It may not officially be autumn yet, but the autumn migration is already in full swing. This movement is highly dependent on the weather and there is certainly 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 p.m. on Tuesday evening. That number is staggering, but it will grow as we get closer and closer to the peak by the end of the month.
Huge tracts of land are about to slip away in darkness and cold as the planet moves in its orbit around the sun and the birds that have benefited from all the sunlight and heat of summer must now flee south, where summer conditions persist all year round. All surviving adults who traveled to Canada’s northern borders are now moving south with all their surviving offspring. However, these birds do not travel “together”. They just move at the same time; traveling above the landscape while most of us snuggle in our beds. How wonderful can the natural world 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 about the same basic size and shape. The only real obvious differences between them are the colors of their feathers and the songs they sing. A wonderful example of divergent evolution, the warblers have carved the forested areas of the Northern Hemisphere into specialized niches in which each species has 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 warbler (Setophaga virens).
An inhabitant 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 household chores in a way that focuses on their strengths. Males are responsible for establishing and defending territories. They do this by singing, declaring their property, and then confronting other men who may disagree with their claims. In contrast, females will choose the location of her nest and build it herself. Nests are usually located 20 to 35 feet above the ground on a horizontal branch with thick foliage. A compact nest with a deep bowl, it is generally ready in about 4 days and consists of fine roots, mosses, lichens and strips of bark lined with soft fur, feathers and cobwebs.
The female lays 4 to 5 white eggs and then the male and female take turns incubating. During this breeding period it is essential that the male continues to defend his territory (where they will find food for themselves and their chicks) so that he can spend more time pursuing this goal than breeding. However, the female will definitely need time to stretch out her wings and regularly find something to eat. 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 close to the female. A wonderful example of the dual purpose of bird singing, black-throated green clearly has a fight song and a love song.
The incubation period lasts 12 days and then caring for 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 before he had so many mouths to feed. The chicks fledge after about 10 days and after a successful nest attempt, the male will rest for the rest of the summer. Instead, he and his family will focus on their preparations to fly as far south as Colombia and Venezuela. A few black-throated vegetables can be found in the extreme southern tips of Texas and Florida during the winter, but the vast majority are found in the coastal regions of Mexico and Central America and the Caribbean islands.
We live in the heart of the Black-throated Warbler breeding area, but I don’t have enough conifers in my immediate area to attract black-throated greenery to nest. Instead, I see them once the breeding season is over and their pre-migration begins.
The bird in the photo I have provided is most likely a freshman female. Adult males would sport a bold black bib that extends from the base of the bill to the top of the chest. Below it, the sides of the chest and abdomen are decorated with dark black stripes.
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 enough wind to stir the leaves on the trees. Then all you have to do is go outside, look up into the treetops and wait for any sign of movement. Bring your binoculars to any moving target and you may spot a warbler, vireo or other small bird that has just landed after flying through the night. It is amazing that they still have energy after such a flight, but there is no time to rest when you are en route to a destination that is so far away.
Bill Danielson has been a professional writer and wildlife 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 currently teaches biology and physics in high school. For more information, visit his website at www.spokeofnature.com, or visit Speaking of Nature on Facebook.