Demography and Populations

Measures Of Breeding Activity

Age At First Breeding

Captive males and females breed during their first spring. Data unavailable for wild birds. When 2 broods produced, females begin laying second clutch 4–5 d after first has fledged.

Clutch

Mode 4 eggs, 5 frequent; for Ontario, mean ± SD = 4.0 ± 0.78 eggs, range 1–6, may include some incomplete clutches (n = 288 nests; Peck and James 1987); elsewhere 4.3 ± 0.65, range 3–7 (n = 74; Western Foundation Vertebrate Zoology). No significant differences among pair types (Knapton et al. 1984, Tuttle 1993, JGK) or first and second clutches (Lowther and Falls 1968, Tuttle 1993). In Bradley, Maine, clutch size (mean ± SE), 3.81 ± 0.09 for 59 WS females and 3.78 ± 0.07 for 86 TS females (Brent Horton unpubl. data). See also Breeding: phenology, for occurrence of later broods.

Reproductive Success

Median number of young fledged/nest is 3. Reproductive success varies from one year to next. Following data based on 171 nests found from 1986 to 1990 in Algonquin Provincial Park, Ontario, during incubation: nest success ranged from 30.0% in 1990 to 78.3% in 1987 (mean = 55.2%); hatching success ranged from 48.7% in 1990 to 77.8% in 1987 and 1988 (mean = 66.6%); nestling success varied from 52.6% in 1990 to 91.4% in 1987 (mean = 70.1%); fledging success ranged from 25.6% in 1990 to 71.1% in 1987 (mean = 47.5%) (JGK). These and other data (e.g. Knapton et al. 1984) from Algonquin Park show no differences between pair types at any stage.

Tuttle (1993) also found no such differences through the nestling stage in Adirondack Park, NY, but her data show that more and heavier fledglings were produced by WS male x TS female pairs than by TS male x WS female pairs. In a later study from the same area, Formica et al. (2004) found no effect of morph, territory size, number of neighbors or habitat type on breeding success. No differences in fledgling weights between pair types were found in Algonquin Park (Knapton et al. 1984, Whillans and Falls 1990, Kopachena 1992).

In Adirondack Park, nest success (60%, n = 73) and fledging success (49% for WS male x TS female pairs, 38% for TS male x WS female pairs, Tuttle 1993) lie within range found in Algonquin Park. In Bradley, ME, Brent Horton (pers. comm.) reported nest success (nests producing at least one fledgling) as 46% for 63 nests of TS males and 43% for 91 nests of WS males. TS males fledged an average of 1.57 ± 0.23 chicks for 55 nests, compared with 1.38 ± 0.19 for 72 nests of WS males. Comparable numbers for Adirondack Park were 2.50 ± 0.63 for 11 nests of TS males, and 2.72 ± 0.75 for 16 nests of WS males (Formica et al. 2004). Data for annual reproductive success for specific females not available.

Most annual variation in reproductive success is accounted for by predation. See Breeding: Young Birds.

Lifetime Reproductive Success

No data available.

Life Span And Survivorship

Oldest wild bird observed in Algonquin Park was WS male banded as adult in 1982, found dead within 1 km of banding site in 1988 (bird at least 7 yr old). Oldest bird reported from Bird Banding Lab: 9 yr, 8 mo (Klimkiewicz and Fletcher 1987). Return of banded birds from year to year in Adirondack Park gave a Type II survivorship curve (constant mortality in all age classes). Birds up to 6 yr old included (Tuttle 1993). Return rates vary from year to year. Survivorship and reproductive success of TS females are reduced following harsh winters (Elaina Tuttle pers. comm.).

MAPS (IBP’s program – Monitoring Avian Productivity and Survivorship) showed apparent survival probability (from modified Cormack-Jolly-Seber mark-recapture analyses) in the range of 0.26-0.47 in different portions of the range of the species, 1992-2003; see http://www.birdpop.org/nbii/surv/survresults.asp?strRegion=aa&strSpec=wtsp for details.

Disease And Body Parasites

External parasites of adults include 3 louse flies Ornithoica vicina, Ornithomyia fringillina, and Lynchia americana (Bequaert 1954) and three ticks Ixodes brunneus (Cooley and Kohls 1945), Haemaphysalis leporis palustris, and Ixodes dentatus (Levine et al. 1991). The latter 2 ticks, collected in coastal Virginia, were larval or immature stages, indicating White-throated Sparrows as intermediate host. Haemaphysalis carried the spirochaete Borrelia burgdorferi, etiological agent of Lyme disease (Levine et al. 1991). The mite Syringophiloides motacillae has been reported from Manitoba (Bochkov and Galloway 2001). Cloacal chiggers were found in Prince George, B.C. (Scott Ramsay, unpubl. data).

Internal parasites include platyhelminths and nematodes in the gut and blood-borne parasites. Of 26 White-throated Sparrows taken early in the breeding season in Algonquin Park, 22 were infected with platyhelminths (Brooks et al. 1993). Visceral parasites include the cestode Anonchotaenia quiscali, trematodes Tanaisia zarudryi, Zonorchis alveyi, Brachylecithum nanum (Byrd and Denton 1950, Denton and Byrd 1951) and Prosthogonimus macrorchis (Brooks et al. 1993), and nematodes Aproctella stoddardi and Diplotriaena thomasi (Anderson 1957,1959). Of 182 adult White-throated Sparrows examined in Algonquin Park, 124 carried blood-borne parasites of the protozoan genera Leucocytozoon, Trypanosoma, Haemoproteus,and Plasmodium and a filarial nematode. Of 241 immature White-throated Sparrows, 129 were positive for the above protozoa (Bennett and Fallis 1960). For species of blood parasites found in this species, see also Bennett 1961, Bennett and Cameron 1974, Bennett et al. 1982, Bennett 1992, and Burry-Caines and Bennett 1992.

Biting insects taken from adult White-throated Sparrows in Algonquin Park include 6 species each of blackflies (Simuliidae) and sandflies (Ceratopogonidae) and the mosquito Aedes canadensis (Bennett 1960, Lowther and Falls 1968). All three groups have been implicated as vectors of blood parasites (e.g. Bennett and Fallis 1960). Larval botflies (Protocalliphora metallica) occur on nestlings (Bennett 1957), as do mites. Nests collected in Prince George, B.C. contained many oribatid mites which are detritivores, fewer parasitic tromboculid mites and flies, Cutophyllus sp. (Scott Ramsay, pers. comm.).

For occurrence of above parasites in Adirondack Park, N.Y., see also Tuttle 1993.

Although parasites associated with White-throated Sparrows are well documented, little is known about their effects on survival and fitness of nestlings and adults

Causes Of Mortality

Mortality of eggs and young mainly by predation (see Breeding: Young Birds, Causes of Death). Causes of adult mortality less well known (see Behavior: Predation; also Conservation and Management: Effects of Human Activity). Most losses occur in immature stage, during migration and in winter. Possible causes include exposure, starvation, predation, intraspecific competition (see Behavior: Territoriality and Dominance). Change in morph ratios in sexes from fledging to breeding suggestive of differential mortality with greater losses of WS females and TS males but see Population Status: Numbers.

Range

Natal Philopatry And Dispersal

No information available. Of 65 nestlings banded in Algonquin Park in 1980, none subsequently seen (Knapton et al. 1984)

Fidelity To Breeding Site

Information scanty. In Algonquin Park, Ontario, males more likely than females to return to same breeding locality. Of 47 breeding males banded in 1959, 19 returned the following year, whereas only 3 of 30 females returned (Lowther and Falls 1968). Of 132 males banded in 1979 and 1980, 63 returned the next year; only 7 of 98 females returned (no differences found between morphs). Four females returned to territory occupied the previous year (Knapton et al. 1984). Returns from both studies: 46% for males, 8% for females. By contrast, return rates in Adirondack Park were similar for males and females (overall 60% vs. 46%; for WS 66% vs 40% , TS 66% vs 50%,). On average these birds returned 2.04 yr ± 1.36 SD (n = 51) (Tuttle 1993). Most returning birds occupy their previous territories, but some males and females move; thus return rates are conservative. The difference between localities might reflect comparative isolation of Adirondack Park population.

Fidelity To Winter Range

White-throated Sparrows often return yearly to distinct home ranges that are maintained all winter. See Behavior: Spacing, Territoriality and Dominance for details.

Population Status

Numbers

Breeding Bird Survey (BBS) data provide relative abundance (birds per route) for various jurisdictions, 1966-2006. For the White-throated Sparrow the highest relative abundance averages, for routes on which it occurs, are 71.9 birds (Quebec), 47.2 (New Brunswick), 40.9 (Ontario), 37.7 (Newfoundland), 30.3 (Nova Scotia), 29.0 (Maine). High densities estimated from census plots: 94.7 males/km2 (11 plots) at Harricanaw River near s. James Bay (James et al. 1982); 152 males/km² in a plot in black spruce (Picea mariana) in Algonquin Park (Martin 1960). Averages for plots in various boreal forest habitats: spruce stands 1–86 males/ km², lowest numbers in subarctic and n. British Columbia, highest in young to medium-mature stands from Nova Scotia to n. Saskatchewan; balsam fir dominated stands 15–67; other conifers to 35; western conifers 2; poplar (Populus spp.)-birch stands 1–18 males/km² (Erskine 1977).

Point counts in Ontario show areas of greatest abundance in a broad band across the northern part of the province (McLaren 2007). Erskine (1992) estimated population in Maritime Provinces as 660,000 ± 130,000 pairs. Extrapolation of this figure to main part of breeding range gives a rough estimate of 10–20 million pairs for total North American population. The Atlas of the Breeding Birds of Ontario (Cadman et al. 2007) estimates the population in the province at 12 million birds, which is consistent with Erskine’s overall estimate. Abundance may be underestimated by most methods because cryptic TS males sing infrequently.

Maximum early winter encounter rate 21.9 individuals/h (Root 1988). Highest winter abundances associated with open water; e.g., highest concentration in Alabama around W. Dannelly Reservoir.

Trends

A small significant decline has occurred in the 44% of the breeding range covered by BBS (-0.6%/yr, p=0.00, 1966-2006; -0.8%, p=0.27, 1966-1979; -0.6%, p=0.00, 1980-2006). Trends are highly variable geographically with the main areas of increase in ne. Newfoundland, parts of Quebec, Ontario, Michigan, se. Manitoba, n. Alberta, and especially in British Columbia (Fig. 8). Of 21 jurisdictions with BBS coverage, 5 show non-significant increases, 9 show non-significant decreases and 7 (Connecticut, Maine, Massachusetts, New Brunswick, New Hampshire, Nova Scotia, Vermont) show significant decreases (BBS Trend Results 1966-2006). The greatest declines are in New England and the Maritime Provinces.

Migration stations, especially the Canadian Migration Monitoring Network (CMMN), provide some indication of trends in more than half of the breeding range in boreal forest north of BBS coverage. From the mid-1990s to 2005, 6 of 11 CMMN stations showed declines in spring while 7 of 13 stations showed declines in fall. Long Point Bird Observatory in Ontario, with the longest record (1967-2005), showed small non-significant variation in spring and significant increases in fall (T. Crewe, pers. comm.). Long Point data were moderately negative from the 1960s through the late 1980s (Hussell et al. 1992); since 1987 in fall and 1991 in spring the trend has been positive (D. Hussell pers. comm.). CBC data that cover 95% of the winter range show small non-significant declines (-0.25%/yr, 1965-2005) (Butcher and Niven 2007). Overall, populations of the White-throated Sparrow are stable or show a small decrease with considerable variation across the range.

MAPS programs have attempted to track population change in this species over various parts of its range; as one example -- northeastern US, 1994-2002 -- see http://www.birdpop.org/nbii/prod/prodresults.asp?strRegion=ne&strSpec=wtsp&strYears=9293&strCount=11.

A major extension of the breeding range into central British Columbia has occurred since the 1940s and the winter range has extended into sw. British Columbia since the mid 1960s (Campbell et al. 2001). Slight extensions in summer range reported in New York (Anderle and Carroll 1988) and Vermont (Laughlin and Kibbe 1985) and in winter range west of longitude 95°W into se. Great Plains (Root 1988); winter range otherwise limited by precipitation and average minimum Jan temperatures.

Population Regulation

With limited data, we can only speculate. Suitable habitat is influenced by fire, insect outbreaks, clearing, and forestry practices. Populations tend to increase as forest opens up and decrease as it closes. Increases with spruce budworm (Choristoneura fumiferana) outbreaks and may decrease after cold winters (JBF). Weather affects exposure and food availability (insects, seeds). Predation on eggs and young may or may not have density-dependent effect. Within suitable habitat, territoriality may limit breeding population. Floaters of both morphs, capable of breeding, replace removals of males, females, and pairs prior to incubation and males throughout the breeding season (D. J. Loncke and JBF unpubl. data).