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Jeff Green | May 22, 2008

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For many nature lovers, the third week of May is the big show. The period from about May 14 to May 21 is about the peak of the spring migration of warblers, tanagers, orioles, vireos and other groups of birds that are collectively known as Neotropical migrants. In recognition of the importance of this week in the birder’s calendar, today’s subject is the 300 or so species of Neotropical migratory birds that head north to breed and raise young in the United States and Canada. Once the breeding season is over in late summer or fall they return to warmer climates in tropical regions of Mexico, Central America, South America, and the Caribbean. While we often consider these to be “our” birds, they actually spend more time on their wintering grounds than they do here in Canada. I suppose this makes them “their” birds – or perhaps shared custody is a better way to describe the arrangements.

Scientists generally agree that these patterns of migration evolved between 10 and 30 million years ago, and are still going strong. For many species of birds, it appears that the benefits of exploiting the explosion of summer insect life in our forests, fields and wetlands outweigh the risks of flying thousands of kilometers north to mate and raise their young.

I would like to explore one particular group of Neotropical migrants – the wood warblers – in a bit more detail. There are almost 40 different species of these pintsized bundles of energy that either pass through or breed in the Land O’Lakes area. Warblers inhabit virtually every possible habitat in our area – seek out Common Yellowthroats in shrubs around the edges of wetlands, Ovenbirds on the ground in undisturbed forests, and Golden-winged Warblers in scrubby second growth forests.

Some warblers are quite common – for example the familiar Yellow Warbler was discovered to be the 6th most common breeding bird in the Kingston region, and can be readily found in its open brush, willow and even suburban habitat. I recall as a boy my grandmother, who was born in Yorkshire, England used to refer to Yellow Warblers flitting about her Montreal area garden as “wild canaries”.

Last May, I was doing some birding on our woodland property east of Sharbot Lake during the peak of spring migration. While standing in an area of young saplings between an old beaver pond and a semi-mature stand of forest, I was able to pick out the calls of 7 different warbler species over a period of about 5 minutes – American Redstart, Yellow Warbler, Black and White Warbler, Common Yellowthroat, Chestnut-sided Warbler, Ovenbird and Black-throated Green Warbler. Unfortunately I had to move on because I was about to be carried off by swarming black flies, which is the major downside of birding in mid-May.

Many birders do the majority of their songbird identification during breeding season by ear. An experienced birder with a good ear for bird songs can walk through a forest in spring and check off birds they have identified with 100 percent certainty without once raising their binoculars. I personally enjoy both the sights and sounds of birds, so while I often first identify a bird by its song, I usually try to hunt it down to catch a good look at it as well.

Much has been written about the conservation of Neotropical migrants, as the populations of many species have shown pronounced declines over recent decades. Scientists have pointed to a number of possible explanations, including habitat fragmentation on their northern breeding grounds, habitat loss on their wintering grounds in the tropics and pesticide use throughout both their winter and summer ranges. While there are a variety of factors that explain why some species are declining, one common thread is the increasing amount of space and resources that people are using to satisfy the needs of our own growing population. In effect, it seems that people may be “crowding out” many of our avian friends.

On the brighter side, the numbers of some previously rare warblers in our area may be gradually increasing. For example, while still not plentiful, the Cerulean Warbler has benefited from the regrowth of mature deciduous forests on some parts of the southern Canadian Shield, including the Land O’Lakes area. And in some cases, the opening up of the forests by European settlers provided certain species with their preferred habitat. For example the now common Chestnut-sided Warbler appears to have greatly increased in numbers thanks to a huge expansion in its preferred open second growth forest habitat. It has been reported that the famous early ornithologist John James Audubon saw only one Chestnut-sided Warbler during his lifetime of observing birds in eastern North America!

Here is a fascinating fact to illustrate the biological wonder that is the migration of the Neotropical migrants. Many Blackpoll Warblers, which nest in the boreal forests of North America, are reported to make an 86-hour, 4000-kilometer nonstop flight across the Atlantic to their wintering grounds in South America. These tiny birds must double their weight in the late summer and early fall to build up enough body fat to make this long-distance flight.

Neotropical migratory birds – truly the stars of the May spring migration.