Demography and Populations
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Measures of Breeding Activity
Duration Of The Breeding Season And Within-Season Re-Occupation Of Nesting Cavities
The probability of renesting varies greatly; strong evidence of differences based on latitude. In eastern N. America, breeding season lasts 108 d in the most northern part of the range, 149 d in the southern part; in the south at 30°N, nesting boxes had a 17-33% higher probability of repeated nestings than nesting boxes at 48°N (Cooper et al. 2005).
Age At First Breeding; Intervals Between Breeding
In S. Carolina in 1977–1979, mean age at first breeding for females was 305.6 d (n = 26); for males, 320.6 d (n = 34; Gowaty 1980). In Tennessee, mean age at first breeding 312 d (range 243–370, n = 24); older females lay 11 d earlier, on average, than first-year females (Laskey 1943a).
Interval between fledging of 1 brood and laying of the next can be as long as several weeks. Typically, female time to renesting is about 2 wk after successful fledging. We have observed a female still feeding nestlings, but building a new nest in an adjacent nesting box and laying eggs in it before young from her previous brood fledged (PAG, JHP).
Modal clutch size varies by latitude and by early versus late brood periods within nesting seasons (Peakall 1970, Dhondt et al. 2002). In S. Carolina, 5-egg clutches are most common in spring, 4-egg clutches in summer (PAG). In Wisconsin, mean clutch size for 1968–1994 was 4.2–4.6 eggs, except for 1980, when it was 3.9 (Radunzel et al. 1997a). See also Breeding: eggs, above.
Data from unbanded birds reported by citizen scientists show that at southern latitudes, where Eastern Bluebirds are year-round residents or partial migrants, clutch size increases until near the mid-season, after which it declines. In the north, by contrast, where Eastern Bluebirds are resident only during the breeding season, clutch size is largest at the start of the season and declines continuously thereafter (Dhondt et al. 2002). Constraints on breeders are likely to be much more important for non-resident than resident breeders, so that strong interaction effects between age (experience) and migratory habits of females may explain the geographical variation in clutch size.
Annual And Lifetime Reproductive Success
About 83% of eggs laid hatch, and 75–90% of hatchlings fledge. Overall nest success (nests fledging young) = 55–84%. See the extensive summary of data from multiple studies in Radunzel et al. 1997a.
Average number of young/successful clutch and pair in Michigan (Pinkowski 1977b) 3.73 ± 1.13 SD (n = 299 nests); in S. Carolina, 1985–1991, 3.61 ± 1.16 SD (n = 1,520; PAG, JHP).
In S. Carolina and Michigan, no differences in percentage of successful nests of SY (second-year) or ASY (after-second-year – fully adult) females or males, including survival rates of fledglings of SY or ASY birds or of clutch sizes of females, nor as a function of whether females were paired with SY or ASY males (Pinkowski 1977b, Gowaty 1980). However, productivity of SY adults was 4.3 young/pair/season; for ASY adults, 5.7 (Pinkowski 1979a). In S. Carolina, SY birds fledged fewer offspring than older breeders, and ASY birds were more likely to successfully complete nesting attempts begun early in the nesting season (Plissner and Gowaty 1996). SY males are more likely to have young from extra-pair paternity in nests on their territories (Gowaty and Bridges 1991a), although SY and ASY females are equally likely to have multiply-sired broods. Individuals that nest near their natal sites are as successful as those that disperse farther (Plissner and Gowaty 1996).
No published data on lifetime reproductive success or differences between males and females—a surprising fact, given how frequently individuals and populations are banded. There are also no comparative data on lifetime reproductive careers, showing the means and variances in durations over a lifetime of significant breeding events for individuals.
Number Of Broods Normally Reared Per Season
In Canadian boreal forest and in Florida, most individuals attempt only 1 brood/season; in most localities, 2 broods/yr are most common; in central portions of the range, 3 brood attempts are not uncommon. Occasional records of females with 4 and 5 successful broods/season. In Michigan, season-long residents average 2.14 nests/yr (n = 357), but only 21% of 357 birds successfully reared 2 clutches (Pinkowski 1979a). In contrast, in a more sedentary S. Carolina population, 1985–1991, 53.8% of 923 successful females had 1 nest/season, 36.5% had 2, 9.4% had 3, and 2% had 4; on average over 7 yr, mean 71 females ± 30.5 SD had 1 successful brood; 48.1 ± 19.5 SD had 2; 12.4 ± 7.1 SD had 3; and 1 had 4. Mean number of successful nests/female during 1993 in Athens, GA, was 1.61 ± 0.65 SD (n = 34; PAG and JHP).
Sex Ratio Of Nestlings And Fledglings
On the basis of multiple studies from many study sites with sampling throughout the local nesting season, sex ratio at hatching and fledging is consistently biased toward females (Gowaty 1993), in contrast to results of studies that did not control for partial-season records (Lombardo 1982). Evidence of parental manipulation of sex ratios in some studies indicates that parents “adjust” numbers of male and female offspring in response to resource levels (Pinkowski 1977b, S. B. Isenberg, R. C. Hensley, and K. G. Smith unpubl. data) or parental circumstances (Gowaty 1980). In S. Carolina during the late 1970s, spring broods significantly favored female offspring: Sex ratio for first-brood nesting attempts was 42% males (n = 437 sexed nestlings). In addition, 5-egg clutches, the most common clutch size in spring broods, had significantly more females (45.6% males, n = 498 sexed nestlings; Gowaty 1991). In S. Carolina, 1985–1991, female offspring significantly outnumbered males in broods fledging before the median fledging date for each year. In early-season broods, males are more likely than females to return to natal areas to breed (Plissner 1994), so potential competition between male offspring and parents is a likely selection pressure favoring female-biased sex ratios in these broods. In S. Carolina, significant negative correlations found between spring and summer nestling sex ratios for uniquely marked individual parents rejected the hypothesis that parents “specialized” and always produced male-biased or female-biased sex ratios (Gowaty 1980).
Mechanisms associated with sex ratio variation are not understood for this or other avian species, but negative correlation between sex ratios in broods of individual mothers for earlier and later broods within seasons suggests that experimental manipulations of earlier-brood sex ratios may affect broods produced later (Gowaty Gowaty 1980, Gowaty and Plissner 1997).
Other forms of sex ratio bias include differential feeding to sons versus daughters as a function of the “quality” of one sex of parent or the other. In an experimental study of parental bias in feeding behavior towards “fledglings” (nestlings near fledgling age), both fathers and mothers more often fed their sons if their mate was more ornamented. In other words, fathers fed their sons more often than their daughters, if their mother was more ornamented. Mothers fed their sons more often than their daughters, if their father was more ornamented (Ligon and Hill 2010).
Life Span and Survivorship
A female and a male, purchased in adult plumage, lived 10 yr in captivity (Butler 1907). From an examination of published bird banding records, 1 wild bird lived a minimum of 6 yr and 6 mo. Study sites at Clemson, SC, have yielded multiple records of 6- and 7-yr-old adult females and males, and 1 record of an 8-yr-old male (PAG, JHP).
In S. Carolina during a 6 yr study of uniquely color-marked individuals, weighted means for between-year survivorship of breeding females (38.2%, n = 760) were not significantly different from survivorship of breeding males (40.7%, n = 717; Plissner and Gowaty 1996). In S. Carolina study observers sampled large study sites in 2 contiguous counties 6 km apart as well as a third study area about 18 km from the other 2 sites. Intensive, systematic sampling off the main study sites reduced the likelihood that “dispersal” was mislabeled as mortality. In Wisconsin, 47% between-year adult survivorship (Pinkowski 1971a).
In a study of adult and first year survival rates of Eastern Bluebirds in two piedmont locations (Athens, GA and Clemson, SC), 2001-2004 (PAG, P. Lang). Based on 2,692 banded and recaptured adults, investigators used program Mark to estimate survival of adults by age and sex. Between-year variability was greater in Clemson than in Athens. In Athens and Clemson, survival probabilities were low (10 and 20% in Athens, but consistently less in Clemson). In Athens and in Clemson males had greater probabilities of survival than females.
MAPS survivorship data are extensive for this species (http://www.birdpop.org/nbii/surv/survresults.asp?strRegion=ne&strSpec=eabl), with ca. 46% annual survival suggested for the Northeast.
Disease and Body Parasites
External parasites include lice (Philopterus sialii, Ricinus sp.); flies (Ornithomyia anchineuria) and mites (Analgopsis sp. and Dermanyssus prognephilus; Peters 1936); eye worms (Oxyspirura pusillae; Pence 1972a); nasal mites (Sternostoma siliphilus and Boydaia spatulata; Pence 1973b); and trematodes (Collyriclum faba; Kibler 1968, Pinkowski 1976c). In some (but not all) populations of Eastern Bluebirds, blowflies (Diptera: Calliphoridae of genus Protocalliphora) parasitize nestlings and suck on blood while attached to wing- and tail feathers, feet, legs, bill, and abdomen (Mason 1936). In Michigan during the early 1970s, 82% of nests in natural cavities (n = 17 nests) and 85% in nesting boxes (n = 84 nests) were parasitized, but mean number of larvae in nests in artificial cavities (91.4) was greater than for natural-cavity nests (51.5; Pinkowski 1977a). Interaction of the presence of blood-sucking blowfly larvae, cold weather, and rain affect the likelihood of catastrophic nestling loss, because parasitized nestlings require more brooding, which becomes extremely difficult when adults must forage under severe weather conditions (Mason 1944). In a large-scale (325 nests) study in NY State, however, there were no statistical differences in survival or rate of development of parasitized or unparasitized nestlings (Wittman and Beason 1992). A secondary parasite, a carrion fly (Mormoniella sp.), is attracted to smellier nests, such as those with dead birds or very large numbers of occupants.
In a nicely designed, large-scale experiment in N. Carolina in which investigators erected nest boxes with easily defendable two-cavity territories, the boxes were on poles 1 m apart so that settling bluebirds could choose to nest between the two. Thus, at the beginning of the season settling bluebirds could choose between identical cavities in the same place. After successful nesting, half of nests were removed and half left as a test of the prediction that bluebirds will switch to cleaner cavities when they are available, perhaps as a way to avoid exposure to parasites left in previously used nesting material. In this study population bluebirds preferred to nest in clean cavities when clean cavities were easily available and required no further territorial defense (Stanback and Dervan 2001). This study demonstrated that given an opportunity to avoid further exposure of adults or future exposure of nestlings to parasites, bluebirds behaviorally adjust their exposure to parasites. Another explanation is that it is easier for females to build an entirely new nest than to remodel or build over an existing nest.
Exposure and starvation account for most anecdotal reports of bluebirds dying in groups during excessively cold weather (PAG). In Tennessee during the cold winter of 1977, 19 bluebirds died in a communal roost; all lacked fat reserves and were emaciated; their weights ranged from 19.5 to 25.9 g (mean 22.2; Pitts 1977b). About 10% of nestlings die in the nesting cavity as result of starvation, exposure, or abandonment. Prolonged, unseasonally cold rain in Clemson, SC, on 9 May 1992 resulted in 100% loss of nestlings > 7 d old; females were apparently able to keep younger but not older broods warm and fed (PAG).
In Wisconsin, predation accounted for loss of up to about 10% of all eggs laid (n = 4,609): 6.1% of all eggs laid failed to hatch; 5.9% died because of abandonment; 4.6% went missing from nests; about 4% were destroyed or depredated by House Sparrows, Tree Swallows, House Wrens, or mammals; 2.7% were buried under nesting material (Kruger 1985b). In S. Carolina, depredation accounts for about 2% of all nestling deaths (n = 1,832 nests with hatched eggs over 7 yr; PAG). During the breeding season, females are sometimes killed on the nest by ants, raccoons, and snakes (see Behavior: predation, above).
Competition With Other Species
Competition with other species may account for mortality among hatch-year birds, which experience highest mortality during Oct, Dec, and Mar; Oct mortalities are highest, probably because of dispersal and/or migration through unknown territories (Pinkowski 1971a).
Bluebird females occasionally kill other females in combat over access to nesting cavities, as do House Sparrows, Tree Swallows, and House Wrens. During the breeding season, males are killed by House Sparrows and Tree Swallows; House Wrens kill male bluebirds in combat over nesting cavities. In Michigan, 67% of adult mortality (n = 93) was attributed to breeding-season casualties (Pinkowski 1971a). See Behavior: agonistic behavior, above.
Food competition with other organisms that prey on the same ground arthropods as Eastern Bluebirds may be a source of morbidity or mortality in bluebirds in some parts of their breeding range. Red imported fire-ants Solenopsis invicta are common invasives in the se. US; and the presence of fire ants correlates with variation in behavior (foraging, mate guarding, and extra pair paternity). In addition in experimental studies comparing the behavior and survival of bluebirds on territories with fire ants removed and control territories, fire-ant removal had positive effects on the number of offspring fledged. Nevertheless, a large scale comparison of effects on adult and offspring survival (Gowaty and Lang unpubl.) indicated that weather patterns were more likely the cause of survival differences between bluebirds in geographic areas with and without fire ants.
Initial Dispersal From Natal Site
Post-independence juveniles form flocks consisting of several families (Krieg 1971, PAG and JHP). Flocks range widely but remain in the general vicinity of the natal area although there are a few reports of juveniles recovered during the natal breeding season 20–300 km from natal areas (Pinkowski 1971a, BBL data). In resident populations, juveniles from final broods often remain with parents as a family unit in the vicinity of the nest site throughout the winter (Low 1934a, Pinkowski 1974b, Plissner 1994, Plissner and Gowaty 1996). Single and multifamily flocks of juveniles range widely and may make flights of several kilometers at a time, flying 50–100 m above ground and then dropping suddenly back to ground level.
Return rates to natal areas vary; typically lower in northern portions of the range. In Minnesota, 11% of banded and fledged individuals returned to the vicinity of their natal sites (Fiedler 1974). In Clemson, SC, 1985–1991, of 3,798 banded and fledged individuals, 13.3% returned to study sites: 11.9% of females (n = 1,900) and 14.6% of males (n = 1,898; Plissner and Gowaty 1996). Of these fledged individuals, only 0.58% bred at their natal box (0.8% of males, 0.37% of females); 1.7% bred for first time next to their natal territories (1.9% of males, 1.6% of females); 3.7% of all individuals (4% of males, 3.3% of females) bred elsewhere within their natal population. In S. Carolina, median natal dispersal distance was 2 territories, on the basis of 335 returns: Males moved an average of 2.3 territories—or, measured as linear distance, mean 1.032 km (median 0.564, n = 173)—and were thus more philopatric than females, who moved on average 3 territories, or mean linear distance of 1.269 km (median 0.768, n = 168). Long-distance dispersals (> 500 km) have been reported for both sexes. Individuals from final broods are more philopatric than older young from the same hatch-year (Gowaty 1993, Plissner 1994).
Fidelity To Breeding Site And Winter Home Range
In Minnesota, 26% of 170 individuals banded as adults returned to breed at their previous year's nesting site (Fiedler 1974). In S. Carolina, of 210 females that bred on study sites in 1988, 39% returned in 1989; of 197 males that bred on study sites in 1988, 44% returned in 1989 (PAG). In an experimental study designed to evaluate correlates of breeding dispersal, 56% of individuals that were successful (n = 24 nests) returned to the same breeding site the following year, while only 15.4% of those unsuccessful in a previous year (n = 10 nests) returned (Gowaty and Plissner 1997). In S. Carolina, of 36 females failing in their first nesting attempt, 69% moved for their second; in comparison, of 87 females succeeding in their first attempt, only 39% moved, a significant difference (PAG in Rohwer 1986d).
Of birds moving between nest sites, females are more likely than males to leave the breeding area to renest (Pinkowski 1977b, PAG and JHP). Movements of adults between years were not sex biased in S. Carolina (JHP, PAG), but return rates were higher for males in Michigan (Pinkowski 1974a). Both males and females occasionally breed >100 km from nest locations of the previous year (BBL data).
In Michigan, radio-collared breeding bluebirds had territories averaging 1.1 ha (n = 3; Carlson 1976). At the Savannah River, SC, site, breeding radio-tracked males had home ranges averaging 19.2 ha ± 4.4 SE (n = 5), females 13.7 ha ± 4.4 SE (n = 5); during winter, ranges averaged 105.9 ha ± 15.5 SE (n = 5) for males and 120.8 ha ± 16.6 SE (n = 5) for females (Allen 1988).
Extent of postbreeding and winter home ranges of individuals and flocks remains poorly known for most habitats, although pairs in resident populations sometimes remain near previous nest sites (JHP and PAG). At the Savannah River (SC) site, 4 radio-collared males, tracked immediately after breeding, had home ranges averaging 30.8 ha; 2 females, tracked Feb–Apr, had average home range size of 22.4 ha (Savereno 1991)
Density estimates vary in different parts of the breeding and wintering ranges. Estimates based on Breeding Bird Survey (BBS) data along the sides of roads probably enhance detectability of Eastern Bluebirds attracted to nesting-box trails and in other cases decrease detectability if local populations are in open woods away from roads. Numbers increase in local habitats as suitable nesting cavities increase. See Population Regulation, below.
For relative abundance based on BBS data, see Figure 9. Note areas of highest abundance (>10-30 individuals seen/route) in the Southeast and mid-Atlantic states.
Figure 10. See http://www.mbr-pwrc.usgs.gov/cgi-bin/atlasa12.pl?07660&1&12 for BBS data showing details for selected regions, 2002-2012. Note most areas show declining populations for the decade, except New England and the mid-Atlantic coast.
Numbers have fluctuated, with notable highs and lows (Bourne 1957b). Historically, population numbers were high because of the absence of mammalian nest predators, plus open areas of coastal grassland and an abundance of natural cavities in indigenous Bermuda cedar (Juniperus bermudianus) and in rock crevices. Bluebird numbers fell after the introduction of House Sparrows to the island in the late 1800s (Webster-Prentiss 1896), and again after the introduction of European Starlings and Great Kiskadees (Pitangus sulphuratus) in the 1950s; cutting of Bermuda cedars accounted for declines in the 1990s.
More recently, population numbers have remained stable (D. Wingate pers.com.). Three factors potentially limit Bermuda bluebird populations: access to nesting sites, competition over nesting cavities with house sparrows, and recently introduced poultry mites.
The Bermuda Audubon Society maintains an active role in building nesting boxes and promoting their use throughout the island. “The result has been that just about everybody who owns property or cares the least bit about nature and conservation has a nest box installed in their garden” (D. Wingate pers. com.) Saturation with nesting boxes seems to have enabled the Bermuda population to continue to thrive in numbers that Wingate estimates to be near or equal to the numbers of individuals observed in the 1990s, despite the pace of urbanization. The presence of House Sparrows on Bermuda remains a potential threat to Bermuda bluebirds, because of competition over access to nesting cavities. A final potential limiting factor for Bermuda bluebirds is poultry mites; since the 1980s feral chickens have increased explosively on the island, facilitating the spread of mites to passerines, including Bermuda bluebirds. The mites apparently slow nestling development in bluebirds. Government efforts to control feral chickens seems to have reduced the passerine mite problem, but this deserves research attention.
Few data; needs study. Known as a common to fairly common resident breeder from northern interior Mexico south to Nicaragua and Belize, but reliable counts are lacking; “colonies” noted in Belize, s. Tamaulipas, and El Salvador (Howell and Webb 1996).
Factors that limit numbers vary in different portions of North America. In some parts of the range, the species is limited by nest-site availability, in others by winter food availability, and in still others by food availability during the breeding season. Individuals are vulnerable to very cold weather, especially ice storms; > 50% of the wintering Illinois bluebird population was lost during ice storms of spring 1940 (Musselman 1941). Similar decreases occurred during the late 1970s throughout the e. U.S.
Individual mortality during cold winters appears associated with the difficulty in foraging on ice-covered fruits. In the middle of their range, numbers have increased where nesting boxes were supplied in appropriate habitat. In Athens, GA, during 1993, bluebirds used 69 of 204 boxes in 97 different nesting attempts; in 1994, 108 of 204 boxes in 167 attempts; in 1995, 114 of 204 boxes in 190 attempts; in 1996, 116 of 204 boxes in 197 attempts; and in 1997, 114 of 204 boxes in 208 attempts (PAG unpubl. data).