Demography and Populations

Age At First Breeding

About half the yearling males who remain as local residents for at least 28 d (time to complete a breeding effort from nest building to the departure of young from the nest) succeed in breeding; unsuccessful males sometimes move to a new territory, and nearly all older males attract a female and breed (Payne 1982, 1989, Payne et al. 1988). Females regularly breed in their first year (known-age females, banded as nestlings, Payne 1989), and nest repeatedly through a season; renesting intervals are longer early in the breeding season.

Clutch

Usually 3 or 4 eggs, less often 1 or 2. One-egg clutches usually abandoned. The 1- and 2-egg clutches may have had an egg removed by a cowbird. Females nest repeatedly in a breeding season and some lay as many as four clutches (RBP).

Clutch size varies with season, locality, and cowbird activity. In two populations, the most common clutch size was 3; larger clutches were more common early in the season. In nests with no cowbird eggs, clutch size averaged 3.41 ± 0.58 (SD; n = 87 nests) at the George Reserve and 3.13 ± 0.59 (SD; n = 174 nests) at Niles. In nests with ≥ 1 cowbird eggs, the number of bunting eggs was reduced (Reserve, mean = 2.35 ± 0.82 SD, n = 37; Niles, mean = 2.36 ± 0.75 SD, n = 97; RBP). Clutch size is similar in Ontario, where the mode is three (n = 126 nests, mean = 3.23 ± 0.77 SD; Peck and James 1987).

Annual Reproductive Success

Most successful nests produce either 3 or 4 young buntings, though many have 2 and a few fledge only 1, sometimes with a cowbird as well. All females appear to breed in every year they return to the breeding grounds. Most females nest repeatedly, either after a failed nesting or after a successful nesting. In 830 female-years in southern Michigan, where breeding success was determined from birds banded 1979 through 1987, 69% successfully reared young buntings from one nest or more. For all female-years, 56.5% fledged one brood, 12.8% fledged two broods, and 0.1% (one female) fledged three broods (she was a yearling, banded locally as a nestling in the previous year; RBP). These proportions are probably minimal because a female may breed elsewhere in the same season (especially if her earlier nest is unsuccessful); some females had 6 and 7 nests in a season. Marked females often nest again on the same territory; others move > 1 km within a season. Breeding success is independent of age, except that first-year males have lower success than older males in some populations (Carey and Nolan 1979, Payne 1989).

In South Carolina, 59% of nesting females (not banded, but tracked by timing of their nests on a territory, assuming a later nest was by the same female) successfully fledged at least one bunting in a season. Mean number of fledglings per season 2.0 for females nesting in territories with a standard amount of edge habitat (rectangles of 1.4 ha), 1.4 for females nesting in same-area blocks of habitat with 50% more edge, the different success due to nest predation, and not to clutch size or parental care (feeding rates) or number of fledglings per successful nest (Weldon and Haddad 2005).

Lifetime Reproductive Success

From their first year as a yearling through the last year a bird was observed, the number of buntings fledged from nests was determined in two populations in Michigan. The distribution of lifetime success across individuals was similar for the sexes and areas: males, Niles, n = 173, range = 0–26, mean = 4.32 ± 4.58 SD; George Reserve, n = 184, range = 0–22, mean = 3.33 ± 4.21. Females, Niles, n = 182, range = 0–27, mean = 4.36 ± 4.32; George Reserve, n = 178, range = 0–20, mean = 3.20 ± 3.41. A few birds were still alive at the end of study and may have dispersed and bred elsewhere. Although most breeding birds of ≥ 2 years were present on the study area in their first spring, about 30% of the fledglings in any year were produced by buntings that had immigrated into the area at a later age, usually in their second breeding season. The means and variances observed in lifetime breeding success were about the same in males and females, as were the highest values (26 and 27 fledged young, respectively) (Payne 1989). Mean longevity and lifetime reproductive success also were the same in birds breeding on their natal area and in birds that immigrated in their first or second year from another area (Payne and Payne 1993a).

Reproductive success might be a function of both the individual bird and the familiarity and compatibility of members of a bunting pair. 16 pairs bred together for ≥ 3 yr, including 2 pairs for 4 yr and 1 pair for 5 yr. The effect of mate familiarity and long-term pair duration was tested in pairs that bred together for ≥ 3 yr. In 4 of the 16 pairs, a female reunited with a previous mate after being with another male for a year; and in pairs that reunited over several years, the twice-chosen mate was not always the first mating partner for either partner, so females were not notably constant to the old mate. A common theme was that each member of a pair returned to the same territory. In these 16 pairs, reproductive success (number of fledglings) in the season did not change with the number of years the pair had bred together, nor did the more familiar pairs breed earlier in the season, allowing for age (older males breed earlier than first-year males on average). There is neither advantage nor cost in reuniting with a previous mate (Payne and Payne 1996).

Reproductive success also might be accounted for by female assessment of a male or nesting site early in the season. Success of a female with males of large body size or with male size differing in relation to her own size, or males with genetic (allozymes) similarity or dissimiliarity, does not differ from success of females that choose males with alternative traits. Reproductive success is not associated with habitat or with the neighborhood song theme. Buntings of either sex that mate with native, locally-born partners do not differ in success from buntings that mate with immigrant partners. Females that breed with an already-mated male (or a male that later gets an additional female to mate with him) do not differ in reproductive success from females in socially monogamous pairs, perhaps because of the minimal paternal care in any case (Payne et al. 1987, 1988, Payne and Westneat 1988, Payne 1991, 1996, Payne and Payne 1993b, 1996).

In the field, the oldest bunting of known age was observed for 11 yr. Both males and females are known to breed for 9 yr (Leberman et al. 1985, Payne and Payne 1990, RBP).

In Michigan, buntings were successful in fledging in 47.6% of the broods (Payne 1989, Payne and Payne 1998 . No observations on first-year survival, because the young leave their natal area and migrate, and only some return to the natal area. First-year birds are less likely to return in second breeding season in some populations, not in others (Blake 1969, Carey and Nolan 1979, Payne 1989). Survival of older adults is independent of age, with nearly 50% of the color-banded birds disappearing from year to year. For males banded as yearlings in 1979 through 1984 and observed in the same breeding population in Michigan through 1990, expectation of further life (S₀) was 2.08 yr at the George Reserve (n = 260) and 2.41 yr at Niles; annual survival (P_S; % males surviving from one breeding season to the next) was 0.52 ± 0.02 SE and 0.059 ± 0.02 SE. For females, expectation of life (S₀) was 1.50 yr and 1.87 yr, and annual survival P_S was 0.34 ± 0.04 and 0.47 ± 0.02 SE (n = 226 at the George Reserve, 243 at Niles). The lower estimates for females may be due either to mortality or to breeding dispersal, or to both; a few females were seen in a year after they were missed (Payne 1989, Payne and Payne 1990). In another local banded population in North Carolina, annual survival was 0.58 (Blake 1969).

Analyses of BBL recoveries of birds banded from 1955 through 1984 (mainly birds captured in migration and not including local returns) give similar results for male buntings, S₀ = 2.34 years, P_S = 0.57 ± 0.04 SE (n = 60 birds recovered away from the banding site). The similarity of survival estimates from observations of local breeding birds and recoveries of banded birds indicates that estimates of survival in local populations were not biased by local dispersal (Payne 1990, Payne and Payne 1990).

Mortality

No quantititative data on sources of mortality. Breeding males found dead on road, hit by trucks and cars (in UMMZ, seven males and two females were found injured or killed), likely while the males sang. Nocturnal migrants are killed flying into lights, windows, and television towers (Tordoff and Mengel 1956, FMNH). Light weight and lack of fat in birds after arriving migration (Moore and Yong 1991, Moore 1999 suggests stress and risk of starvation in migration. More than 400 bunting specimens in FMNH were killed by flying into one building in migration season, recovered in early morning after nocturnal flight, and four were found dead washed onto shore, apparently killed at night in a storm while flying over Lake Michigan (Dave Willard, pers. comm., FMNH). Wintering buntings in Mexico are trapped by local people for local people; banded birds are unreported (G. Hill, pers. comm.).

Diseases And Body Parasites

Hippoboscid flies occur on some adult buntings. Details of diseases and parasites unknown. Buntings are hardy songbirds. One male, banded as a yearling with one eye missing, returned the next year to the same breeding area (RBP).

Initial Dispersal From Natal Site

In a population where > 150 banded bunting nestlings have returned to their natal area, about 10% of banded fledglings return to breed within 1-2 km of the natal site (Payne et al. 1987, Payne 1991, Payne and Payne 1997). Most individual birds that bred in two study areas of 4 km²and 10 km², where all the surviving nestlings had been banded for the previous four years, were not born on the same area and so had immigrated. About 12% (Niles) and 2% (George Reserve) of the breeding adult birds during the next 3 yr were born on the area where they bred. The 12% estimate for Niles is minimal for an undisturbed population, because many nestlings were biopsied, and these birds were less likely than the unbiopsied nestlings to return next year (Westneat et al. 1986). For birds that return to their natal area, the site of the first territory is independent of the return of kin, either same-sex or other-sex parent or sibling. Inbreeding is rare, and the low proportion (1% of matings observed where kinship was known) is a consequence of natal dispersal; nearly all birds settle away from their natal territory (Payne 1991, Payne and Payne 1997).

The low proportion of birds returning to breed in their natal area and the mortality rate of adults indicate that most surviving birds banded as nestlings settle and breed far from their natal site. Most breeding birds in the local area were immigrants born outside the study area. Other survivors of their cohorts, as inferred from demographic parameters, apparently breed > 1 km from their natal area. Long-distance natal dispersal from birth to breeding area is known directly by two banded nestlings that were recovered at 52 and 350 km from the natal site (Payne 1991). The overall distribution of disperal distances beyond the natal area is unknown.

Fidelity To Breeding Site And To Mate: Breeding Dispersal

Buntings often return to breed on territory of previous year, but some birds settle on a new site. In Michigan, breeding dispersal varies with sex and age more than with breeding success. Males that return to their breeding area usually settle on their territory, regardless of previous breeding success in the previous year. For males that were territorial as yearlings and returned the following year, most (165 of 232, 71%) returned to their old territory and 29% settled on a different one in thier second year. For 316 returns of color-marked males that were ≥ 2yr, only 13 (4%) moved to a different territory the next year (Payne and Payne 1993a). A few males, mostly first-year birds, move from one territory to another within a breeding season, defending each in turn and not holding two territories at the same time (Carey and Nolan 1979, Payne et al. 1988, Payne and Payne 1989, Payne 1989).

Returning females were more likely to settle on a different territory than were males. For 313 marked females that returned to their breeding area in successive years, nearly half (139 or 44%) moved to a new site. Females that were successful in the previous year returned to their old site in 141 cases of 244 (58%) and unsuccessful females returned to their old site in 33 of 69 cases (48%); the difference was not statistically significant (Payne and Payne 1993a).

What accounts for variation in breeding dispersal? Three hypotheses were tested to account for breeding dispersal of returning buntings between seasons. First, if variation in dispersal is heritable, then buntings whose parents were born elsewhere should disperse from their natal area to their breeding area, and buntings whose parents returned to their own natal area should return to the same area. Second, if females seek out their successful mate of the previous year, then they are more likely to re-mate with him when he returns, and to choose a new male when he does not. Finally, if buntings disperse in relation to their previous breeding success on the site, then they are likely to return after a successful season, and disperse after an unsuccessful season, even though they are more familiar with the old site.

To test heritability, behavior of a parent and offspring were compared in the two study areas in s. Michigan. In 30 cases both the returning nestling and its parent of the same sex survived to return at least two seasons. There was no significant association of breeding dispersal between offspring and parent of the same sex (Payne and Payne 1993a), and so no evidence of heritable behavior differences.

To compare the effects of mate and breeding success on a nesting site, breeding dispersal was compared with the availability of the old mate and the success of the birds on the old site. Nearly half the males returning from a first-year season settled in their second season on a new site, but nearly all the older adult males returned to their old site. First-year males were more likely to disperse after a failed breeding season than after a successful season; older adult males were not more likely to disperse. Females tended to return to the old site, yet they were not significantly more so when they were successful than when they were unsuccessful. At Niles, 64 of the 96 successful females that returned tot he old site mated with the old male, and only one of these successful females mated with a new male. Nine of 26 unsuccessful females that returned to the old site were with the old male, and all 22 on the new site had a new mate. At the George Reserve, 29 of the 45 successful females that had returned to tehir old site had the same mate, and all but one of the 39 females on the a new site had a new mate. Five of the 11 unsuccessful females on their old site had the same mate, and all 10 on a new site had a new mate. In summary, female return to the breeding site depended less on her breeding success in the previous season than on whether her old mate returned to the site.

Females appear to be more strongly attracted to a familiar site than to an old mate, regardless of past success on the old site. The advantage of site familiarity may be more important in choice of a nesting site than the past loss of a brood on the site due to a one-time hit by a predator. Female breeding dispersal and mate change also may be determined by a year-to-year change in the details of habitat on their breeding sites. By way of comparison, male breeding dispersal may be constrained by the benefits of social familiairty with their male neighbors, as when they share song themes (Payne and Payne 1993a, 1996).

Fidelity To Winter Site

Birds remain on winter site within a season, and birds are recaptured through winter in the same site (Loftin et al. 1966, Johnston and Downer 1968, Fisk 1979, Kricher et al. 1986). Banded birds returned in successive winters to that site in Florica, Jamaica, Mexico, Belize, Guatemala, Honduras, and Costa Rica (Van Tyne 1932, 1935, Nickell 1968, Johnston and Downer 1968, Downer 1972, Fisk 1979, Rappole and Warner 1980; BBL).

In Jamaica, at least 30 of 172 (17%) of buntings banded at a feeding station one winter and returned the next winter, and seven returned for four winters (Johnston and Downer 1968). In Belize (Great Fall Estate) 9 of 576 buntings banded one winter were recaptured four and five years later at the same site (BBL). In southern Florida, where migrants and wintering birds were banded at a feeder, 130 of 666 (20%) banded buntings returned from year to year, and one returned for seven years (Fisk 1979). One bunting moved to a different site in successive winters: a male banded and recaptured through the winter on a barrier island off Florida in the Gulf of Mexico was recovered next winter on Andros Island in the Bahamas—he was recovered a year later in the breeding season in Nova Scotia (Stedman and Stedman 1988).

Numbers

Breeding bird censuses across eastern North America suggest average density about 35 pairs per 100 ha (Johnston 1965, Taber and Johnson 1968). Estimates vary with region, habitat, and method of census. In linear transects, 6.9 adults estimated per 100 ha in s. Illinois, 3.2 in central Illinois, and 1.6 in northern Illinois, with the highest densities in edge and shrub habitats (119 per 100 ha is s. Illinois; Graber and Graber 1963). In area sampling, densities ranged from 2 to 25 per 100 ha in Maryland (Stewart and Robbins 1958 and averaged 18 pairs per 100 ha in Missouri (three years after forest clearing; Kabrick et al. 2004), 28 per 100 ha in North Dakota (Faanes and Andrew 1983, and 38 per 100 ha in West Virginia (Hall 1983). In color-marked populations in s. Michigan, about 35 breeding pairs per 100 ha near Niles, and 15 pairs per 100 ha near the George Reserve (Payne et al. 1988, Payne 1996, Payne and Payne 1996, 1997).

Breeding bird surveys (BBS; Robbins et al. 1986; Fig. 7) from 1966 to 1988 showed an average of 21.97 birds per route (mean of 12 routes) for all BBS routes within 100 km of the Niles study area (Michigan and Indiana routes), and 9.61 birds per route (mean of 12 routes in Michigan) for all routes within 100 km of the George Reserve. On sample routes taken with an experienced BBS surveyor, all but one bunting detected were singing males, half were noted within 100 m of the sample spot, and all but one were within 200 m, so the likelihood of detecting a bunting decreased between 100 and 200 m. If half the breeding pairs present were detected within 200 m at each stop, then 200 m is the 50% detection distance or radius r (Payne 1990). The area within which 50% of the pairs are sampled at a stop is π r², or 0.126 km². The number of pairs per route of 50 stops is 50 π r², or 6.3 km². Where n is the number of birds per BBS route, the density of pairs is 2 n / 6.3 km². With this reasoning, the density of buntings near Niles is 2 x 21.97/6.3 km², or 6.7 pairs/km², and density near the George Reserve is 2 x 9.61/6.3 km², or 2.89 pairs/km², similar to those in Illinois (Graber and Graber 1963). Bunting densities were higher in the study areas than in s. Michigan at large.

To estimate the population size of buntings in North America, the number recorded in a year of BBS were compared with land area and with the detectability of birds in roadside counts. A total of 19,413 birds were counted in 1,832 BBS routes in 1977 (Robbins et al. 1986). The total land area is about 1.2 x 10⁷km². An approximate population size is estimated as:

2 x 19,413 x 1.2 x 10⁷km²/1,832 x 6.28 km²

or about 40 million pairs. This may be an overestimate because BBS survey routes were concentrated in e. North America where urbanized areas are under-represented in the routes; nevertheless the survey routes are biased in favor of the buntings as these routes sample roadsides, which buntings prefer for habitat. Ten million pairs may be a better estimate.

BBS data show highest densities in s. Missouri, Kentucky and Tennessee, where this species may be the most common of all passerines (Robbins et al. 1986) and on coastal plains in Virginia and North Carolina. Local in w. North America. Wintering densities in tropics appear higher than breeding densities; wintering area is smaller than breeding area, and the young of the year swell the winter populations. On the Yucatán Peninsula (Mexico), Indigo Buntings are "extremely abundant in old fields, thickets, and heavily disturbed forests through-out," with densities as high as 137 birds per 100 ha (Waide et al. 1980).

Trends

Geographic range and breeding densities increased through most of the twentieth century, on a continental basis. Historical densities, compared in linear transects at 50-year intervals, estimated to have increased by 47% from 1909 to 1958 in s. Illinois (Graber and Graber 1963 . Numbers increased or remained nearly the same through 1965-1979 in eastern and central North America (Robbins et al. 1986). Over 50 years buntings increased by more than 30% in the southern Appalachians, when forests were cut and brush habitat became available (Haney et al. 2001). Local decreases and extinctions occurred mainly in areas of increasing forest growth in the Northeast (Taber and Johnston 1968). Range and numbers increased in Florida from 1900 through the early 1960s (Johnston 1965), New Mexico (populations established in the early 1960s, Hubbard 1978, in Arizona first noted in 1940s and bred locally by 1970s (Phillips et al. 1964, Monson and Phillips 1981), and locally elsewhere in w. North America. However in the last quarter of the twentieth century, numbers declined in e. North America as indicated in BBS (Robbins et a. 1989, Sauer et al. 2003, http://www.mbr-pwrc.usgs.gov/cgi-bin/atlasa99.pl?05980%1%05), and in mist-net monitoring of migrant birds in spring and in autumn (Lloyd-Evans and Atwood 2004). In wintering area, bunting numbers increased in the middle of the century; they were first observed in Jamaica in 1946, second in 1947; abundant there in the 1960s (Tordoff 1952, Johnston and Downer 1968).

No social regulation of natal recruitment suspected, for the following reasons: buntings are not colonial breeders, male territory size is highly variable, no aggressive behavior is seen betwen females, all females generally breed (Payne 1989), more than one female may nest on a male’s territory (Carey and Nolan 1979, Payne 1982, Payne et al. 1988), and fertilization is not always effected by the resident male (Westneat 1987b, 1990, Payne and Payne 1989). Numbers appear to depend largely on the availability of suitable brushy and weedy habitat.