The Marvel of Nocturnal Bird Migration

Blackpoll Warbler. Photo by Alix d’Entremont

Twice a year, one of the greatest spectacles of nature passes over our heads as we sleep. Tens of thousands of birds in the spring, and hundreds of thousands of birds in the autumn, fly over the Tri-County area. Continent-wide, the count is in the billions. Most of these birds are small songbirds like warblers and sparrows that are known as neo-tropical migrants, which nest during the summer in Canada, and winter in the southern United States, Mexico, Central America, and South America.

Birds migrate north to benefit from the explosion of insects and plant buds that occurs each spring, and where there is also an abundance of suitable nesting habitat and a long daylight period to take care of their young. In the autumn, the birds leave to avoid the harsh winter conditions that will be coming later.

One of the great mysteries of bird migration is how birds find their way over these vast distances without flying off course. Science is now beginning to unlock this mystery and has found that there are a variety of ways that birds navigate. These include getting their bearings from observing the sun and stars, sensing the magnetic fields of the earth, and following landmarks like coastlines and mountain ridges.

There are many challenges to be met in long-distance migration. Migrating birds need to know where to stop to refuel during their voyage. Like athletes, birds must maintain a strong body condition to complete their journey. One possible mechanism is to follow known routes or pathways where abundant and high-quality food will be most available.  The likelihood of favourable weather for a safe flight is another factor determining the location of these routes. Routes which have fewer predators would also be advantageous.

By flying at night, birds are better equipped to meet these challenges. In general winds and turbulence are less at night, there are fewer predators, air temperatures are cooler for such high energy activity, and it is easier to find food for refueling during the day.

As they fly, the birds call to each other. These calls are high pitched tweets that sound virtually identical to each other for us humans. But scientific studies indicate that the birds know the individual identity and gender of the bird they are hearing. The reason for the calling is not well understood, but it is believed that it is a form of communication that may keep species and family groups together, provide information on changes in weather conditions, and maintain flight headings. This is supported by the observation that calling increases when birds become disoriented in fog and bright lights.

For more than ten years, I have been recording these flight calls at different locations throughout Nova Scotia using specially-designed microphones. The microphones amplify the sound so that calls as high as 300 meters in altitude can be heard. The greatest density of flight calls in my studies has been here in the Tri-County area where I record 30,000-40,000 calls in just one location like Cape Forchu and Beaver River over the course of the autumn.

Blackpoll Warbler. Photo by Mark Dennis

Birds arriving to or leaving from the Tri-County area have a big decision to make. Do they fly over the Bay Fundy or do they take the longer but more direct route between Nova Scotia and New England across the Gulf of Maine? Birds hesitate to travel over large bodies of water because if something goes wrong, their options are limited. Using radar and tiny transmitters, called geolocators, attached to the birds, science has found that birds, even within the same species, will make different decisions as to whether they will take the short route or the longer route. These decisions may be governed by a variety of factors such as weather, the age of the bird, or their body condition.

For the Blackpoll Warbler, this decision is even greater. The Blackpoll Warbler nests across northern Canada. In the autumn, it travels eastward, feeding along the way. By the time it reaches Nova Scotia, it may have doubled its body weight. But in addition to the two over water options just discussed, it has a third option: a non-stop flight over the Atlantic to Haiti, the Dominican Republic, or Puerto Rico. This journey is 2,500 kilometers and takes three days. After refueling, it moves on to Columbia and Venezuela to spend the winter. For a creature with a normal weight of 10 to 12 grams (same as one AAA battery), it is indeed a marvel.

Leave a Reply

Your email address will not be published. Required fields are marked *