Signs of Spring 9: Bluebirds!

Photo by K. Thomas, Public Domain

One of our great signs of spring is the return of eastern bluebirds to our field and edge ecosystems. A group of my students monitoring the birds on our campus through the winter and spring saw their first bluebird in early March long before the snows had started to melt or the temperatures had begun to moderate.

The eastern bluebird (Sialia sialis) is a small thrush that is also called the common bluebird, Wilson’s bluebird, and the American bluebird. It is six to seven inches long (smaller than a robin) with a wingspan of ten to thirteen inches and a slender, black beak that is well adapted to its predominantly insectivorous lifestyle. The brilliant blue of its wings and tail (males have entirely blue wings and tails, females are more gray with only the edges of their wings and tails colored blue) is the source of its name. Contrasting with the blue is its rusty-red chest and white abdomen.

In spring and summer eastern bluebirds can be found north and west into Saskatchewan and eastward to Nova Scotia in Canada. Their range then extends to the south all across the United States and into Mexico. They are bounded on the west by the Rocky Mountains. In winter, all of the northernmost birds and many of the middle latitude birds migrate into the southern United States, Mexico, and Central America. The bluebirds return to their northern ranges in March or April to establish their mating territories.

Photo by Sandyphotos 2009, Wikimedia Commons

Bluebirds eat insects (about two thirds of their summer diet) and fruit. Beetles, crickets, caterpillars, and grasshoppers are some common insect foods. They primarily forage for their insects on the ground rather than snapping up them up in flight (like tree swallows, for example). Blackberries, raspberries, honeysuckle berries, wild grapes, dogwood fruits, and red cedar fruits are commonly ingested fruits.

Bluebirds prefer “savanna-like” ecosystems with open, grassy fields and scattered trees for perching and nesting. Edge ecotones between grasslands and forests are also highly preferred habitats. A wide range of tree types and ages is necessary to provide the birds with adequate tree holes and sheltered snags for nesting sites. This requirement has been frequently modified via the inclusion of human-made, bluebird boxes. Bluebirds seem to prefer sites near running water (creeks, streams, etc) in preference to sites near ponds, lakes, or birdbaths. This preference may be due to subtle differences in water quality between ponds and flowing streams or to the observation that flowing water freezes much later in the fall and winter and thaws much earlier in the spring.

Bluebirds are said to be monogamous, but many other “monogamous” bird species have been shown, after closer observation, to actually have very complex patterns of courtship, mating, and reproduction. It is likely that bluebirds, like many of the other thrushes, would also resist simple behavioral labels. Males establish a mating territory of 0.01 to 0.08 square kilometers after returning to their summer ranges in the early spring. Sometimes the males even begin building a nest in order to attract a female. Females arrive in the summer territories later in the season and are “wooed” by the males with songs, grooming behaviors, and offers of food.

Photo by K. Thomas, Public Domain

Nests are typically built in a tree hole or snag (or in a fortunately placed bluebird box!). Ideally these nests are located high off of the ground and have narrow (one to one and half inch) openings into the nesting chamber. Location and entry hole size restrictions are essential protections against nest predators and parasites. Abandoned woodpecker holes are often chosen by a mating bluebird pair for their nesting site. The female will either entirely construct the nest, or, possibly, finish the nest begun by the male in his mating display. The nest will have an outer structure of coarse grasses, hay, and pine needles and an inner lining of softer grasses, hair, and feathers. Nest construction takes about ten days. Nests are only used once.

Females lay one egg a day for three to seven days for each of their broods. Here in Western Pennsylvania there can be two clutches each season (early spring and mid-summer) and incubation is between thirteen and sixteen days. After hatching, the female broods the nestlings for about a week and then joins the male in the Herculean task of gathering sufficient food for the rapidly growing nestlings. More than half of the gathered foods are insects. After fifteen days the nestlings are ready to fledge and very quickly after that are able to feed and care for themselves. At one year of age, the birds are ready to reproduce. Life spans of bluebirds in the wild possibly average six years for those individuals who successfully fledge but only about 65% of the eggs that are laid, actually yield successful fledges.

Bluebirds may be preyed on by bears, raccoons, and house and feral cats. Blow flies also frequently infest nests and nestlings. Nest predation by cats, chipmunks, and squirrels and also English sparrows and European starlings (two exotic, invasive species) cause substantial egg and nestling loss. The influx of the English sparrows and the European starlings along with the habitat spread of the nest parasite, the brown headed cowbird, are major reasons for the eastern bluebird’s numerical and distributional decline throughout the twentieth century. Human destruction of nesting and feeding habitats has also been a contributing factor to this decline. Human efforts to provide existing bluebird populations with suitable and secure nesting sites (“bluebird boxes”) have, however, been extremely successful in bringing this beautiful species back from the brink of extreme decline. The North American Breeding Bird Survey reports that since 1966 eastern bluebird populations have increased by nearly two percent a year! The Cornell Laboratory of Ornithology estimates the worldwide population of eastern bluebirds (80% of which spend at least some time in the United States) at 22 million individuals.

Photo by D. Sillman

Deborah and I have recently joined a group up at Harrison Hills Park to monitoring a set of 38 bluebird boxes that were originally established in 2005. In 2006 observers counted 90 bluebirds fledged from the nests in these boxes! These boxes have not been repaired or monitored over the past eight so a group of volunteers led by Kristi Cihill and her husband Justin have jumped in to refit as many houses as possible and to watch and record the nesting and fledgling activity over the summer. I would like to call our group the “Cavity Nesting Team” since ALL native bird species who might utilize our nest boxes are going to be very highly prized. Tree swallows, nuthatches, chickadees, titmice, house wrens and Carolina wrens are all possible (if not likely) cavity nesters in our area whom we hope to see throughout the summer. The accompanying picture of four bluebird eggs was taken by Deborah last weekend at one of our boxes!

So, it’s bluebirds and more this summer! What a wonderful Spring has finally happened!