Effects of Human Activity
See Distribution: historical changes, and Demography and Populations: population status, above. Cutting of vast tracts of longleaf pine in the southeast US may have initially reduced ancestral habitats for bluebirds. However cutting of eastern deciduous forest for conversion to agricultural fields and orchards during the 1800s and early 1900s probably increased availability of habitat suitable for foraging and nesting, and likely also increased edges along which bluebirds might disperse. Thus, despite concerns about 20th-century fluctuations in numbers, populations may have been more abundant in the 20th and early 21st centuries than in any other period of North American history since the Pleistocene (PAG).
Recent non-weather-related declines in bluebird numbers are a product of interactions between bluebird populations and other human commensal species, such as introduced European Starlings and House Sparrows, and possibly of exposure to pesticides and other indirect effects of humans on the arthropod food sources that sustain bluebirds.
Nesting-box campaigns begun in 1960s and 1970s increased availability of artificial cavities that typically excluded European Starlings; placement of boxes away from animal-holding buildings and grain sources decreases interactions with House Sparrows. For some populations, therefore, design of boxes and placement of artificial cavities have eliminated nest-cavity limitation as a dominant mechanism of population regulation.
Pesticides And Other Contaminants/Toxics
No systematic, prospective studies, but many anecdotal reports claim pesticide-caused deaths of nestlings, perhaps because adults differentially forage on pesticide-weakened insects, thereby increasing toxic load on nestlings. On Cape Cod, MA, during 1965, carbaryl (trade name Sevin) may have accounted for nestling deaths (Bednarek and Davidson 1967). Some anecdotal reports suggest that direct ingestion of pesticide-loaded insects have caused death of adult bluebirds.
Endocrine Disruption From Pesticides
In a comparative study in Canada of potential endocrine disruption on Tree Swallows (Tachycineta bicolor) and Eastern Bluebirds as a function of exposure to DDT and other pesticides, the effects on the bluebirds were more severe, likely owing to differences in the foraging behavior of the two species. In contrast to aerial foraging swallows, eastern bluebirds forage extensively on ground arthropods, prey whose own foraging habits guarantee that their bodies concentrate DDT and other pesticides that fall to the ground. Historically, bluebird eggs from sprayed orchards contained higher DDT concentrations than eggs from non-sprayed, control sites. In addition, Eastern Bluebirds also showed evidence of chronic elevations in corticosterone (the stress hormone), which can cause neuronal cell death and decrease size because of the suppression of growth (Mayne et al. 2004). In addition, 12 d old nestlings exposed during development to the same DDT persistent metabolites in soil (along with other common pesticides) exhibited morphological and physiological signs of thyroid injury consistent with thyroid disease (Mayne et al. 2005). In as much as the thyroid is sometimes called “the master gland”, thyroid disease in wild-living Eastern Bluebirds may be associated with poorer survival and breeding performance. Thus it is not surprising that bluebirds nesting in N. Carolina golf courses where pesticides are routinely sprayed have lower nest success than bluebirds in rural, non-agricultural areas (Stanback and Seifert 2005).
Investigators in Indiana showed that other well-known endocrine disrupting chemicals of humans, namely polychlorinated biphenyl's (PCBs) significantly affect external heart deformities of songbirds, including eastern bluebirds from nests on reference sites (DeWitt et al. 2006). The study is significant in underscoring the persistent effects of PCBs because their use has declined dramatically since 1977 when they were banned in the US; however PCBs continued to be released into the environment. Wildlife, including eastern bluebirds continue to suffer the effects of exposure to PCBs, effects that are mechanistically associated with the effects of endocrine disrupting chemicals on growth in affected individuals.
Direct Human/Research Impacts
Human visits to nests increase the probability of mortality of egg and nestling losses due to predation; predation increases by >11% on the first day after a human visitation, probably because these visits leave scent trails that cue mammalian and other predators, such as snakes (Bart 1977). In S. Carolina, this effect is strongest when nests are visited after noon (PAG); thus, it is advisable for observers to visit nests in morning and to band nestlings away from the immediate nest vicinity.
A probable by-product of selection exerted by historical nest-site limitation is this species' habit of exploring cavities of any sort. Bluebirds often die in smokestacks, asphyxiated by toxic fumes, and after entering chimneys when they are unable to locate exits. Reduced number of rotting fence posts with cavities available for secondary-cavity nesters, because of conversion to metal fences, is responsible for some local declines in Eastern Bluebird numbers. Management practices that reduce availability of snags (dead trees) are detrimental, but prescribed burning is beneficial because it increases snag availability and forest openings, by decreasing thickness of ground cover, understory plants, and midstory plants (Pinkowski 1977a).
Bluebird nesting trails are often located along gradients of human activity, which logically could affect the behavior, reproductive success, and survival of Eastern Bluebirds (Kight and Swaddle 2007). In a semi-experimental study in which investigators erected nesting boxes along a gradient of human activity and observed the behavior and reproductive success of adults, bluebirds unexpectedly had higher nest success at intermediate levels of disturbance. Where human activity was intense and variable, the self-maintenance activities of adults decreased, while attention to eggs and nestlings increased, a result consistent with the idea that bluebirds buffer their eggs and nestlings from deleterious human activities, if they can (Kight and Swaddle 2007). The consequences of such buffering, if it is general, may have important consequences for survival of adult birds from one season to another.
The effects of human noise on eastern bluebird productivity (Kight et al. 2012) are potentially important, indicating that more low-pitched louder human-caused noise (such as from machines) is associated with smaller clutch sizes and an lower numbers of fledglings.
Investigators often capture and handle breeding female bluebirds, whose response to handling varies, often decreasing on second capture (PAG pers. obs.), something that could be interpreted as an effect on individuals from learning that capture and handling are not dangerous. Endocrinologists studying the stress response associated with release of corticosterone showed that females being handled for a second time had attenuated corticosterone release (Lynn et al. 2010), and concluded that there was a long term effect of initial corticosterone release. It would now be interesting for investigators to evaluate if such corticosterone attenuation is associated with all potentially dangerous situations or only repeats of ones about which females are already familiar and from which they earlier “escaped”, when the investigators released their subjects the first time.
Not Threatened or Endangered in U.S. or Canada. Unevaluated in Central America. Common and increasing in e. North America. Threatened in Bermuda (D. Wingate pers. comm.).
Measures Proposed And Taken
Silvicultural practices in longleaf and slash pine forests that encourage nesting include cutting that allows accumulation of standing dead trees. Landowners and foresters should minimize salvage cutting in these forests; management for snags (dead trees) is critical for populations in north-central and northeastern forests (Evans and Conner 1979).
Nests in holes in fence posts; in Virginia, cavities in fence posts excavated by woodpeckers were most frequently used. Landowners might provide old fence posts with cavities and allow them to remain standing (Pierson and Scanlon 1986).
See also Breeding: nest site, above. Where nesting boxes are placed in suitable habitat, bluebirds sometimes appear so rapidly that it has been joked that they “spontaneously generate at the site of boxes” (PAG). A key element in successful management is placement of suitable nesting cavities. Importantly, there is no statistical difference in likelihood of successful nesting in natural cavities or in artificial nesting boxes (Pinkowski Pinkowski 1976f, Pinkowski 1977b).
Management practices, reducing competitive advantage of European Starlings and House Sparrows while favoring bluebird numbers, including providing nest boxes with entrances no wider than 3.8 cm, a size that excludes European Starlings and also inhibits Brown-headed Cowbirds that occasionally parasitize nests. In Michigan, nesting boxes with 2-cm “predator” guards around the nest-hole entrance and a top with a 9-cm-diameter screened hole were significantly more productive than standard nesting boxes (Radunzel et al. 1997a), an effect attributed to reduction in takeovers by House Wrens. In S. Carolina, in a systematically controlled study, no significant difference was found in rates of takeover or occupation by House Sparrows in standard boxes or boxes with open-screen tops (PAG). Systematic evidence shows that small nest-hole diameters discourage use of nesting boxes by European Starlings (Davis et al. 1986); front-slot entrances with width of 30 mm admit entrance by Eastern Bluebirds, but exclude use by European Starlings; notably slot entrances of 38, 35, and 31 mm do not exclude starlings. Systematic studies of differential requirements of House Sparrows are a priority for those interested in increasing breeding-season success of Eastern Bluebirds.
Peaceful coexistence of bluebirds and other secondary-cavity nesters can be facilitated by placement of nesting boxes specifically for all potential local competitors. Put boxes for bluebirds in open areas; hang Tufted Titmouse boxes in trees; put small boxes for chickadees on low posts under oak trees; place shallow House Wren boxes near edges under eaves of low buildings, on or near porches, or inside barns and sheds (Thomas 1946a, Hardin and Evans 1977). Competition between Eastern Bluebirds, Tree Swallows and House Wrens can be reduced by placement of bluebird nesting boxes on posts ≥10 m from high perches in open habitat with a field of view >0.4 ha from nest-hole entrance, about 30 m from forest edge; such sites are used more than expected by chance by bluebirds, not used more than expected by swallows, and used only very rarely by wrens (Patterson 1969; Rendell and Robertson 1990). Some evidence suggests that placement of multiple nesting boxes in close proximity reduces interspecific interference (PAG), a management technique that requires further systematic, controlled study.
Nests built in larger boxes (floor size 143 cm2) are higher and have greater volume than nests built in smaller boxes (floor size 71.5 cm2); in Tennessee, however, nesting-box size had no effect on mean clutch size, number of eggs hatching, or fledging rate (Pitts 1988).
Use of predator guards is recommended by amateur observers and the North American Bluebird Society. Professional conservation biologists indicate that management practices should be aimed at maintaining, rather than disrupting, natural predation regimes (Meffe and Carroll 1994). Notably, in one well-controlled study there was no statistical difference between predation rates in natural and unguarded artificial box cavities (Pinkowski 1976f).
Although data from controlled, comparative studies do not exist, metal cones, “conical metal baffles,” or protective metal sheeting around posts supporting nesting boxes may prevent raccoons and snakes from climbing posts. An additional 1.2-cm thickness of wood placed around the entrance hole to increase thickness may decrease the likelihood that mammalian predators, particularly domestic cats, reach an incubating or brooding female and her eggs or nestlings; anecdotal evidence suggests that this sort of deterrent is not effective in all populations.
Concerns about predation on Eastern Bluebirds might be most effectively directed at reducing the deleterious effects of human commensals such as domestic cats.
Maintenance of bluebird trails—nesting boxes along fencerows, country roads, and highways, maintained by hundreds of amateur ornithologists—is no doubt a significant factor in increasing numbers and populations of this species. Lawrence Zeleny, founder of North American Bluebird Society (Zeleny 1976) encouraged trail development and maintenance, something that has brought the attention of Eastern Bluebirds to amateur naturalists and conservationists, and contributed to the growing knowledge of the species.
Nesting trials on golf courses have been controversial, partly because of the potential exposure of Eastern Bluebird breeders and young to the herbicides and pesticides that are commonly used on intensively managed fairways. In contrast to this expectation, in a study of bluebird adults and nests on and off a golf course in Virginia, investigators reported (LeClerc, Che et al. 2005) that contrary to their own predictions, golf courses received 28% more eggs and produced 17% more fledglings than in non-golf course sites. In addition bluebird nestlings on golf courses in this study exhibited more “developmental stability” than on non-golf course sites.
Although very encouraging, these results should be looked at with caution, in that there were no controls in this study for ages or settling history of adult bluebirds. In addition, 6 years of observations of Eastern Bluebirds nesting in 250 nest boxes on golf courses and in 150 nest boxes on rural sites in N. Carolina came to different conclusions. “Bluebirds nesting on golf courses initiated their first nests an average of 1 day later and laid slightly smaller clutches (4.4 vs 4.5) than pairs nesting in non-golf habitat. The mean time interval between spring and summer nests was 3.5 d longer for bluebirds on golf courses. Brood size did not differ significantly between golf and non-golf habitat. Nestlings (the shortest-winged chick in each brood) on golf courses were in slightly but significantly poorer condition than those in non-golf habitat. … adults of both sexes breeding on golf courses were similar to those nesting in non-golf habitat” (Stanback and Seifert 2005). More studies of the potential deleterious effects of golf courses on bluebirds reproductive success and survival would provide needed information to better inform management decisions.
In areas where bluebirds migrate, nesting boxes placed out before mean arrival dates attract settlers from among early-arriving individuals. In areas in which bluebirds overwinter such as S. Carolina, nesting boxes placed out in October, when size of the local population is at its peak (due to influx of migrant bluebirds), attract new bluebirds into a potential nesting area (PAG). In Bermuda, nest box occupancy by House Sparrows is discouraged by removal of nestling cavities at the end of the breeding season in July and re-erection at start of breeding season around 1 Mar (D. Wingate pers. comm.).
Removal of old nesting material after fledging allows definitive observation of new nesting activity and may decrease numbers of ectoparasites. This practice may lead to counter-intuitive effects on bluebirds, however. Removal of old nests removes parasitic wasps (Nasonia vitripennis) that feed on larvae of blood-sucking blowflies (Protocalliphora sialis). Blowflies do not overwinter in nesting boxes, but parasitic wasps do; thus, removal of old nests may actually lead to increases, rather than decreases, in blowfly larvae parasitism of nestlings. Therefore, old nesting material left in boxes until just before start of each new breeding season may discourage subsequent blowfly larvae parasitism of nestlings (Mason 1944, Davis et al. 1994b). This last suggestion gains some additional merit from a comparison of blowfly parasitism of natural and artificial nesting cavities in Michigan in which parasitism was significantly greater in artificial cavities from which nesting material was removed (Pinkowski 1977a). See also Demography and Populations: disease and body parasites, above.
Because some trail operators recommend salvaging contents of partially predated or otherwise abandoned nesting attempts by moving eggs and/or chicks into active nests of Eastern Bluebirds or even other species, systematic, controlled experiments designed to evaluate the effects of this management practice on hosts is needed. Though this practice may reduce the likelihood of immediate mortality for “salvaged” eggs and nestlings, they may have other anti-intuitive, perhaps deleterious consequences on development of salvaged individuals and on the survival and reproductive success of their hosts.
Fire ants, Solenopsis invicta, which have recently colonized the se. U.S., occasionally prey directly on bluebird nestlings, eating them alive. However, fire ants prey primarily on ground arthropods that constitute major foods of breeding and wintering Eastern Bluebirds. In areas in which fire ants had reached large densities before the mid-1990s (Athens, GA), compared with areas where fireants were absent (Clemson, SC) in the mid-1990s, some adults failed at breeding. Breeding failure was associated with apparent insect availability in that females in such areas made fewer kills per foraging attempt. Similarly in areas with a long history of fire ant colonies, overall nest production was lower, nestling growth and development was relatively retarded, and nestling competition and starvation were more common (PAG unpubl. data).