C. Michael hogan PhD
November 27, 2009
Sialia currucoides is a chiefly insectivorous bird, whose habitat is limited by availability of suitable nesting sites in western North America. The species has experienced broad population decline in the last century, triggered by agricultural land conversion and timber management practices that zealously extract snags and that unnaturally supress periodic natural fires.
DISTRIBUTION AND MIGRATION
The species breeding range is in the western USA and Canada from Utah and northern Nevada northward to western Manitoba, southern Yukon and east-central Alaska. The wintering range extends from California west to the Black Hills of Montana and to eastern Kansas south to Texas, Baja California, southern Chihuahua and central Nuevo Leon. Sometimes the Mountain Bluebird migrates alone, but more often it moves in flocks of up to fifty birds, stopping frequently for feeding. In some high altitude habitats of southern Utah and northern Arizona such as locations in Zion National Park and the Kaibab National Forest, the Mountain Bluebird is a year around resident.
The breeding range of the Western Bluebird, S. mexicana, overlaps that of the Mountain Bluebird, but the two species are rarely found in the same location due to differential habitat preferences. S. currucoides arrives at its western breeding areas in February and March, typically in mixed flocks; (Herlugson) continent interior locations are typically populated for breeding in March and April, with males characteristically arriving earlier than females. Fall migratory flocks gather in October and November, and these flocks typically stay intact over the winter.
S. currucoides attains a length of 15 to 18 cm. Males manifest a deep sky blue plumage with paler blue breast and whitish stomach; females exhibit a duller gray-blue wing color with gray throat, crown and back. The female sports a white eye-ring. Both sexes manifest black eyes; the adult male exhibits a thin bill. Juveniles show spotted chests and unspotted backs, with blue in wings and tail feathers; otherwise juvenile plumage is similar to adults, albeit duller.
S. currucoides select a territory where food consumption, courtship, copulation and nesting take place. Nest building may occur in snags of Western Larch (Larix occidentalis), Whitebark Pine (Pinus albicaulis), or Ponderosa Pine (Pinus ponderosa); also used are rock crevices, woodpecker holes, abandoned nests of the American dipper (Cinclus mexicanus) and nest boxes. Territories may be as small as 10,000 square meters. Most commonly the male arrives first into a territory and selects a nesting site; the male then flits about its territory, calling to attract a female. After mating, the female constructs a nest from plant fiber, the male guarding the nest site
Typical clutch size is three to eight eggs, (Ehrlich et al.) which are incubated chiefly by the female for about two weeks. Hatchlings are blind, featherless and altricial. Natal down appears on the second day. Eyes begin to open by the fourth day. Mountain bluebird nestlings are fed by both parents. Chicks fledge at about 22 days, but are not totally independent of parental feedings until age six to seven weeks.
DIET AND BEHAVIOR
S. Currucoides is chiefly insectivorous, but consume fruit in the non-breeding season (Taverner) and when insects are less active (at dawn, dusk and in cold weather). Principal insects taken are grasshoppers, beetles, caterpillars, ants, weevils, bees and wasps. (Beal) Berries consumed include Juniper Mistletoe (Phoradendron juniperinum), Netleaf hackberry (Celtis reticulata), Saskatoon serviceberry (Amelanchier alnifolia), and Roundleaf buffaloberry (Shepherdia rotundifolia).
S. currucoides may perch or hover as they scan for insect prey; after a sighting the bird will drop to the surface for capturing its prey. In addition this species often forages in a hopping-gleaning mode, where the vegetative cover is relatively open.
HABITAT AND ECOLOGY
S. currucoides prefers an ecotone between woodland and open area with sparse vegetation. (Pinkowski) The Mountain bluebird displays an affinity for burned woodland, likely due to an adaptation for using dead and burned snags for nesting sites.Such a predilection does not apply to areas that have been clearcut or burned so extensively that the landscape is devoid of snags.
Chief predators of S. currucoides are certain raptors, but some deaths occur during interspecific nest competition. The most common raptors involved are Accipiter cooperi, A. striatus, Buteo jamicensis and Circus cyaneus. The House Sparrow (Passer domesticus) is one of the main killers of the adult Mountain Bluebird in nest competition. Egg or nestling predation is mades by species such as the American Crow (Corvus brachyrhynchos) and the Deer Mouse (Peromyscus maniculatus). Nestlings have also been killed by infestations of ants.
The range of S. currucoides has shrunk since the early 1900s, at which time the distribution extended eastware into the Great Plains; moreover, the species numbers have declined in Alberta, Nevada, (DeSante and George). Idaho and Utah; the Nevada decline since 1973 reverses an earlier trend. There are many locations where firm population trendlines are simply not understood. Chief reasons for population decline have been agricultural land conversion, (Wittbecker) clearcutting and other forestry management practices that reduce the number of woodland snags; suppression of fire is a further reason for the species decline, since S. currucoides has adapted to periodic natural burn conditions. Salvage harvesting of a burned woodland is destructive to cavity nesting species, particularly woodpeckers and secondary cavity nesters such as S. currucoides.
One should note that S. currucoides is one of the birds which is actually most resilient to forest destruction, since it has a foraging strategy that aids its competition in open ground. With reduced numbers of snags for nesting, interspecific nest site competition has become a proximate cause of S. currucoides population decline; significant species engaging in this pressure are the Northern Flicker (Cloaptes auratus), Violet-green Swallow (Tachycinera thalassina) and Yellow-pine Chipmunk (Tamias amoenus); moreover, since introduction of the European Starling (Sturnus vulgaris), that invader has become an important contributor to decline of the Mountain Bluebird.
Agricultural land conversion should be avoided in order to preserve further S. currucoides population declines. For future western forest management consideration should be given to (a) avoidance of mass clearcutting where snags and dead trees are removed along with the principal harvest; (b) avoidance of salvage harvest of burned snags and logs; (c) reducing numbers of the invasive European Starling; (d) leaving old-growth forest of up to ten percent in any genral area of timber harvesting; (e) prohibiting firewood cutting or limiting such activity to snags of less than 35 cm in diameter; (f) providing birdboxes with 7.6 cm aperture as additional nesting sites, set in forest/clearing ecotones.
* Christopher Herlugson.1980. Biology of sympatric populations of western and mountain bluebirds. Washington State University. Dissertation.
* Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye. 1988. The birders handbook: a field guide to the natural history of North American birds''. New York: Simon & Schuster, Inc. 785 p.
* P.A. Taverner. 1919. The birds of the Red Deer River, Alberta. Auk. 36: 248-265.
* F.E.L Beal,. 1915. Food of the robins and bluebirds of the United States. Bulletin No. 171. 31 p. U.S. Department of Agriculture. Washington, DC
* Benedict C. Pinkowski. 1979. Foraging ecology and habitat utilization in the genus Sialia. in: The role of insectivorous birds in forest ecosystems: Proceedings of a symposium; 1978 July 13-14; eds. J.G.Dickson, R.N.Conner, R.R.Fleet et al., Academic Press, Inc. Nacogdoches, TX. New York: pp165-190
* C. Michael Hogan and Michael P. Frankis. 2009. Utah Juniper: Juniperus osteosperma, GlobalTwitcher.com, ed. N. Stromberg
* David F. DeSante and T. Luke George,. 1994. Population trends in The landbirds of western North America. in: A century of avifaunal change in western North America; Proceedings of an international symposium at the centennial meeting of the Cooper Ornithological Society, Studies in Avian Biology No.15, pp. 173-190, eds. Joseph R. Jehl Jr.and Ned K.Johnson, April 17, 1993, Sacramento, Ca..
* Alan E. Wittbecker. 1987. Behavioral ecology of mountain bluebirds in northern Idaho. Bulletin of the Ecological Society of America. 68(3): 451