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Wildlife, Animals, and Plants
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Introductory
WILDLIFE SPECIES: Sialia sialis | Eastern Bluebird
ABBREVIATION :
SISI
COMMON NAMES :
eastern bluebird
TAXONOMY :
The currently accepted scientific name for eastern bluebird is Sialia
sialis (Linnaeus) [3,59]. It is a member of the family Muscicapidae
[59].
Subspecies recognized in the American Ornithologist's Union 1957
checklist (the last edition including subspecies) include [2]:
S. s. sialis Linnnaeus, eastern bluebird
S. s. grata Bangs, Florida bluebird
S. s. episcopus Oberholser, Tamaulipas bluebird
S. s. fulva Brewster, azure bluebird
Eastern bluebird interbreed with mountain bluebird (S. currucoides) [46].
ORDER :
Passeriformes
CLASS :
Bird
FEDERAL LEGAL STATUS :
No special status
OTHER STATUS :
Atwood [4] compiled state status lists of western landbirds; eastern
bluebird was listed as of special concern in Montana and a "watch"
species in North Dakota. Eastern bluebird was on the Audubon Society's
Blue List (1986) in 1972, from 1978 to 1982, and of Special Concern in
1986 [74]. In a 1979 compilation of state lists eastern bluebird was
listed as rare and/or endangered in Connecticut (1976), existing in
limited numbers in Massachussetts, and uncommon and declining in New
Hampshire [75]. It was listed in the South Dakota Natural Heritage
Database as a species monitored by the South Dakota Department of Game,
Fish, and Parks; apparently secure in the breeding season in South
Dakota, though rare statewide and with cause for long-term concern. It
was rated globally secure though rare in parts of its range [76].
In Canada, the eastern bluebird is listed as vulnerable in Manitoba, New
Brunswick, Nova Scotia, Ontario, Quebec, and Saskatchewan [77].
COMPILED BY AND DATE :
Janet Sullivan, July 1995
LAST REVISED BY AND DATE :
NO-ENTRY
AUTHORSHIP AND CITATION :
Sullivan, Janet. 1995. Sialia sialis. In: Remainder of Citation
WILDLIFE DISTRIBUTION AND OCCURRENCE
WILDLIFE SPECIES: Sialia sialis | Eastern Bluebird
GENERAL DISTRIBUTION :
Eastern bluebirds breed from southern Saskatchewan east to southern Nova
Scotia and south through the eastern United States [2]. In the West it
occurs casually along the eastern foothills of the Rocky Mountains in
Montana, Wyoming, and Colorado, more commonly in the Dakotas through
central Nebraska, central Kansas, and central Oklahoma to central and
southeastern Texas, and south to most of Mexico [41,70]. Eastern
bluebird was listed in 1969 as a species whose present occurrence in
Arizona is limited, unknown, or only suspected [64], although more
recent work [46] lists Arizona as within its range.
Eastern bluebirds winter in the middle parts of the United States south
to Nuevo Leon, the Gulf Coast, and southern Florida, and rarely to
western Cuba [2].
Ranges of subspecies are as follows:
Florida bluebird: Resident throughout peninsular Florida
Tamaulipas bluebird: Tamaulipas north to the Rio Grande valley in
south-central Texas
Azure bluebird: Transition zone from the mountains of southern Arizona
south to Jalisco, Oaxaca, and Vera Cruz (Guerrero). Winters south to
northern Guatemala [2,6].
ECOSYSTEMS :
FRES10 White-red-jack pine
FRES12 Longleaf-slash pine
FRES13 Loblolly-shortleaf pine
FRES14 Oak-pine
FRES15 Oak-hickory
FRES39 Prairie
STATES :
| AL |
AK |
AZ |
AR |
CO |
CT |
DE |
FL |
GA |
IL |
| IN |
IA |
KS |
KY |
LA |
ME |
MD |
MA |
MI |
MN |
| MS |
MO |
MT |
NE |
NH |
NJ |
NY |
NC |
ND |
OH |
| OK |
PA |
RI |
SC |
SD |
TN |
TX |
UT |
VT |
VA |
| WV |
WI |
WY |
DC |
BLM PHYSIOGRAPHIC REGIONS :
13 Rocky Mountain Piedmont
14 Great Plains
15 Black Hills Uplift
16 Upper Missouri Basin and Broken Lands
KUCHLER PLANT ASSOCIATIONS :
K070 Sandsage-bluestem prairie
K074 Bluestem prairie
K075 Nebraska Sandhills prairie
K076 Blackland prairie
K077 Bluestem-sacahuista prairie
K081 Oak savanna
K082 Mosaic of K074 and K100
K083 Cedar glades
K084 Cross Timbers
K088 Fayette prairie
K089 Black Belt
K095 Great Lakes pine forest
K100 Oak-hickory forest
K104 Appalachian oak forest
K110 Northeastern oak-pine forest
K111 Oak-hickory-pine forest
K112 Southern mixed forest
K116 Subtropical pine forest
SAF COVER TYPES :
1 Jack pine
14 Northern pin oak
15 Red pine
20 White pine-northern red oak-red maple
21 Eastern white pine
40 Post oak-blackjack oak
42 Bur oak
43 Bear oak
44 Chestnut oak
45 Pitch pine
46 Eastern redcedar
50 Black locust
51 White pine-chestnut oak
52 White oak-black oak-northern red oak
53 White oak
55 Northern red oak
64 Sassafras-persimmon
67 Mohrs (shin) oak
70 Longleaf pine
71 Longleaf pine-scrub oak
72 Southern scrub oak
74 Cabbage palmetto
75 Shortleaf pine
76 Shortleaf pine-oak
78 Virginia pine-oak
79 Virginia pine
80 Loblolly pine-shortleaf pine
81 Loblolly pine
82 Loblolly pine-hardwood
83 Longleaf pine-slash pine
84 Slash pine
85 Slash pine-hardwood
109 Hawthorn
110 Black oak
111 South Florida slash pine
SRM (RANGELAND) COVER TYPES :
601 Bluestem prairie
602 Bluestem-prairie sandreed
603 Prairie sandreed-needlegrass
604 Bluestem-grama prairie
606 Wheatgrass-bluestem-needlegrass
608 Wheatgrass-grama-needlegrass
609 Wheatgrass-grama
610 Wheatgrass
611 Blue grama-buffalograss
612 Sagebrush-grass
708 Bluestem-dropseed
709 Bluestem-grama
710 Bluestem prairie
711 Bluestem-sacahuista prairie
717 Little bluestem-Indiangrass-Texas wintergrass
720 Sand bluestem-little bluestem (dunes)
721 Sand bluestem-little bluestem (plains)
731 Cross timbers-Oklahoma
732 Cross timbers-Texas (little bluestem-post oak)
801 Savanna
802 Missouri prairie
804 Tall fescue
809 Mixed hardwood and pine
810 Longleaf pine-turkey oak hills
811 South Florida flatwoods
812 North Florida flatwoods
PLANT COMMUNITIES :
Eastern bluebirds are found in a wide range of plant communities with
open overstories and in openings within woodlands. They are usually
found at low elevations and are frequently observed in oak (Quercus
spp.) woodlands [46]. In southern Michigan, oak-pine (Pinus spp.)
woodlands frequented by eastern bluebirds were dominated by black oak
(Quercus velutina), pin oak (Q. palustris), northern red oak (Q. rubra),
white oak (Q. alba), eastern white pine (Pinus strobus), red pine (P.
resinosa), jack pine (P. banksiana), and Scotch pine (P. sylvestris).
Eastern bluebirds are also commonly found in old fields characterized by
hawthorns (Crataegus spp.), black walnut (Juglans nigra), black locust
(Robinia pseudoacacia), black cherry (Prunus serotina), chokecherry (P.
virginiana), staghorn sumac (Rhus typhina), and multiflora rose (Rosa
multiflora) [46]. Other eastern bluebird sites include the jack pine
plains of Huron National Forest, dominated by jack pine, northern pin
oak (Quercus ellipsoidalis), and scarlet oak (Q. coccinea). Bracken
fern (Pteridium aquilinum), blueberries (Vaccinium spp.), and sweet fern
(Comptonia peregrina) are characteristic understory components [41].
Eastern bluebirds are also present in cutover and burned pine-oak
woodlands [46] and in Wisconsin jack pine savanna. The latter habitat
type is maintained by frequent low-severity fire [68]. Eastern
bluebirds are common in regenerating stands of central and southeastern
oak-pine forests [17].
In western Virginia eastern bluebirds were observed in the Appalachian
Mountains in lower and midslope forests dominated by scarlet oak, black
oak, chestnut oak (Q. prinus), and white oak [11].
Wintering areas used by eastern bluebirds in the Apalachicola National
Forest included pasture, open longleaf pine (Pinus palustris) woods, and
pine-oak woodlands usually dominated by live oak (Q. virginiana) with
Spanish moss (Tillandsia usneoides) [41].
REFERENCES :
NO-ENTRY
BIOLOGICAL DATA AND HABITAT REQUIREMENTS
WILDLIFE SPECIES: Sialia sialis | Eastern Bluebird
TIMING OF MAJOR LIFE HISTORY EVENTS :
The eastern bluebird is nonmigratory in many parts of its range. This
trait confers an advantage over migratory competitors for limited nest
sites. Eastern bluebirds and mountain bluebirds (Sialia currucoides)
may occupy the same nest site in different years [46].
Spring Migration and Territory Establishment: Eastern bluebirds and
western bluebirds (Sialia mexicana) are similar in habitat requirements
and exhibit interspecific territoriality [37], as do eastern bluebirds
and mountain bluebirds in areas of sympatry. Eastern bluebird territory
size varies seasonally. In Michigan eastern bluebird territories are
largest in March and become progressively smaller through April, May,
and June [46]. Nonmigratory males establish territories from February
20 to March 10. Other males arrive and establish territories from late
March to early April [48]. Females arrived after males in 1970, 1971,
and 1974 [46]. Krieg [29] however, reported that in northwestern New
York, the general pattern was a staggered arrival of male and female
birds from mid-March to early June. Some birds arrived already paired,
others were unpaired [29]. In New York males choose territories in
mid-March [60]. Each territory is established around at least one nest
site and is usually expanded to include several potential nest sites
[29]. Territories may be established as long as 6 weeks or as little as
an hour before nest building occurs [71]. The males display and sing at
several nest sites to attract females. After a female accepts a
particular site, she builds the nest out of dry grasses or other plant
materials; sometimes several nests are built in different sites before
eggs are laid in the final site [29,41]. Nest building usually takes 5
or 6 days, but may take 2 weeks or more; on occasion nests are built in
1 day or less and egg-laying commences immediately [71]. Eastern
bluebirds do not exhibit strong nest-site fidelity [48].
Eggs: Egg laying normally starts soon after the nest completion, but
delays of more than a week are not unusual [71]. In southern Michigan
egg-laying activity begins in early April, with complete clutches
present as early as April 8. Egg-laying activity (in a population)
usually peaks the third week of April and again the third week of June.
One egg is laid per day, usually with 1 or more days between eggs. The
typical eastern bluebird clutch is three to five eggs, occasionally six
[41,71]. Incubation only begins after the clutch is complete [71]. The
female usually does all the incubation, and is either fed by the male or
takes short foraging trips [41,45]. Incubation lasts 13 to 15 days, and
ranges from 12 to 21 days [41]. In New York first broods usually hatch
at 14 days and later broods take 13 days, probably because of higher
temperatures [71].
Development of Young: In New York first broods hatch in early May. The
naked, altricial hatchlings are blind [60]. They are brooded almost
constantly by the female for the first few days; brooding becomes more
sporadic after feather growth commences, and ceases a few days before
the young leave the nest. Brooding intensity varies with weather [71].
Both parents feed nestlings and remove fecal sacs [22,60]. Fledging
occurs from 15 to 20 days after hatching [41], most commonly at 17 or 18
days. First flights are usually directly to a perch, and are 50 to 100
feet (15-30 m) in length. Fledglings stay near the nest and each other,
roosting together at night. Parents feed the fledglings, who begin to
find their own food about 2 weeks after fledging and achieve
independence 3 to 3.5 weeks after fledging. Widowed females often
continue to raise the brood, often with help from unmated females and
immatures (often members of earlier broods). Widowed males attempt to
raise broods if nestlings are well feathered; they also often have
helpers [71].
About 10 days after the first brood fledges, the female usually builds a
new nest (often in the same site) and lays a second clutch. The male
continues to feed the young of the first brood; when the second brood
hatches the male and sometimes members of the first brood help feed the
nestlings [29,60].
Multiple Broods: Eastern bluebirds often produce three broods in one
season in the central part of their range. They are single-brooded on
the northern periphery, and usually double-brooded elsewhere [40]. In
southern Michigan there are two main nesting periods. The spring
nesting period peaks from April 6 to May 14 and is fairly synchronized
(that is, most eastern bluebirds in the area are nesting at this time).
The summer nesting period occurs from June 7 to July 23. Eastern
bluebirds nesting during the spring period usually attempt a second
brood in the summer period. However, there is an intermediate period,
from May 15 to June 6. Eastern bluebirds nesting in this period are
usually only able to raise a single brood [47]. Nesting success is
greatest in the intermediate period, but spring broods are bigger;
overall more fledglings are produced from spring broods [45].
Two or three broods in a season is typical for many populations of
eastern bluebirds; however, there are a number of reported instances of
the production of four clutches or broods. A single pair of eastern
bluebirds produced four clutches in one season but only the first three
produced fledglings [32]. In Alabama a male eastern bluebird
successfully reared four broods in 1987. This male apparently mated
with two different females, raising two broods with each female. It is
possible that he was a helper rather than a parent with one of the
females; genetic relationships were undetermined [65]. In northeastern
Texas a single pair of eastern bluebirds (nonmigratory) successfully
raised four broods: four eggs, one hatchling, one fledgling April 14;
five eggs, two hatchlings, two fledglings June 1; five eggs, one
hatchling, one fledgling July 12; and four eggs, four hatchlings, and
four fledglings August 22 [30]. Eastern bluebirds are predominantly
monogamous (one male and one female are the genetic parents of all
members of a brood) but polygyny has also been observed [23,24].
Fall Migration: The presence of fruit determines eastern bluebird
distribution in winter [41]. In winter eastern bluebirds are found
mainly in the southern half or two-thirds of the breeding range [71].
In southern Michigan probably close to 95 percent of eastern bluebirds
migrate southward; in some areas all of the eastern bluebirds migrate
[41,45]. The proportion of eastern bluebirds migrating decreases with
decreasing latitude [41]. Southward migration of eastern bluebird
flocks is usually leisurely as the birds search for food [71]. In New
York parents and both broods remain together in summer and fall, then
join larger flocks in the fall which move southward, stopping frequently
where food is plentiful. New York State eastern bluebirds travel as far
south as Virginia and North Carolina [60]. In Tennessee eastern
bluebirds that are present during the breeding season are usually
nonmigratory. They form small flocks of adults and immature birds in
late summer and stay together most of the nonbreeding season [52].
Nesting Success and Productivity: Pinkowski [45] reported that eastern
bluebird achieved 56 percent nesting success in Michigan (nesting
success is defined as the proportion of nests producing at least one
fledgling). Mean annual productivity for eastern bluebirds nesting in
southern Michigan, over a 10-year study, was five fledged young per pair
per season. Estimated survival rates were 82 percent between fledging
and independence, 33 percent between independence and the start of the
next breeding season, and 50 percent thereafter (on an annual basis)
[47]. In west-central Wisconsin nest boxes placed on managed lands, 45
eastern bluebird nests produced an average of 1.2 fledged young per
nesting attempt [67]. Blowfly parasitism reduces nest productivity; it
is a serious problem in some areas [73].
PREFERRED HABITAT :
Nesting and Foraging Habitat: Eastern bluebirds usually forage near the
nest site. In Ohio eastern bluebirds traveled up to 1,320 feet (400 m)
from the nest on foraging trips [22]. Eastern bluebirds generally
prefer early successional habitats or open stands. In Arkansas several
plots representing various stages of oldfield succession were surveyed
for birds. Eastern bluebirds were observed only in burned fields and in
woody fields and not in more heavily wooded plots [58]. Within the
oak-hickory (Carya spp.) forest, eastern bluebirds prefer to nest in
savanna and savanna-like habitats such as pastures with scattered small
trees and bushes, usually near a lake or other body of water [55]. In
southern Michigan eastern bluebirds forage in open terrain [47]. They
are most commonly found in old fields dominated by forbs and grasses
with scattered trees and shrubs [44], oak and pine woodlands, open woods
with brushy undergrowth, areas of tall weeds, roads and roadsides,
recently plowed ground, and lawns. Feeding perches adjacent to open
areas are essential; rolling terrain is preferred over completely flat
areas. A dependable fruit supply is also important, particularly in
early spring when insect availability is low [41]. In the northern
sections of eastern bluebird range, key nesting habitat discriminators
in order of importance include topographic relief, presence of evergreen
shrubs, number of genera of deciduous trees, distance to nearest edge,
and slope. In the central sections, key discriminators were topographic
relief, presence of evergreen shrubs, number of genera of deciduous
shrubs, relative eastern bluebird densities, and number of genera of
deciduous trees. Nest box placement recommendations in relation to key
habitat parameters for the northern sections of eastern bluebird range
include presence of evergreen shrubs within a 100-foot (30 m) radius of
the box and presence of 5 to 13 genera of deciduous trees within a
100-foot (30 m) radius. Distance to the nearest edge should be between
86 and 452 feet (26-137 m) and slope should be between 0 and 8 percent.
For the central sections it is recommended that there be no evergreen
shrubs in a 100-foot (30 m) radius, none or only one genus of deciduous
shrubs, and zero to four genera of deciduous trees within a 100-foot (30
m) radius [39].
Pinkowski [41] stated that dry, sandy, acid soil deficient in lime and
organic matter creates near-optimum conditions for eastern bluebirds.
In the Appalachian Mountains of western Virginia, eastern bluebirds were
more common in areas of dense, low vegetation with scattered residual
pole-sized trees and areas with open overstories and brushy
understories. Nests were in snags in recent clearcuts [11]. Conner and
Adkinson [9] reported that eastern bluebirds prefer areas near
clearcuts. In Virginia eastern bluebirds were observed in clearcuts
formerly occupied by oaks. Nests were located in 1- to 2-year-old
clearcuts which were sparsely stocked with young hardwoods (oaks,
hickories [Carya spp.], black locust, sassafras [Sassafras albidum], and
flowering dogwood [Cornus florida]) about 3.3 feet (1 m) tall; in
5-year-old stands moderately stocked with 6.6-foot (2 m) tall hardwoods;
and one nest was located in a 12-year-old, densely stocked stand of 13-
to 16.5-foot (4-5 m) tall oaks and hickories. All nests were in standing
dead trees. Eastern bluebirds were not present in 15-year-old stands
[8,9].
In Illinois eastern bluebird densities in sampled habitats were as
follows: 30 individuals per 100 acres (40 ha) in orchards, 34
individuals per 100 acres in edge communities with shrubs, 25
individuals per 100 acres in residential areas (lawns etc.), and 13
individuals per 100 acres in second-growth woods [25].
In central hardwood forests eastern bluebirds are common in regenerating
stands (seedling-shrub stage) but are not present in later successional
stages. In loblolly-shortleaf pine (Pinus taeda-P. palustris) forests,
they are uncommon in regeneration and sapling stages and present in
old-growth stages but not present in pole and mature stands (possibly
because of the presence of snags) [17].
In the Southeast, red-cockaded woodpecker (Picoides borealis) holes are
used successively by other woodpeckers (Picidae), flycatchers (Empidonax
spp.), titmice (Parus spp.), and eastern bluebirds. Squirrels and
chipmunks (Sciuridae) are also aggressive users of woodpecker holes
[15]. Pileated woodpeckers (Drycopus pileatus) sometimes make holes too
large to be used by some species, but eastern bluebirds use most of the
holes [5].
Winter habitats used by eastern bluebirds usually contain fruit-bearing
plants; these include open pine-oak woodlands, pastures (especially saw
palmetto (Serenoa repens)-pineland threeawn (Aristida stricta) in the
Southeast), open pine woodlands, and old fields [41].
COVER REQUIREMENTS :
Nesting: Eastern bluebirds nest in tree cavities, nest boxes, and
crevices. Many nests are built in abandoned woodpecker (Picidae) holes,
knotholes, and cavities formed by decay and/or fire wounds. Eastern
bluebirds rarely nest in open situations. In southern Michigan eastern
bluebirds apparently prefer nest boxes to natural cavities but fidelity
to one type of nest site is not strong. Nest boxes may be more
attractive than natural cavities because they are usually placed in
suitable locations and have optimal dimensions. Most changes from one
type of nest site to another occur after nest failures [45,48].
Most natural cavities used by eastern bluebirds are in oaks or American
elm (Ulmus americana) stubs [45,48]. Fire-scarred snags are also
commonly used [42]. In South Carolina longleaf pine-loblolly pine
stands, optimal d.b.h. of snags for cavity trees is 8 inches (20 cm)
[27]. In oak-pine woodlands the average height of cavities in pine
snags used by eastern bluebirds was 12 feet (3.6 m) and ranged from 1.7
to 55 feet (0.5-16.8 m). Cavity depth averaged 7.8 inches (19.8 cm) and
ranged from 3 to 19 inches (7.6-48.8 cm), entrance diameter averaged 2.4
inches (6.1 cm) and ranged from 1.5 to 5.2 inches (3.7-13.3 cm), and
interior diameter averaged 3.6 inches (9.2 cm) and ranged from 2.2 to
6.3 inches (5.7-15.9 cm). Entrance hole orientation does not affect
nest site use, although there is a slight tendency to choose cavities
facing southwest [41,42].
Foraging: Perches near open areas, with an unobstructed view of air and
ground and sparse ground cover, are favored foraging sites. Dead
branches are preferred over live ones, presumably for greater prey
visibility. Where natural vegetation is tall, eastern bluebirds prefer
mowed areas to unmowed areas. Areas with dry, nonfertile soils, low
vegetation, and much bare ground are also favored [22,46].
Roosting: Night roosting sites are commonly in pine, oak, or pine-oak
woodlands with fairly large trees [41]. In winter, eastern bluebirds
use nest boxes for roosting only on very cold days (in Tennessee they
roosted in nest boxes on days when the temperature was 14 degrees
Fahrenheit [-10 deg C] or lower). Numbers of eastern bluebirds in nest
box roosts ranged from 1 to 16 [50,51].
FOOD HABITS :
A large proportion of the eastern bluebird diet consists of arthropods,
most frequently grasshoppers and crickets, but also butterflies and
moths, spiders, and beetles [46]. Preferences of captive eastern
bluebirds included (in order) mealworms, crickets, grasshoppers, leaf
hoppers, locusts, sow bugs, and stink bugs. Wild eastern bluebirds also
consume earthworms, May beetles, and caterpillars [41]. In Tennessee
eastern bluebirds continue to hunt for arthropods in winter; even on
cold days some arthropods, especially spiders, may be active on
south-facing slopes. When air and soil temperatures are below 32 degrees
Fahrenheit (0 deg C), insect activity (and therefore availability) is
extremely limited [52]. In northwestern Tennessee droppings collected
from nest boxes used as roost sites on the coldest days in winter
contained only plant materials. Insects are inactive on these very cold
days [51,52].
Eastern bluebirds rely heavily on fruit in nonbreeding seasons when it
is available. Fruit is usually scarce in the early part of the breeding
season. In southern Michigan staghorn sumac and smooth sumac (Rhus
glabra) fruits are frequently consumed [44,46]. Fruits of chokecherry,
black cherry, multiflora rose, and flameleaf sumac (R. coppalina) are
also common dietary components [41]. In northwestern Tennessee in
winter, eastern bluebirds consume the fruit of sumacs (Rhus spp.),
flowering dogwood, grapes (Vitis spp.), Japanese honeysuckle (Lonicera
japonica), hackberry (Celtis spp.), rose (Rosa spp.), deciduous holly
(Ilex decidua), pokeweed (Phytolacca americana), greenbriers (Smilax
spp.), and climbing bittersweet (Celastrus scandens) [52]. In Florida
fruits eaten in the nonbreeding seasons include those of greenbriers,
smooth sumac, and juneberry (Amelanchier spp.). Earthworms and
caterpillars are also consumed when available [41].
Nestlings are fed caterpillars, grasshoppers and crickets, spiders, and
(usually for older nestlings and fledglings) succulent early-maturing
fruits such as mulberries (Morus spp.), raspberries (Rubus spp.),
dogwood (Cornus spp.) fruits, cherries (Prunus spp.), and honeysuckle
berries (Lonicera spp.) [46].
Foraging Techniques: The most common foraging technique is dropping
from a perch to capture already-spotted prey on or near the ground.
Eastern bluebirds also forage by flycatching, foliage gleaning
(hover-gleaning), and rarely, hopping and drop-gleaning (spot prey on
foliage from a perch) [22,41,42,44,46].
PREDATORS :
Most nest failures are due to predation and/or nest site competitors.
In southern Michigan 23 percent of nest box failures was due to house
wrens (Troglodytes aedon) puncturing eggs or removing them; 18.8 percent
was due to raccoons (Procyon lotor) or other mammals, and 14 percent to
weather [45]. In that study raccoons had to be controlled before
successful eastern bluebird nesting occurred [41]. Eastern bluebird
nestlings were killed and partially eaten by an eastern chipmunks
(Tamias striatus) [38]. Red squirrels (Tamasciurus hudsonicus) kill
adult eastern bluebirds and rob eggs. House sparrows (Passer
domesticus) kill adult eastern bluebirds in nest boxes in competition
for nest sites [41].
MANAGEMENT CONSIDERATIONS :
Historical Populations and Range Expansion: There has been speculation
the genus Sialia evolved in the mountains of the southwestern United
States; the eastern bluebird was probably abundant in the sparse
pine-oak woods of Mexico and the southern states prior to European
settlement. Native American burning practices probably created areas
suitable for eastern bluebirds [41]. From the early 1600's to about
1957 eastern bluebirds probably increased in eastern North America due
to increased amounts of edge and oldfield habitats from human activity.
Logging and slash burning create or enhance eastern bluebird habitat in
many areas. In the 1900's the range of eastern bluebird expanded into
the Great Plains and more recently into the southwestern United States,
possibly as a response to logging [70].
Population Status and Trends: In the late 1950's and early 1960's
eastern bluebirds declined to 17 percent of their previous numbers.
Eastern bluebird numbers were reported as very low in Pennsylvania where
they were formerly abundant [34]. James [20] referred to eastern
bluebirds as a "disaster species" after the severe winter of 1961. An
unpublished 1964 American Ornithologists' Union report stated that the
eastern bluebird was thriving in the wilder northern parts of its range
but had suffered severely in more developed areas [69]. An analysis of
Breeding Bird Survey (BBS) data for the years 1965 to 1979 indicated
that eastern bluebirds declined significantly in the eastern and central
regions. Declines and increases varied by subregions, showing a decline
when averaged over the entire continent [54]. Recoveries following
declines in 1976 and 1977 were documented [56]. During the period 1978
to 1987, BBS data indicated significant increases over much of the
eastern united States, with nonsignificant decreases in the lower
Mississippi Valley.
Trends for the entire period 1966 to 1987 are a mosaic of nonsignificant
increases and decreases: the southern United States had nonsignificant
increases in populations of eastern bluebirds; more northerly regions
had significant declines; and seven states showed significant increases.
The authors pointed out that for bluebirds, selection of the interval
from which trends are estimated can greatly affect perceptions of the
trend. Survey data can be greatly influenced by short-term effects, and
population trends estimated from variable populations contain an
environmentally induced "noise" component that is sometimes larger than
the long-term trend [56]. Currently eastern bluebirds are uncommon to
rare in the Northeast [14] but are either year-round residents or
short-distance migrants. However, eastern bluebirds have recently
experienced a statistically significant increase in the region [61].
Causes of Declines: Factors influencing eastern bluebird populations
include nest site availability, predators, diseases, parasites,
pesticides, and land use patterns [51]. James [20] analyzed Audubon
Society Christmas Bird Count data and concluded that there had been
long-term declines in wintering eastern bluebird populations from the
mid-1940's to 1961 and that there was a strong correlation between cold
winters and eastern bluebird declines. The decline in eastern bluebird
numbers in the 1950's and 1960's was attributed to severe winters (a
late spring freeze in southern Michigan killed large numbers of eastern
bluebirds in 1958), use of heptachlor to control Argentine fire ants,
and to other biocidal agents [20,41,51]. There is no evidence that any
of these factors changed in the 1970's with the possible exception of
land use patterns. Conversion of pasture into soybean production
reduced the amount of suitable eastern bluebird habitat [51]. Extreme
local declines [46] and extreme regional declines [51] have been noted
in association with severe weather during 1976 and 1977.
Low nesting success (including nest losses) has been attributed to
competition with European starlings (Sturnus vulgaris), house wrens, and
house sparrows, particularly where natural cavities are scarce. Further
losses are caused by blowfly parasitism and loss of nest sites
[41,71,72].
Habitat Suitability and Nest Site Availability: Many agricultural areas
formerly occupied by eastern bluebirds are now suboptimal for eastern
bluebirds because of a lack of nest sites [8]. In the southeastern
United States, a common cause of tree death is lightning. Lightning
damaged or killed trees are quickly infested with beetles and other
insects. Many species of birds, including eastern bluebird, are
attracted to these trees [28]. In open areas, particularly in
residential developments, humans usually remove dead limbs and trees.
Wooden fenceposts, once an abundant source of nest cavities for eastern
bluebirds, have largely been replaced by metal posts [26].
Competition for Nest Sites: A number of authors report that the
availability of suitable nest or perch sites is the factor immediately
limiting to populations of hole-nesting birds, including eastern
bluebirds [5]. Niche overlap between eastern bluebirds and house
sparrows in south-central Oklahoma was greater than would be expected by
chance. House sparrows have the potential to greatly influence the
nesting success of eastern bluebirds [53]. Competition for nest sites
has forced eastern bluebirds away from centers of human activity where
house sparrows and European starlings are relatively abundant [8].
House sparrows were able to usurp seven out of nine nest boxes occupied
by eastern bluebirds in a south-central Oklahoma study; they are
sometimes able to kill the eastern bluebird occupant of a desired nest
box [53].
The introduction of the European starling has greatly increased
competition for natural cavities with large holes. The aggressive and
abundant European starling restricts eastern bluebirds to nest sites
with small holes [41,72], especially where peak use of nest sites by
European starlings coincides with the period in which eastern bluebirds
are establishing territories and choosing nest sites. In Maryland an
investigation of competition between European starlings and eastern
bluebirds was conducted in a residential area using a nest box with a
large entrance hole. This box was placed in an area containing nest
boxes with small holes, many of which were used by eastern bluebirds.
European starlings were so abundant in the area that they were probably
saturating all suitable nest sites. Trapping and killing of European
starlings at the test box over the course of 3 years (a total of 335
European starlings) has reduced the numbers of European starlings seen
at the box, and it is hoped by the author that in a few more years
European starling populations may become low enough to reduce
competition with eastern bluebirds for large-hole cavities [72]. In
Kentucky, a study of 45 nest boxes over 3 nesting seasons evaluated
eastern bluebird productivity in the presence of European starlings.
Eastern bluebirds fledged almost as many young in 1983 when European
starlings used 60 percent of the boxes as in 1985 when European
starlings were excluded. This result was attributed to the difference
in timing of nest box use. European starlings produced one brood in the
nest box and eastern bluebirds produced broods both before and after
European starling use each year. No eastern bluebird nests were taken
over or destroyed by European starlings in this study; it was therefore
concluded that European starlings had little impact on eastern bluebird
nesting success [12].
Other competitors of eastern bluebird include mountain and western
bluebirds in areas of sympatry, particularly in the Great Plains
grasslands where eastern and mountain bluebirds have expanded their
ranges [46].
Nest Box Programs: Nest box programs have become popular in many areas
[41]. Pinkowski [45] reported that the nesting success rate (proportion
of nests that produce at least one fledgling) was similar in natural
cavities and in nest boxes. Recommended dimensions for nest boxes are
available [26,60,72,73]. Blowfly parasitism rates are similar in
natural cavities and in next boxes, but nest boxes often have a higher
number of blowflies per nest [73]. Nest boxes are most effective in
open areas. In south-central Oklahoma eastern bluebirds chose nest
boxes in open areas with few trees [53].
Environmental Toxins: In Wisconsin treatment of eastern bluebird
nesting areas with paper mill sludge containing dioxin had no measurable
adverse effects on growth or reproduction of eastern bluebirds as
compared to untreated plots. Eggs collected from treated study plots
had dioxin levels ranging from 6.6 to 11 picograms per gram of egg
(1 ppt). Eastern bluebird eggs injected with 1,000 ppt of dioxin
exhibited no toxic effects (i.e., eggs hatched at the normal rate), but
eggs injected with 10,000 ppt did not hatch [63].
Management Recommendations: In South Carolina pine and pine-hardwoods,
10 snags per 100 acres (40 ha) is optimal for maintaining eastern
bluebird populations at 3.2 pairs per 100 acres (40 ha). However,
maintenance of all cavity-nesting species present at their average
population levels would require 311 snags per 100 acres. The principal
cause of tree death and snag production in this area is lightning;
currently these trees are rapidly removed for firewood. The authors
recommend severely limiting the removal of lightning- and insect-killed
trees in order to provide more snags for wildlife use. Other
recommendations include retention of snags in clearcuts, leaving large
snags instead of small ones, and protection of drainage systems by
leaving strips of unmanaged forest along creeks to provide cavity trees
[27]. Clearcuts in oak-hickory forest can be good nesting areas for
eastern bluebirds provided that snags are present [26].
REFERENCES :
NO-ENTRY
FIRE EFFECTS AND USE
WILDLIFE SPECIES: Sialia sialis | Eastern Bluebird
DIRECT FIRE EFFECTS ON ANIMALS :
It is very probable that few eastern bluebirds die in fires [36].
Adults easily escape fire, and nests and nestlings are only vulnerable to
severe fire that destroys nest sites.
In Minnesota prescribed fire conducted during the breeding season was
evaluated for direct effects on eastern bluebird nesting activity. Nest
boxes 4 and 4.6 feet (1.2 and 1.4 m) above ground were monitored on the
day of the fire. Flame lengths were 3.3 feet (1 m) or less. Adult
eastern bluebirds left the nest boxes as the fire front approached; one
pair was observed hovering over the nest as the fire passed underneath
the box. The adults returned to the nest boxes when the fire had
passed. No eggs were harmed by this fire and there was 100 percent nest
success (all eggs produced fledglings) for both boxes. In the area the
success rate for 23 nests was 93 percent [10].
HABITAT RELATED FIRE EFFECTS :
The eastern bluebird is known as a fire-follower [1,16,35,62]; the low
sparse vegetation on fresh burns and plentiful natural cavities caused
by decay in fire injured or killed trees are important for eastern
bluebird habitat [36,41]. In southern pine forests, fire retards
succession, reduces midstory hardwoods and shrubs, and favors herbaceous
vegetation; all of these effects enhance eastern bluebird habitat [16].
An important consideration is the effect of fire on food sources. Fire
often reduces understory fruit production [33]. There are conflicting
reports on the effect of fire on arthropod populations [16]. The
effects of fire on invertebrate populations may be transitory or long
lasting. There is usually an immediate decrease in invertebrates due to
direct mortality and indirectly due to loss of food supplies and
shelter. In some instances flying insects are attracted by heat, smoke,
or killed or damaged trees, and therefore populations of some species may
increase during and after a fire. Fires reduce the populations of most
soil fauna (animals that spend most of their time on the forest floor or
mineral soils). The length of time of this effect varies with fire
severity and postfire vegetation [36].
FIRE USE :
Savannas and open stands are natural bluebird habitat that usually
require recurrent fire for maintenance. Prescribed fire is usually
beneficial to eastern bluebirds, especially if it controls shrubs and
understory hardwoods [46].
REFERENCES :
NO-ENTRY
REFERENCES
WILDLIFE SPECIES: Sialia sialis | Eastern Bluebird
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Index
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