter in California (AOU 1957). However, the exact location of winter habitat and the extent to which it is threatened are not known.

For both species, habitat loss is a known threat and it is current, ongoing, and widespread across the historical ranges.

Habitat Degradation

For both species, many potential habitat areas have been degraded due to disturbance from increased human activity, changes in agricultural practices, or ingrowth by both native and non-native vegetation (Campbell et al. 2001). Most remaining open areas (e.g. city parks, golf courses, backyards, gardens) contain shrubs, turf, and non-native grasses unsuitable for ground-nesting birds (Jones and Bock 2002). Development has fragmented or destroyed most suitable habitat in British Columbia (Campbell et al. 1997), and continuing pressures from development will likely destroy most of the remaining habitat (Dawe et al. 2001).

At the Nanaimo airport, the existing breeding Vesper Sparrow affinis population uses an area that is away from the vicinity of buildings and human activity. However, habitat in this area is vulnerable to potential future activities, such as airport expansion (new or longer runways), construction of new airport infrastructure (airport buildings, aircraft hangars, parking areas for vehicles and equipment), and expansion of ancillary commercial operations (expanded recreational vehicle sales or other new businesses).  As this site is the only extant breeding location for the subspecies in Canada, future development at this site may pose a threat to the persistence of the species in Canada.  The extent to which any particular proposed development might negatively impact Vesper Sparrow affinis or its habitat would depend on the specific nature and on-site location of the development.

Habitat Fragmentation

Human activities in the region have resulted in changes to the size and spatial configuration of suitable habitat patches. The minimum patch size for the Horned Lark strigata may vary depending on the landscape context. Although the Horned Lark strigata may require patches no larger than 5–10 ha within an open landscape (e.g. dunes, coastal island or coastal prairie habitats), a suitable habitat patch would likely need to be larger if surrounded by forest, buildings or other tall structures that reduce visibility. In general, U.S. data suggest that smaller patches are suitable only in habitats directly adjacent to the coast, whereas inland sites must be much larger (S. Pearson and B. Altman, pers. comm.).

The Vesper Sparrow affinis appears to tolerate smaller areas of suitable habitat (B. Altman, pers. comm.). For example, at the Nanaimo airport, the population is breeding in an area less than 10 ha in size (Beauchesne 2004a). Similar sized patches of suitable habitat within close proximity to the existing population appear to be lacking, and this may reduce the chances of the current population expanding into neighbouring sites (Beauchesne 2003, 2004a).

Consequently, for both species, the threat of fragmentation of habitat is suspected, but not known, although fragmentation of habitat is ongoing and widespread across the historic ranges.

Agricultural Practices

Agriculture probably benefited both species from the late 19th to early 20th century through creation of open short-grass habitats, some of which would have been suitable for breeding (COSEWIC 2003). However, intensified agriculture typical of the latter half of the 20th century has reduced the suitability of some farmland areas that were previously used for nesting. Agricultural practices that involve mechanical procedures (e.g. tilling, mowing) can destroy active nests, significantly reducing reproductive success of ground-nesting birds. Modern crop “improvements,” involving more rapid growth with earlier and more frequent harvest, exacerbates this risk to ground-nesting birds. For the Vesper Sparrow affinis, the modern “clean farming” practice of removing shrubby hedgerows eliminates an important habitat structural feature, reducing the suitability of some pasture habitat (Rodenhouse et al. 1993; Sauer et al. 2004). In addition, industrial agricultural practices increasingly rely on chemicals, which likely have detrimental consequences for most species of birds (Gard et al. 1993).

Intensive grazing, where livestock are concentrated, reduces the suitability of the habitat if grazing is too severe, and there is an increased likelihood of nests being trampled or predated (Bock et al. 1999; Nack and Ribic 2005). While grassland species are known to reproduce successfully in grazed areas in the interior of British Columbia, livestock stocking density is generally higher in coastal areas, potentially resulting in an increased risk of nest destruction through trampling.

Finally, manipulation of water levels in an agricultural context may also pose a threat, as flooding during the breeding season may result in nest failure.

While agricultural practices have resulted in known loss and degradation of habitat for both species, no extant or potential suitable sites are known to be currently threatened by agricultural activities.

1.5.2 Changes in Ecological Dynamics or Natural Process

Fire Suppression

For both species, suitable habitat in the Georgia Basin may have been more abundant in the past due to more frequent fires, both natural and of First Nations origin. Fire suppression has resulted in the infilling of previously suitable habitats by native plants, such as Douglas-fir (Pseudotsuga menziesii) and common snowberry (Symphoricarpos albus). In a recent controlled burn experiment in the United States, the Horned Lark strigata was more abundant in burned plots versus control plots in the post-breeding period (Pearson and Hopey 2005). Late summer burns appear to be more beneficial for the Horned Lark strigata, as regeneration of vegetation is not as vigorous, possibly allowing birds to locate and capture invertebrate prey more easily.

The impacts of fire suppression are thus suspected, but not known. Fire suppression has resulted in the loss of suitable habitat in the past and remains a concern, especially in Garry oak and affiliated ecosystems.

Dyking

Improved dyking of the Fraser River delta in the latter half of the 20th century has reduced the amount of sparsely vegetated sandy shoreline along the edges of the Fraser River. Coastal sand dunes are an important breeding habitat for the Horned Lark strigata in Washington State (S. Pearson, pers. comm.) and were one of the known breeding habitats used in British Columbia (Campbell et al. 1997).

Dyking is a known, but historical, threat that reduced available habitat for the Horned Lark strigata along the lower Fraser River.

Predators

The effects of predators may also limit these species. Predation was the primary source of nest failure in the Horned Lark strigata at nearly all sites studied in the Puget Lowlands, Washington coast, Columbia River, and Willamette Valley (n = 166 nests: Altman 1999; Pearson and Hopey 2005. Vesper Sparrow researchers have found that predation is a major cause of nest failure, with rates of up to 63% (see numerous studies cited in Jones and Cornely 2002).

Known predators on both adult and young grassland birds include corvids, raptors, snakes, and small mammals. American Crows (Corvus brachyrhynchos)and garter snakes (Thamnophis sp.) have been observed depredating Horned Lark strigata nests (Pearson and Hopey 2005). Breeding male Horned Larks may be more vulnerable to aerial predators than Vesper Sparrows, owing to their flight display behaviour. For both species, urbanization and other human activities tend to lead to increased predation pressure from introduced predators, particularly domestic and feral cats (George 1974; Cooper 1993; Coleman et al. undated). No nest predation information is available for the Vesper Sparrow affinis population at the Nanaimo airport, although domestic and feral cats have been observed at the site (S. Beauchesne, pers. comm.).

Nest predation is a known and ongoing threat for both species, wherever they occur. For both species, the proximity of any known or potential suitable sites to urbanized areas increases the risk of predation from introduced predators.

Infilling of Open Habitats by Shrubs and Exotic Grasses

Fire suppression is a prevalent policy around developed areas. Historically, some ecosystems burned frequently, but current practices have allowed natural succession to infill open habitats that were historically maintained by fire. The introduction of invasive plants such as Scotch broom, common gorse (Ulex europaeus), Himalayan blackberry (Rubus discolor), and tall non-native grasses has exacerbated the situation, as these species are well suited to rapidly colonize open spaces. This change in vegetation structure has reduced the suitability of most of the remaining old-field, Garry oak, and sand dune habitats for the Horned Lark strigata (Fraser et al. 1999).

Because the Vesper Sparrow affinis requires a combination of short vegetation, bare ground, and shrub cover, this species tolerates some infilling by invasive species such as Scotch broom and will even use that plant as a singing perch and for escape and nest cover (Beauchesne 2002a, 2003, 2004a). However, Vesper Sparrows are excluded from areas where shrubs completely dominate open areas (Jones and Cornely 2002). If left unmanaged, Scotch broom can degrade formerly open habitats to the point where they become unsuitable. Other invasive plant species may have structural similarities to native species and may be beneficial to breeding birds of either species. For example, Vesper Sparrow affinis nests at the Nanaimo airport site have been situated directly adjacent to English plantain (Beauchesne 2006).

1.5.3 Disturbance

Human Disturbance

Within the range of both species in British Columbia, rapid human population growth has increased recreational usage in the few remaining areas with suitable habitat. Most of these areas are small. Small, multi-use areas are generally not compatible with ground-nesting birds. U.S. researchers report that the Horned Lark strigata will flush when approached within 30 m by people or dogs (S. Pearson and B. Altman, pers. comm.). Recreational activities such as hiking, dog walking, bird watching, horseback riding, bike riding, fireworks, all-terrain vehicle use, or “sand-bogging” in any form of vehicle can result in destruction of nests or cause disturbances leading to nest failure if too frequent (Rogers 2000; Pearson and Altman 2005).

Researchers in Colorado found that although some habitat within or adjacent to urban areas appeared suitable, Vesper Sparrows tended to occur in lower density in these areas than further away from the urban edges where human disturbance is presumably lower (Bock et al. 1999). Elsewhere, researchers have found that this species was significantly more abundant on control transects away from disturbance than on transects along recreational trails (see Miller et al. 1998  in Jones and Cornely 2002).

At least in part, preference for low levels of disturbance likely explains the persistence of the Vesper Sparrow affinis at the Nanaimo airport. The birds occur almost entirely within a restricted area where they are subjected to noise from planes but very little foot or vehicular traffic. Although planes and helicopters are very loud, they do not come in close contact with the birds and do not cause flushing behaviour. These birds do flush when approached by researchers on foot or if they are close to the roadside when a vehicle passes (S. Beauchesne, pers. comm.). However, Vesper Sparrows appear to be less likely to flush than Horned Larks, and researchers have indicated anecdotally that Vesper Sparrows flush from the nest only when individuals approach within a few metres (S. Beauchesne and P. Krannitz, pers. comm.).

While human disturbance is a well-documented ongoing threat for both species elsewhere in their global range, as it results in the destruction of nests and nest failure, the extant population of the Vesper Sparrow affinis at the Nanaimo airport appears to experience fairly low levels of human disturbance.

1.5.4 Accidental Mortality

Bird Strikes

U.S. military reports indicate that Horned Larks are the most commonly reported aviation bird strike victim of any species (BASH 2006). Horned Lark flight behaviour, including a tendency to fly at considerable altitude during mating displays, likely exposes them to a greater risk than that experienced by other grassland species. Horned Larks (strigata subspecies) have been found dead along the runways of military airport breeding locations in Washington State (Pearson and Altman 2005). During the breeding season, the Horned Lark strigata is also vulnerable to automobile strikes, as it often selects nest sites on or directly adjacent to gravel roads (S. Pearson and B. Altman, pers. comm.).

As the Vesper Sparrow affinis is not known to frequent roadside habitats, the risk of automobile strikes for this species is likely lower. Despite the subspecies' presence at the Nanaimo airport, air strikes are also less likely, as Vesper Sparrows do not engage in high-altitude flights to the same extent as do larks. Also, they tend to frequent areas farther away from runways at this site, since the shrub component is removed from areas within 100 m of the strip.

Although mortality from bird strikes has been documented for the Horned Lark strigata elsewhere in its range, it remains merely a suspected threat for the Vesper Sparrow affinis, despite the proximity of the population to aircraft.

Small Population/Distribution Effects

A recent estimate of Horned Lark strigata population size in the United States, based on breeding and winter surveys, by U.S. researchers put the population at approximately 774 individuals (Pearson and Altman 2005). Preliminary genetic work suggests that the remaining birds have little genetic diversity. Samples all shared the same haplotype, whereas all other Horned Lark subspecies exhibited multiple haplotypes (Drovetski et al. 2005). Genetic data do not currently exist for the Vesper Sparrow affinis, although patterns of genetic differentiation for other coastal grassland species suggest that this subspecies will be genetically distinct and isolated from subspecies occurring elsewhere in North America (Ruegg and Smith 2002; Drovetski et al. 2005). Small population size and low genetic diversity cause these subspecies to be especially vulnerable to stochastic events, such as severe weather events or disease outbreaks. Given that source populations for the Horned Lark strigata in British Columbia are now remote (i.e. nearest populations are in south Puget Sound in Washington State), the probability of recolonization after a catastrophic event would be extremely low. The nearest U.S. population of the Vesper Sparrow affinis is on the San Juan Islands, but that population has suffered severe declines, and only a few individuals are thought to remain there. The Vesper Sparrow affinis population in the Willamette Valley in Oregon State is comparable with that of the Horned Lark strigata there (i.e. about 400 individuals) (B. Altman, pers. comm.). However, as the Vesper Sparrow affinis is more abundant at the southern end of the range, the total population of the subspecies is estimated to be larger than that of the Horned Lark strigata.

Because the Vesper Sparrow affinis currently occurs in very small numbers at a single site in Canada, this population is particularly vulnerable to extirpation. A single catastrophic event on the airport grounds or adverse weather conditions on the wintering grounds have the potential to eliminate the entire breeding population.

The risk of extirpation due to stochastic events is known, is ongoing, and would affect any extant or future subpopulations of either species.

1.6 Actions Already Completed or Under Way

  1. A stewardship agreement was in place with the Nanaimo Regional Airport, designed to promote protection of Vesper Sparrow nesting habitat (Beauchesne 2002c) but has since lapsed.
  2. The Nanaimo Area Land Trust has developed a stewardship program with some of the local landowners adjacent to the Nanaimo Regional Airport.
  3. The Garry Oak Ecosystems Recovery Team is coordinating conservation and recovery of Garry oak ecosystems in British Columbia. The Vertebrates at Risk Recovery Implementation Group of the Garry Oak Ecosystems Recovery Team is coordinating recovery actions for five species of birds associated with these ecosystems, including the Horned Lark strigata and the Vesper Sparrow affinis (Beauchesne 2004b).
  4. Habitat restoration work is under way in a number of protected areas within the range of the Horned Lark strigata and the Vesper Sparrow affinis, some of which will benefit these species and other ground-nesting birds.
  5. Research is under way to better document habitat used by the breeding Vesper Sparrow affinis at the Nanaimo airport (Beauchesne 2002a, 2003, 2004a, 2006).
  6. Inventory of the breeding population and habitats used by individual Vesper Sparrows at the Nanaimo airport is being enhanced in 2005 through colour banding of adults and nestlings.

1.7 Knowledge Gaps

In addition to the knowledge gaps preventing complete identification of critical habitat, outlined in Section 2.6.1 below, a number of knowledge gaps not pertaining directly to identification of critical habitat have been identified.

For both species, the feasibility of either passive or active reintroduction is not known. The likelihood of passive introduction of either species to suitable habitat from existing populations in the United States or Canada through natural dispersal mechanisms is unknown. Knowledge of dispersal distances would be useful to determine optimal inter-patch distances when selecting potential new sites for the Vesper Sparrow affinis. For the Horned Lark strigata, more knowledge of dispersal distances may inform habitat restoration decision-making regarding investments in northern Puget Sound sites (should this prove feasible) relative to Canadian sites. Active introduction techniques have not been developed for grassland bird species.

For the Vesper Sparrow affinis, minimum patch size requirements are essentially unknown and would help to determine the suitability of sites for protection and restoration. Data on productivity, survival, and mortality of the Nanaimo airport population are also lacking. These data could be used to develop a population model, which in turn could be used to help identify key limiting factors. Also unknown is the extent to which the population may respond and adapt to future changes to the habitat at the existing site, and whether techniques such as call-playback can successfully be used to encourage individuals to settle in areas not being considered for future development. Events on the wintering grounds may also be limiting the Vesper Sparrow affinis, but nothing is known about wintering locations of the Canadian population.

The primary nest predators have not been identified for Horned Lark strigata populations in the United States, although nest predation has been identified as the primary source of nest failure (Pearson and Altman 2005). Because this subspecies is migratory, events on the wintering grounds (i.e. primarily the Willamette Valley) may also be limiting populations. Nothing is known about the relative importance of wintering versus breeding events in regulating populations of larks in the United States.

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