Demography and Populations

Measures Of Breeding Activity

Age at first breeding; Intervals between breeding

Apparently breed as 1 year olds—first spring after fledging (MC). Breed annually.

Clutch

Number of eggs: Nests found before completion of egg laying: in Missouri, mean = 3.96 ± 0.59 SD, mode = 4, range 3–5, n = 47. Five-egg clutches not found after mid-Jun; Jul/Aug clutches typically 3 eggs (DEB). In Pennsylvania (1987-2006): mean = 3.65 ± 0.59 SD, mode = 4, range 2–5, n = 469. Five egg clutches not found after May; 76 of 81 July clutches (94%) 2-3 eggs (mode = 3) (MC). In Michigan (1939-1948): mean = 3.37, mode = 4, range 2-5, n = 446 (Walkinshaw 1978).

Number of clutches per breeding season: Nest predation or desertion force frequent renestings. In Iowa, for pairs mated all season, there was an average of 4 nesting attempts/pair (Crooks and Hendrickson 1953); in Pennsylvania a mean of 2.9 ± 0.91 SD nesting attempts/season/female (range 1–5, n = 98; MC). Best (1974b) described one female that made 10 nesting attempts in one season, laying a total of 28 eggs. If early nests (before end of June in Pennsylvania) are successful, a second brood is initiated (MC).

Annual and lifetime reproductive success

Table 3 summarizes data on annual reproductive success in 2 long term studies (Walkinshaw 1978, MC). For all measures, mean success in Pennsylvania was higher than in Michigan (e.g. fledging success [proportion of nests fledging young] in Pennsylvania - 45%; in Michigan – 38%). Comparable fledging success rates have been found in other short term studies (44% - Batts 1961; 35% - Crooks 1948; 27% - Nolan 1963). In an Illinois population, however, only 10% of nests successfully fledged young (Best 1978).

Hatching success (proportion of nests hatching young) was 29% higher than fledging success in Pennsylvania; in Michigan, 40% higher. Difference indicates that in the Pennsylvania population a higher proportion of nests failed prior to hatching than in Michigan. In Pennsylvania fewer nests failed completely after hatching – 139/939 - 14% (MC); in Michigan 93/379 – 25% (Walkinshaw 1978). In Pennsylvania, 312 of 430 breeding females (73%) fledged at least one young each year; the average female fledged 3.0 young per year (see Table 3).

There was considerable annual variation in fledging success (and other success measures) in the 2 populations, ranging from 20-63% in Pennsylvania; 29-62% in Michigan (Table 3). In Pennsylvania, fledging success showed a significant decline over the 20 yr of the study (r = -0.46, df = 18, p = 0.04). Hatching success and proportion of successful females showed near significant declines with year; p for both = 0.06 (MC). Significant successional changes occurred on the Pennsylvania fields leading to steady decreases in population size with year (see below). The decrease in annual reproductive success is likely related to this successional decrease in habitat quality (MC). The Michigan population showed no significant change in any measure of reproductive success with year, e.g. fledging success: r = -0.43, df = 10, p = 0.16 (Walkinshaw 1978). Walkinshaw (1978) in his habitat description made no mention of successional changes in his study site. This possible lack of successional habitat change during the Michigan study may be the reason for the absence of significant changes in reproductive success measures.

In Pennsylvania using only banded birds: female age (see LifeSpan and Survivorship, below) had no effect on annual reproductive success. Minimum fledgling production in 1st time breeders: mean = 3.04 ± 2.22 SD, n = 249; maximum in females ≥ 5 years: mean 4.33 ± 2.65 SD; but no significant differences with female age (Kruskal-Wallis ANOVA, H = 6.63, df = 4, p = 0.16 (MC). First year males on the site had lower fledgling production, mean = 2.40 ± 2.24 SD, n = 246; males older than one year in all age classes produced on average more than 3.5 fledglings/yr. The difference is significant (Kruskal-Wallis ANOVA, H = 21.72, df = 5, p < 0.001). The difference results primarily from many 1st year males remaining unpaired during their time on the study site; almost all older males acquired a mate (MC).

In Pennsylvania, lifetime reproductive success (LRS) -- measured as number of fledglings produced per adult over all breeding years on the study site -- ranged from 0-35 for males; 0-31 females. LRS is strongly associated with lifespan (number of breeding years on the site) in both males and females. Males breeding only one year on site: mean fledgling production = 2.73 ± 2.07 SD, n = 104; males breeding ≥7 yr on site: mean = 29.5 ± 5.65 SD, n = 4. Females breeding 1 yr on site: mean = 2.62 ± 2.27 SD, n = 189; females ≥ 5 yr: mean = 23.5 ± 6.89 SD, n = 6. Correlation between LRS and number of years breeding on site is highly significant for both genders. Males: r = 0.84, df = 204, p < 0.001; females: r = 0.83, df = 273, p < 0.001).

Life Span And Survivorship

Determined by return rates of banded birds to study site in subsequent years:

Juvenile Survivorship

No data between hatch year and first breeding year; with few exceptions, young birds do not return to the natal site to breed (Walkinshaw 1978, MC). Walkinshaw (1978) estimated that ~50% of fledglings died before fall migration, but gave no data to support that estimate.

Adult Survivorship

Males: In Pennsylvania (1987-2006): 206 banded males defended territory on the site for all or part of a breeding season; 102 (50%) returned for a second season; 53 (26%) for 3 yr; 27 (13%) – 4 yr; 13 (6%) – 5 yr; 7 (3%) -- 6 yr; and 2 each for 7 and 8 yr (MC). Annual variation in male return rates: 18-80%; on average 53% ± 19 SD of males breeding in one year returned to breed in the next year, regardless of age (MC). No correlation between annual male return rates and year (MC). Walkinshaw (1978) had similar return rates in Michigan; maximum male life span – 6 yr. Male return rates are probably a good estimate of survivorship because breeding males (especially after their first year) typically stay on territory for the entire season, and such males return to virtually the same territory year after year (MC).

Females: In Pennsylvania (1987-2006): Female return rates lower than male. 274 banded females built at least 1 nest on the site; 85 (31%) returned for a second season; 34 (12%) for 3 yr; 16 (6%) – 4 yr; 6 (2%) – 5 yr; 4 – 6 yr; and 1 – 7 yr (MC). Annual female return rates ranged from 15-48%; on average 36% ± 10 SD of females breeding in one year returned in the next (MC). Female return rates not associated with female age or year (MC). Walkinshaw (1978) also had lower female return rates; maximum female life span - 6 yr. Females are less likely to remain on the breeding site for an entire season, often disappearing after nest failure (MC). They are also less likely to return to the same breeding territory in subsequent years; thus some females bred on the study site, were absent for 1-2 yr, then bred on the site again - no such males were seen (MC). Female return rates, then, are likely an underestimate of actual female survivorship.

Maximum longevity record at the US Bird Banding Laboratory is 8 yr-9 mo (See http://www.pwrc.usgs.gov/BBL/homepage/long4930.htm). Annual survival rate estimates (genders not separated) from the MAPS banding program range from 0.35 ± 0.06 SE in the Southeast region to 0.48 ±0.03 SE in the South-central region; North-central region estimate – 0.43 ± 0.04 SE; no data for Northeast region (see Michel et al. (2006) for methods and protocols).

Disease And Body Parasites

Fifteen species of ectoparasites and one species Plasmodium reported (Peters 1936, Herman 1944, Malcomson 1960, Sutton 1960, Nolan 1963); none found in any nest in PA (n = 913, MC). In Pennsylvania, R. Smith (pers. comm.) found a mean of < 25 feather mites (Acari: Astigmata)/adult wing (n = 52).

Causes Of Mortality

In Pennsylvania (1987-2006), adult mortality on the breeding site was minimal: one adult known to have died (cause unknown); 4 more disappeared from active nests and were presumed to have died (MC). Fretwell (1968) considered conditions on the winter range to be the major cause of adult mortality.

Exposure

Cold temperatures sometimes cause nest failure, e.g., in Iowa, a 3-d cold spell with rain, sleet, and snow caused desertion of 3 nests with eggs and loss of 7 of 8 fledglings (Crooks and Hendrickson 1953); similar conditions in late May 1992 in Pennsylvania led to desertion of 14 of 18 active nests (MC). Hot sun on poorly shaded nests can cause early fledging and death of young (Walkinshaw 1968a). Severe storms may topple nests; 1993 flooding destroyed nests in Missouri (DEB).

Predation

The major cause of nest mortality; 112/147 nests predated in Illinois (Best 1978), 24/66 in Indiana (Nolan 1963), 377/939 in Pennsylvania (MC). Probability of predation highest during incubation (Best 1978, MC). On rare occasions brooding or incubating females also taken by predators (Walkinshaw 1968a). Fretwell (1968) noted a trend toward decreased nesting success with increased nest density, but Best (1978) found no such correlation in Illinois. In Pennsylvania, population has decreased annually, but nest success has also decreased slightly (MC).

Human/Research Impacts

Infrequent victim of tower kills during migration compared to other species; 90 of 96 killed over 25 yr at a Florida TV tower were found during fall migration (Crawford 1981). Many probably killed by cars, but no data. Deaths attributed to trapping, banding, and handling are negligible. Unknown if investigator visits cue predators to locations of nests. Interruption of blue racers preying on nestlings by researchers in two separate events did not deter snake from returning (Best 1974a, DEB).

Range

Initial Dispersal From Natal Site

Rarely return to natal sites to breed. Of 100 fledgings banded in N. Carolina, one recovered on site in winter, one other nested on natal site the following summer (Fretwell 1968); 6 of 1,826 banded in Pennsylvania recovered later in natal site (MC); in Michigan 1 of 514 banded nestlings returned (Walkinshaw 1978). Dispersal to new sites apparently occurs prior to fall migration (see Breeding: immature stage).

Fidelity To Breeding Site And Winter Home Range

See Longevity and Survivorship above. Surviving males typically return to same territory used previously: in Illinois (1971–1972), 11/25 (44%) resident males returned to same territory (Best 1977b). In New York (1985–1989), 33/61 (55%) males returned (Nelson 1992). In Pennsylvania (1987–2006), 208/414 (50%) males returned to same territory in following year; annually percentages varied from 18-80% (MC). Female returns over the same period were lower: 146/420 (35%), annually varied from 15-48% (MC). Individuals that did not return were not known to have bred elsewhere and were presumed to have died. Females less likely to return to same breeding territory (Best 1977b, MC).

No data on fidelity to winter range, though birds that do not migrate often stay year-round in the same area (Fretwell 1968).

Population Status

Numbers

Density varies with habitat. In pine planting with bluestem in N. Carolina, spot mapping showed 23 males in 5.95 ha; in pines with pasture, 14 males in 10.12 ha (Fretwell 1970). In Maryland, 17 males/100 ha in damp deciduous scrub with standing dead trees; 195/100 ha in old field; 197/100 ha in apple orchard with unmowed ground cover (Stewart and Robbins 1958). On a 40.5 ha abandoned farm in Michigan (1938-1948), density varied from 22 – 49 males (54-121/100ha), with highest numbers midway through the interval (Walkinshaw 1978).

In Pennsylvania old fields, numbers varied with successional age of the fields: 1) fields abandoned in 1972 (16.4 ha) - 12 breeding territories in 1987 declining fairly steadily to zero in 2003; 2) abandoned in 1978 (12.4 ha) - 17 territories 1991 to 3 in 2006; fields where hay mowing stopped 1987 (5.0 ha) - 4 territories in 1988, to a maximum of 12 in 1994, declining to 2 in 2000 when fields were mowed once again; 4) fields where tomato cultivation stopped in 1998 (12.1 ha) – 1 territory in 2000 rising to a 10 territories in 2006 (MC). The general trend is that Field Sparrows begin breeding within 1-2 years after human uses stop; population sizes rise for perhaps a decade, then decline once again. After ~30 years of old field succession, the habitat is overgrown with trees and shrubs and no longer used for breeding (MC).

Breeding Bird Survey data from 1994-2003 suggest highest densities (>30 birds/BBS route) in Missouri, Oklahoma, e. Nebraska, and in the Ohio/Tennessee River watersheds; lowest densities (<1 bird/BBS route) around the periphery of the species range (see Fig. 6, Sauer et al. 2005).

For numbers in early winter, see Sauer et al. 1996; http://www.mbr-pwrc.usgs.gov/bbs/cbcra/h5630ra.html. Note highest numbers (30-100/CBC route) in e.-central Texas, n. Alabama and Georgia, e.-central Carolinas, and ne. Virginia.

Trends

Species may have had greatest abundance in late 19th century, after felling of eastern forests and before increase in woodland, agriculture, and suburban development (Arbib 1988, Peterjohn and Rice 1991). See Numbers, above.

Based on Breeding Bird Survey data, significant declines in breeding populations, 3.2% per year (p < 0.00001) between 1966 and 2003 (1,789 BBS routes). Declines at a rate >1.5% per year through most of the species range. Numbers increasing only in areas around the periphery of the range (small numbers there). Within the range, increases at rates >1.5% per year are noted only in the prairies, Kansas through S. Dakota (See Fig. 7 - Sauer et al. 2005).

Population Regulation

Breeding success is affected primarily by predators (See Predation, and Measures of Breeding Success above). Although data on fledgling survivorship are minimal, fledging success in Michigan and Pennsylvania appears to be sufficient to maintain stable breeding population sizes (Walkinshaw 1978, MC). Causes of adult mortality not understood, but few adults are lost during breeding (Walkinshaw 1978, MC; see also Causes of Mortality above). Since there is ~50% mortality of adult males from one breeding season to the next (see Longevity and Survivorship above), adult mortality is primarily associated with migrations and/or the winter season (see also Fretwell 1968). No data on adult mortality rates and causes during non-breeding periods.

In much of its range, optimal breeding habitat is ephemeral, and the availability of such habitat appears to be the primary factor affecting breeding numbers. In Pennsylvania old fields, a few individuals began breeding 1-2 years after mowing or tillage ceased. But on fields 30 years out of cultivation, no breeding Field Sparrows remained. Numbers peaked ~10 years after human use ceased (MC). In New York, Lanyon (1981) also found breeding on fields 1-2 yr old, but after 18 yr the fields were no longer used. Given such short-lived breeding habitat, it is likely that the species can rapidly disperse and colonize new breeding habitat as it becomes available. However, data directly relating to dispersal rates and colonizing abilities are not available.