It may not officially be autumn yet, but the autumn migration is already underway. This movement depends heavily upon the weather and there is definitely a connection between the movement of storm fronts and the movement of large numbers of birds. Last week I checked an internet tool called “BirdCast” and saw that there were 428 million birds in flight at 11:00 PM on Tuesday night. That number is staggeringly large, but it will grow as we get closer and closer to the peak near the end of the month.
Huge tracts of land are about to slip into darkness and cold as the planet moves in its orbit around the sun and the birds that took advantage of all of the sunlight and warmth of summer must now flee to the south where summertime conditions persist throughout the year. All of the surviving adults that traveled up into the northern limits of Canada are now moving south with all of their surviving offspring. These birds do not travel “together,” however. They simply move at once; traveling above the landscape while most of us are snug in our beds. How wondrous the natural world can be.
Among the many species seeking safety to the south will be the warblers. This is a group of small, insectivorous birds that are all roughly the same basic size and shape. The only real obvious differences among them are the colors of their feathers and the songs that they sing. A magnificent example of divergent evolution, the warblers have carved up the forested portions of the Northern Hemisphere into specialized niches in which each species holds an advantage. There are over 50 species of warblers and 35 of them pass through Massachusetts. Among them is a beautiful species known as the black-throated green warbler (Setophaga virens).
A denizen of coniferous forests, the black-throated green is also willing to nest in forests that are a coniferous-deciduous mix. As is often the case with warblers, the sexes seem to divide up the chores of domestic life in a way that focuses on their strengths. Males are responsible for establishing and defending territories. They do this by singing, to declare their ownership and then confronting any other males that may not agree with their claims. In contrast, females will select the location of her nest and build it herself. Nests are usually located 20 to 35 feet off the ground on a horizontal branch with thick foliage. A compact nest with a deep cup, it is generally finished in about 4 days and consists of fine rootlets, mosses, lichens and strips of bark lined with soft fur, feathers and spider webs.
The female will lay 4 to 5 white eggs and then the male and female will take turns incubating. During this incubation period it is essential that the male continues to defend their territory (which is where they will find food for themselves and their chicks), so he may spend more time pursuing this goal than he spends incubating. However, the female will definitely need time to stretch her wings and find something to eat on a regular basis. It has been noticed that the male sings two different types of songs depending on where he is in the pair’s territory. One of the songs is sung at the territory boundaries, while the other is sung when he is close to the female. A wonderful example of the dual purpose of singing in birds, black-throated greens clearly have a fight song and a love song.
The incubation period lasts 12 days and then the care of the chicks begins to take precedence over everything else. The male will continue to sing and defend the territory, but not to the same extent as he did before he had so many mouths to feed. The chicks will fledge in about 10 days and after a successful nesting attempt the male will quiet down for the remainder of the summer. Instead, he and his family will focus on their preparations for flying as far south as Colombia and Venezuela. A few black-throated greens may be found in the extreme southern tips of Texas and Florida during the winter, but the vast majority are found in coastal areas of Mexico and Central America and throughout the Caribbean Islands.
We live in the heart of the black-throated green warbler breeding range, but I don’t have a sufficient percentage of coniferous trees in my immediate surroundings to attract black-throated greens for nesting. Instead, I see them once the breeding season is over and their pre-migratory wandering starts.
The bird in the photo that I have provided is most likely a first-year female. Adult males would have a bold black bib that extends from the base of the beak to the top of the breast. Below that the sides of the breast and belly will be decorated with dark black streaks.
Observing a migrating warbler is simple, but not necessarily easy. All you have to do is get up early on one of those perfect September mornings when the sky is clear and there isn’t sufficient wind to stir the leaves on the trees. Then, you just have to go outside, look up into the treetops and wait for any sign of movement. Bring your binoculars to bear on any moving target and you may see a warbler, a vireo, or some other small bird that has just landed after flying through the night. It is amazing that they have any energy at all after a flight like that, but there is no time to rest when you are bound for a destination so very far away.
Bill Danielson has been a professional writer and nature photographer for 25 years. He has worked for the National Park Service, the US Forest Service, the Nature Conservancy and the Massachusetts State Parks and he currently teaches high school biology and physics. For more in formation visit his website at www.speakingofnature.com, or head over to Speaking of Nature on Facebook.