For the Birds - Purple Martin

Maine provides breeding ground for six species of swallows. Tree Swallows, the first to arrive in the spring, are the most abundant. Barn Swallows are nearly as common. As colonial nesters, Cliff Swallows and Bank Swallows may be locally abundant. Rough-winged Swallows even less common in the state. But the least abundant is the Purple Martin.

Purple Martins are the largest swallows in North America and among the largest in the world. They are an extremely popular bird, recognized by many non-birders. Males are completely covered with a glossy blue black plumage and are the only swallows in North America with a dark belly. Females with brown upperparts and lighter underparts are similar to other female swallows but can be distinguished by their large size and the prominent brown or gray collar around the nape. A martin’s chattering vocalization adds to their charm.

Although Purple Martins are scattered in parts of western North America (parts of the desert Southwest and coastal areas from British Columbia to central California), they are continuously found throughout most of the eastern states. However, they occur discontinuously in New England. In Maine they occur in a broad swath proceeding northeastward from coastal York County to the Calais region of Washington County. This band continues northeasterly into New Brunswick ending at the northwestern tip of Nova Scotia.

The Maine Department of Inland Fisheries and Wildlife estimates there are probably not more than a couple of hundred pairs in the state. Purple Martins are colonial nesters but Maine colonies are typically smaller than the large colonies one sees in southeastern states.

Although Purple Martins nested in natural cavities before the development of the United States by European immigrants, nearly all eastern birds now nest in human-made apartments. Elaborate martin houses may be used as well as gourds, hollowed out, and hung close together. The provision of houses may have caused martins to become colonial. Martins in the west often nest solitarily in natural cavities. Eastern martins have nested exclusively in human-made cavities since 1900. The geographic difference in nest-site choice is one of the most pronounced of any bird over its range.

Because of their popularity, incorrect information about Purple Martins gets published and reprinted, becoming fact in the eyes of unskeptical readers. Some of these myths pertain to the diet of Purple Martins. Manufacturers of martin houses often encourage the purchase of their products by touting the effectiveness of martins in controlling mosquitoes. Don’t believe it!

Martins tend to feed several hundred feet above the ground and always take flying insects. Their prey are mostly large species like dragonflies, wasps, bees, and Japanese beetles. Martins feed only during the day.

Mosquitoes on the other hand tend to be found in damp grass close to the ground during the day and may only be active at night. No study of martin diets has ever shown mosquitoes to be a significant component of their diet. I suspect that martins may even aid mosquitoes by reducing the populations of dragonflies, which are often effective mosquito predators.

Depending on flying insects for sustenance, Purple Martins are subject to the vagaries of the weather, particularly in New England and the Maritime Provinces. Cold or extended rainy weather during the early part of the summer can reduce the number of flying insects and spell disaster for Purple Martins.

The nesting process is slower than in many other songbirds. Incubation takes place for 15-20 days before hatching. The female does most of the incubation although the male may take a turn while the female leaves the nest to feed. The young do not fledge until 28 or 29 days after hatching.

As one would expect, nesting begins earlier in more southern parts of the range. Females begin laying eggs around the end of March in Texas but not until the first ten days of June in Maine and Nova Scotia.

It’s wonderful to have a martin colony on your property. However, getting a colony established in Maine is a tough proposition. Besides their low population numbers in Maine, martins have specific requirements for their nest boxes. The nest box needs to be higher than trees in the local area and no tree should be within 60 feet of the martin house. The martin house can be placed between 30 and 120 feet from human habitation. Research has shown that martin houses further than 120 feet from human houses are usually avoided. Martins have learned that proximity to a human house lowers the risk of predation. Another challenge is keeping House Sparrows and European Starlings out of the nest cavities.

Lots of useful information can be found at two websites: http://www.purplemartin.org and http://www.purplemartins.com/

[This column was originally published on July 1, 2006]

For the Birds - Ruby-throated Hummingbird

Over 320 species of hummingbirds have been described. This family of birds occurs only in the New World and most species are found in Central America and South America. With a little effort, it is not hard to find a dozen species in places like southeastern Arizona. But in eastern North America, we have a single species, the Ruby-throated Hummingbird.

Ruby-throated Hummingbirds have a broad distribution occurring in the United States eastward of the 100th meridian everywhere except the southern tip of the Florida peninsula. In Canada, ruby-throats occur from the Maritime Provinces westward into Saskatchewan. No other species of hummingbird in North America has a broader geographic range.

It’s hard to think of a Maine bird whose summer arrival is more anticipated. Who is not fascinated by these little sprites? The red throat feathers (called a gorget) of a male Ruby-throated Hummingbird may seem to sparkle in the right light. Their throat feathers refract light, giving the bird an iridescence that makes the feathers seem to shimmer as the bird moves its head.

Ruby-throats winter from Mexico south to northern Panama. To migrate to eastern North America, these birds often fly non-stop over the Gulf of Mexico, a flight of 500 miles. Such a trip requires a lot of fuel and Ruby-throated Hummingbirds fatten on insects and nectars, nearly doubling their weight before the trans-Gulf flight.

The mating system of Ruby-throated Hummingbirds appears to be polygyny; one male may mate with many females. No long-lasting pair bonds are forged. A male will court a female through complicated flight displays. When a female flies into his territory, he begins with a dive display, flying U-shaped loops starting from as high as 30 feet above the female. If the female perches, he switches to very fast, side-to-side flights, with his gorget extended, within two feet of the female. If the male is acceptable as a mate, the female will cock her tail feathers to one side and lower her wings, inviting the male to mate with her. Mating lasts only about 2-3 seconds and that is the end of the male’s contribution to the offspring.

The female builds the nest by herself. The base is made of the down from dandelions and thistles and is attached to the upper side of a branch, much like a saddle over the back of a horse. The sides of the nest are made of plant down, bud scales and spider webs. The plant material is woven into the nest with the spider silk.

The eggs are usually two in number and, as you might imagine, are tiny. An average egg is half an inch long.

Incubation takes 12-14 days and the young hatch as naked, blind chicks. Feeding usually begins soon after hatching and the young fledge about 20 days after hatching.

We think of Ruby-throated Hummingbirds as depending on nectar for their sustenance. However, these birds also take spiders and insects (mosquitoes, gnats, fruit flies and small bees). Although no quantitative study has been conducted, some ornithologists have examined ruby-throat feces and estimate that 50-60% of the diet is insects.

As nectar-feeders, hummingbirds, along with many insects, serve as pollinators for a number of plants. Trumpet creeper, a forest vine, seems to be primarily pollinated by Ruby-throated Hummingbirds.

Of course, the fondness of hummingbirds for nectar makes it easy for us to attract hummingbirds to our gardens and backyards. Hummingbirds will visit many types of flowers but generally those with long, tubular flowers are most attractive. Bee balm, for instance, is a favorite of ruby-throats.

An offering of sugar water is an effective way to attract hummingbirds through artificial means. The nectar produced by hummingbird-pollinated flowers is rich in sucrose (table sugar). On the other hand, flowers that are pollinated by songbirds like Cedar Waxwings and European Starlings are rich in two other types of sugar, fructose and glucose. Experiments with Rufous Hummingbirds using solutions made from these three sugars showed a preference of the hummers for sucrose.

The best recipe for making sugar water for hummingbirds is one part sugar to four parts of water. You can heat the water to make the sugar dissolve more easily but heating the water is not necessary.

Don’t add red dye to your sugar water. The most common red food dye, Red Dye Number 40, is approved for human consumption in the U. S. but not in several European countries. We also know that substances that seem to be safe for humans may be harmful to other vertebrates. Tylenol, for example, has strong negative effects on cats. Arguments about the safety of red dyes are moot to me; most hummingbird feeders have some red on them anyway and the addition of red dye does not make the feeder more attractive. At best, red dye is useless. Why bother with it?

[First published on June 17, 2006]

For the Birds - Bicknell's Thrush

Maine has seven members of the thrush family among its breeding birds. These include Eastern Bluebird, American Robin and five species of thrushes with various amounts of spotting on their breast. These spot-breasted thrushes are among the finest of avian songsters. Their flute-like songs are magical, in part owing to the fact that these thrushes can control the left and right side of their syrinx, the organ that produces sound in birds. Thrushes can harmonize with themselves!

The Wood Thrush, more common to our south, but can reliably be found throughout the state. Veeries are found throughout the state. The Veery sings a song that spirals downward in pitch; the name of this thrush comes from its characteristic call note, Veer. Swainson’s Thrush, whose song spirals upward rather than downward like the Veery’s song, is a bird of spruce-fir forests. This species is therefore scarce in the southern third of Maine. Hermit Thrushes are widely distributed in the state, nesting in bogs as well as conifer forests. The final species, Bicknell’s Thrush, is the least known of our thrushes and is restricted to high elevations.

Bicknell’s Thrush has only been recognized as a full species for the past decade. It was formerly considered a subspecies of the widely distributed Gray-cheeked Thrush, which occurs from Alaska to Newfoundland in boreal habitats. Thanks to the work of the late Henri Ouellet, ofthe Canadian Museum of Nature in Ottawa, the Check-list Committee of the American Ornithologists Union elevated Bicknell’s Thrush to species status.

Ouellet built the case for Bicknell’s Thrush being a valid species with several lines of evidence. He began by demonstrating that Bicknell’s Thrushes differ in the color of the upperparts, the tail feathers, the throat and the undersides from Gray-cheeked Thrushes. Ouellet also showed that Bicknell’s Thrushes are smaller than Gray-cheeked Thrushes from the same latitudes. But morphological differences do not necessarily demonstrate that Gray-cheeked Thrushes and Bicknell’s Thrushes do not breed with each other. However, he was able to show that Bicknell’s Thrushes and Gray-cheeked Thrushes do not overlap in either nesting areas or wintering areas.

To really make his case, Ouellet needed to demonstrate that the two forms are reproductively separate. He analyzed the songs of Gray-cheeked and Bicknell’s Thrushes, finding distinct differences. Furthermore, Bicknell’s Thrushes on their breeding grounds did not respond to playbacks of Gray-cheeked Thrushes.

Ouellet’s final piece of evidence was a comparison of the DNA of the two forms. The analysis showed that the DNA differed by an amount similar to differences between other closely related species. The analysis suggested that the ancestral group split into Gray-cheeked Thrush and Bicknell’s Thrush about a million years ago.

The nesting range of Bicknell’s Thrush includes the Adirondack region of New York, and portions of Vermont, New Hampshire, Maine, Nova Scotia, New Brunswick and Quebec.

In northern New England, Bicknell’s Thrushes are generally found above 3000 feet in elevation although these birds may occur as low as 2,050 feet on some Maine peaks. Bicknell’s Thrushes like stunted spruce-fir forest, particularly areas that have been disturbed by fir waves or rime ice accumulation during the winter. They may nest along the edge of ski trails. The thick re-growth of these disturbed areas is often nearly inpenetrable, necessitating patience if you want to see one of these birds.

Male Bicknell’s Thrushes sing throughout the day until the middle of June and then singing stops abruptly. Birds will occasionally call after singing drops off but the dense habitat and secretive nature of the thrushes make them very hard to find. In short, if you want to see a Bicknell’s Thrush on its breeding habitat in Maine this year, you better plan a trip soon!

The entire population of Bicknell’s Thrushes is certainly no more than 50,000 individuals, making Bicknell’s Thrush a species of concern for environmental managers. These birds do nest at fairly high density (average of about 50 pairs per 100 acres) so habitat alteration of even a small area at high altitude can have serious repercussions for the species.

Conservationists worry about winter habitat degradation as well. The entire population winters on only four islands in the Greater Antilles.

In 1992, The Vermont Institute of Natural Sciences (VINS) launched a survey of Bicknell’s Thrushes in the northeastern United States. You can find lots of information on Bicknell’s Thrush at their website including maps of mountains with breeding Bicknell’s Thrush and downloadable recordings of songs and calls. Their URL is: http://www.vinsweb.org/cbd/mtn_birdwatch.html

You can download a copy of Henri Ouellet’s paper and two papers from the VINS research team, all published in the Wilson Bulletin, by going to: http://elibrary.unm.edu/sora/

Type in Bicknell’s Thrush in the search box. The second, third and fourth entries are the ones you will want to see.

[Originally published on June 3, 2006]