Savannah sparrow (princeps subspecies) management plan: chapter 3

SPECIES’ BACKGROUND

2.1 Biology, habitat, and distribution

The reproductive rate of Ipswich Sparrows is very high; they mature in a year, have high annual adult survival rates (28–42%) and high fledging success (72–84%), and raise two to three broods per breeding season (Stobo and McLaren 1975).

Ipswich Sparrows are the only passerines nesting in any numbers on Sable Island. They eat a variety of seeds and insects, especially marram seeds and, in winter, sea oat seeds. Predators include nest predators such as gulls, crows, European Starlings (Sturnus vulgaris), and blackbirds (family Icteridae), and adults are also taken by raptors, probably especially in migration and on the wintering grounds. If it is assumed that no areas of particular abundance have been missed, Ipswich Sparrows are too sparse in the wintering grounds to be ecologically important. On Sable Island, however, they are the dominant landbird, although their importance in ecosystem processes (e.g. whether they are a “keystone” species) is unknown. 

The Ipswich Sparrow nests almost exclusively on Sable Island and likely evolved there in isolation from “typical” Savannah Sparrows (Passerculus sandwichensis sandwichensis) on the mainland. No other breeding location is known or suspected, although occasional breeders in mainland Nova Scotia pair with the “typical” Savannah Sparrow. The vegetated terrain on Sable Island constitutes this bird’s breeding habitat. All available breeding habitat on Sable Island appears to be used, albeit at varying density, depending on plant community and year. Introduction to other locales is inadvisable because of the sparrow’s specialization to its habitat and its potential to interbreed with “typical” mainland Savannah Sparrows.

Ipswich Sparrows winter along the Atlantic seaboard from Nova Scotia to Florida, with highest concentrations in the mid-Atlantic states (see map in Appendix). This distribution has not changed, at least in historic times. Winter habitat needs are poorly understood. The sparrow winters in beach grass on outer dunes, with a possible preference for sites with more access to fresh water, more relief, and a thicker and more extensive cover of beach grass (Stobo and McLaren 1971). Highest concentrations occur between New Jersey and Virginia, but the relative importance of this and the rest of the over 3000 km of coastline where the bird winters is unknown. Most of the population might winter on islands off Virginia, but these have not been surveyed for the bird.

2.2 Threats to population

The main intrinsic limiting factors for Ipswich Sparrows are their restricted breeding range and migratory dispersal across a broad winter range (McLaren 1979; Horn 1999). The population is not currently under any known threat. However, its localized distribution makes it particularly vulnerable to potential threats such as chance events, predation, human activity, and habitat loss. Studies of other grassland sparrows suggest that, in theory, the population could also be threatened by chance demographic effects and loss of genetic variation (Walters et al. 2000). Available evidence suggests that, aside from chance events, none of these factors currently threatens this particular population.

2.2.1 Breeding habitat los

Erosion of vegetated terrain is the primary management concern in most dune habitats and would obviously be damaging to the Ipswich Sparrow population. Evidence that vegetated terrain has been reduced on Sable Island, however, is equivocal; it may have declined (Gray 1992), shifted distribution (McCann and Byrne 1994), or stayed unchanged (Freedman 1996). Detailed analysis of newly available georeferenced air photos (Desjardins 2002) may settle the issue.

Grazing and trampling by horses may contribute to loss of vegetation (e.g. DeStoppelaire et al. 2001). Grazing can also maintain floral diversity, however, by arresting succession (van Dijk 1992). In the case of Ipswich Sparrows, grazing may preserve habitat by arresting succession of herbs by woody shrubs (Horn 1999). Grazing may be beneficial for several species of grassland sparrows, although its effects vary among sites (Walk and Warner 2000). Thus, whether horses are detrimental or beneficial for sparrow habitat will require more research.

2.2.2 Human activity

Any project proposed on Sable Island has the potential to impact the Ipswich Sparrow or its habitat. The most likely type of project that would affect sparrow populations is construction of any structures on the island, such as new buildings, towers, stacks, or wind turbines. Given the sedentary habits of these birds and their resultant familiarity with objects on their territories, collisions with such structures do not pose a significant threat. However, while any given construction footprint will displace only a few sparrow territories, multiple construction projects will, of course, destroy habitat cumulatively. Also, even small-scale excavations can cause widespread erosion, with devastating effects on habitat.

Sable Island’s isolation provides some protection from landings, but its isolation will decrease as better and cheaper navigational aids make trips to the island easier. Most visits, whether project-related (e.g. maintenance of facilities, research) or more informal, are unlikely to affect the sparrow population. Nonetheless, they do carry a low risk of serious inadvertent or intentional damage to sparrow habitat -- for example, through fire, introduction of mammalian predators, or introduction of non-native plants. The latter two threats in particular have characterized the history of Sable Island’s biota.

Offshore petroleum development, which was accelerating when the updated status report was written (Horn 1999), has had no known effect on the sparrow or its habitat; the industry has avoided landing on the island and has adhered to visitor guidelines, including staying off vegetation, when there (Z. Lucas, pers. commun.). However, if offshore development continues, efforts to control its impact will have to be sustained.

A public review of the Sable Offshore Energy Project raised concerns that Ipswich Sparrows approaching or leaving Sable Island might be attracted to, collide with, or be disoriented by offshore gas platforms constructed by the project (Fournier et al. 1997).

Chemical toxins such as diesel or aviation fuel, motor oil, or other liquid fuels may cause sickness or be lethal if ingested or absorbed by Ipswich Sparrows.

2.2.3 Chance events

Harsh winter weather is known to be a threat, as it severely reduced overwinter survival and, in turn, the breeding population in 1977–1978 (McLaren 1979). In theory, harsh weather and disease during the breeding season seem to be the next most likely chance events that could affect the whole population, because of its limited distribution. Population reproductive rate, even within a season, is very high, however, so presumably recovery would be rapid (McLaren 1979; Horn 1999).

2.2.4 Chance demographic effects

Populations fluctuate in size by chance, so as a population becomes smaller relative to these fluctuations, it runs the risk of chance extinction. Data summarized in previous status reports suggested large yearly fluctuations (McLaren 1979; Horn 1999), probably linked to harsh weather during migration and wintering (see section 2.2.3 above). The population’s high reproductive rate probably buffers it against these fluctuations; to confirm this, the size of the fluctuations must be determined more precisely (Smith et al. 2003).

2.2.5 Predation

Possible avian predators of Ipswich Sparrow eggs and nestlings include gulls, crows, starlings, and blackbirds. According to both the original and updated status reports, predation may have caused high nest losses in some years, but not to the extent that would threaten the population (McLaren 1979; Horn 1999). Nest success data from 2001 and 2002 showed virtually no predation in either dense or sparse habitats (Taylor et al. 2001; Horn et al. 2003a).

Introduction of a mammalian predator to Sable Island might be catastrophic, as for other island populations of birds, although historically the population survived the long-term presence of mammalian predators, specifically cats and foxes (Elliot 1968).

2.2.6 Loss of genetic information

The updated status report suggested two potential genetic threats to Ipswich Sparrows: intergradation with mainland Savannah Sparrows and inbreeding within the Sable Island population (Horn 1999); both threats have been shown in many animal populations since then (Frankham et al. 2002). Preliminary results of studies since the updated status report, however, suggest that neither hybridization (Mockford et al. 2003; Zink et al. 2005) nor low genetically effective population size (Temple 2000; Horn et al. 2003a) is currently a pressing threat.

2.3 Monitoring history

From 1967 to 1979, estimates of the breeding population in early June (i.e. after adults arrived but before first eggs hatched) varied from 2100 to 3300 individuals, except for dips to 1700 individuals in 1977 and to 1250 in 1978, probably due to harsh winters that caused low overwinter survival (McLaren 1979). By 1979, the population had recovered to 2000 individuals; the next census was not until 1995, when the population was 3400 individuals (Horn 1999). A 1998 census using different methods yielded 5962 individuals (i.e. almost twice the maximum of the 1967–1979 censuses). This latest figure suggests either that there has been a large population increase or that one or both census methods are flawed.

Wintering populations are surveyed only through Christmas Bird Counts (CBCs). Totals suggest high variability in the amount and/or timing of winter mortality but are so low for any given count area that they may be unreliable (Horn 1999).

2.4 Knowledge gaps

The study of Stobo and McLaren (1975) remains one of the classic studies of any North American bird and provides comprehensive information on the ecology of the Ipswich Sparrow. However, knowledge is still poor in several key areas, listed below.

2.4.1 Breeding habitat use and trends

The importance of sparrow access to varied habitats in buffering population loss in years with high predation is not known, but such knowledge will be required to guide any future mitigative habitat management.

The dynamics of dune ecosystems elsewhere have been well studied but differ widely between sites (Doody 2001) and may be particularly distinct on Sable Island because of its unique location and biota. The factors that maintain long-term stability of the sparrow habitat on Sable Island are not well understood, nor is the role that horses play in the marram–beach pea assemblages. Research is required to clarify processes that create and maintain specific sparrow habitats -- for example, succession and nutrient cycling within dense marram, beach pea, and heath communities.

2.4.2 Winter habitat use and trends

Detailed information on wintering habitat and, more precisely, its use is lacking for this species. Mortality during migration and wintering, apparently the most important limit on the population, is very poorly understood. Limiting factors on the wintering grounds, such as habitat availability, food, and predation, need to be further identified.

Once wintering habitat has been better identified, trends in sparrow wintering habitat require review.

2.4.3 Genetic threats

The extent and stability of the hybrid zone, the success of hybrid pairs, and dispersal patterns on and off the island remain unstudied.

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