The Eastern Bluebird (Sialia sialis) is a beloved North American songbird, known for its striking blue plumage and gentle nature. Understanding their habitat, dietary needs, nesting habits, and behavior is crucial for conservation efforts and for anyone wishing to attract these beautiful birds to their yards. This guide provides a comprehensive overview of the Eastern Bluebird, drawing on expert knowledge and recent research.
Habitat Preferences
Eastern Bluebirds thrive in open country that features scattered trees, with minimal understory and sparse ground cover. Their original habitats likely included frequently burned pine savannas, beaver ponds, mature yet open woodlands, and natural forest clearings. Today, they are commonly found in areas like pastures, agricultural fields, suburban parks, backyards, and golf courses. These environments provide the necessary perching spots for foraging and open areas for insect hunting.
Diet and Foraging Habits
The primary food source for Eastern Bluebirds throughout much of the year consists of insects caught on the ground. Their diet prominently features caterpillars, beetles, crickets, grasshoppers, and spiders. During the fall and winter months, their diet shifts to incorporate a significant amount of fruit. They consume a variety of berries, including those from mistletoe, sumac, blueberries, black cherry, tupelo, currants, wild holly, dogwood, hackberries, honeysuckle, bay, pokeweed, and juniper. On rare occasions, Eastern Bluebirds have been observed preying on small vertebrates such as salamanders, shrews, snakes, lizards, and tree frogs. Their foraging strategy often involves perching on wires or fence posts overlooking open fields, from which they can spot prey from a considerable distance. They then flutter down to the ground to capture insects or, occasionally, catch them in midair.
Nesting Behavior and Requirements
Eastern Bluebirds are cavity nesters, utilizing natural tree hollows or artificial structures like nest boxes. They tend to prefer old woodpecker holes in dead pine or oak trees, often located up to 50 feet above the ground. Studies suggest that older bluebirds may show a preference for nesting boxes, and individual birds can adapt their preferences between nesting attempts. Research indicates that bluebirds may favor slightly smaller nest boxes (4 inches square) with slightly larger entrance holes (1.75 inches in diameter) when given a choice.
Once a male has attracted a female to a potential nest site through courtship displays, the female undertakes the entire nest-building process. She constructs the nest by loosely weaving together grasses and pine needles, then lining it with finer grasses and sometimes incorporating materials like horsehair or turkey feathers. In areas where nest boxes are prevalent, a single territory might offer several suitable nesting holes. Females may build nests in each available cavity but typically only use one. Bluebirds are known to reuse the same nest for multiple broods within a single breeding season.
Key nesting facts include:
- Clutch Size: 2-7 eggs
- Number of Broods: 1-3 broods per year
- Egg Dimensions: Approximately 0.7-0.9 inches in length and 0.6-0.8 inches in width
- Incubation Period: 11-19 days
- Nestling Period: 17-21 days
- Egg Description: Typically pale blue, though white eggs are rarely observed.
- Hatchling Condition: Newly hatched birds are naked except for sparse tufts of dingy gray down, their eyes are closed, and they are physically clumsy.
Social Behavior and Territoriality
Eastern Bluebirds exhibit distinctive foraging behaviors, often perching on elevated spots to scan for prey. Their flight pattern is characterized by relatively low altitude, fast, and somewhat irregular wingbeats. Males can be fiercely territorial, engaging in high-speed chases with rivals, sometimes grappling with their feet, plucking feathers, and striking with their wings.
The competition for suitable nesting cavities is intense. Male bluebirds are known to aggressively defend their nesting sites against other species that require holes for nesting. This includes species like House Sparrows, European Starlings, Tree Swallows, Great Crested Flycatchers, Carolina Chickadees, and Brown-headed Nuthatches. They may also attack non-cavity nesters such as robins, Blue Jays, mockingbirds, and cowbirds if they perceive them as a threat to their nesting territory. Male bluebirds attract females through a display involving carrying nesting material into and out of the cavity. Once a female joins him in the nest hole, the pair bond is typically formed and can last for several seasons. Studies indicate that approximately one in every four or five eggs may have a parent outside of the established pair.
Conservation Status and Efforts
The conservation status of the Eastern Bluebird is generally positive. According to the North American Breeding Bird Survey, populations have shown an increase between 1966 and 2019. Partners in Flight estimates a global breeding population of around 23 million individuals, and the species has a Continental Concern Score of 7 out of 20, indicating low conservation concern.
Historically, Eastern Bluebird populations experienced a decline in the early 20th century. This was largely attributed to competition from aggressive, introduced species like the European Starling and House Sparrow, which increasingly occupied available nesting cavities. However, conservation efforts initiated in the 1960s and 1970s, particularly the establishment of bluebird trails and widespread nest-box campaigns, have significantly helped populations recover. The development of nest boxes specifically designed to exclude larger competitors like the European Starling has been instrumental in this resurgence.
References
- Dunne, P. (2006). Pete Dunne’s essential field guide companion. Houghton Mifflin Harcourt, New York, USA.
- Ehrlich, P. R., D. S. Dobkin, and D. Wheye (1988). The Birder’s Handbook. A Field Guide to the Natural History of North American Birds, Including All Species That Regularly Breed North of Mexico. Simon and Schuster Inc., New York, NY, USA.
- Gowaty, Patricia A. and Jonathan H. Plissner. (2015). Eastern Bluebird (Sialia sialis), version 2.0. In The Birds of North America (P. G. Rodewald, editor). Cornell Lab of Ornithology, Ithaca, New York, USA.
- Lutmerding, J. A. and A. S. Love. (2020). Longevity records of North American birds. Version 2020. Patuxent Wildlife Research Center, Bird Banding Laboratory 2020.
- Partners in Flight. (2020). Avian Conservation Assessment Database, version 2020.
- Sauer, J. R., D. K. Niven, J. E. Hines, D. J. Ziolkowski Jr., K. L. Pardieck, J. E. Fallon, and W. A. Link (2019). The North American Breeding Bird Survey, Results and Analysis 1966–2019. Version 2.07.2019. USGS Patuxent Wildlife Research Center, Laurel, MD, USA.
- Sibley, D. A. (2014). The Sibley Guide to Birds, second edition. Alfred A. Knopf, New York, NY, USA.
Learn more about Eastern Bluebirds at Birds of the World.
