Warblers find their niche by controlling insects

Over the recent years, I've been noticing some migratory birds heading north sooner than they did when I grew up in the area. This got me thinking about migratory warblers and how they are alike and different.

With that in mind, we'll look at three of these small insectivores: the palm, Blackburnian, and black-and-white warblers.

There are dozens of warbler species. They are diminutive, often colorful birds that can't hold still. Since they rely on insects for food, they don't stay in our neighborhood during the colder months.

A few stay during the summer, but the majority just pass through heading north in the spring and south in autumn. Most will breed and nest in the coniferous forests of the northern U.S. and Canada.

Now, I've worked in the northern U.S. and I know there are a lot of bugs up there in the summer, but enough for 50-plus species of warblers? Wouldn't these little birds - weighing roughly a third of an ounce - be getting in turf fights over available food? Nature has worked this out.

Habitats and niches

First, two quick definitions. Habitat is the local environment occupied by an organism. The lake a fish occupies would be its habitat.

Niche is often described as the "role" of a species in its habitat. That is to say, what a critter eats, what it is eaten by, where it lives and so on. A bottom-feeding catfish, for example, plays a different role in the lake than does a predatory bass.

This brings us back to the warblers heading north for the summer. What are the niches for a these miniature bug-eaters?

Palm feeders

Speaking of roles, let me introduce the cast of warblers for this forest production.

We'll start with the palm warbler. It is one of the first migrants to be seen in spring. Yes, it is named after the tropical trees but, unfortunately, this is a poor name for the species. When on its wintering grounds, it is rarely seen in palms, preferring the low-growing saw palmetto.

This preference for feeding low is true during migration as well. It can often be seen feeding on the ground, in shrubs, and along fences. Its diet includes ants, aphids, mosquitoes, flies, grasshoppers, gnats, plus small green caterpillars. While I wouldn't call it a bottom-feeder, feeding low is definitely its role.

The palm warbler is brown above, yellowish below, wearing a sporty chestnut cap.

Some palm warblers stay to breed in northern Wisconsin, Minnesota or Michigan, but most are headed for Canadian bogs. There, they'll build a nest (can you guess?) at the base of a small birch, fir, or spruce. Suffice it to say, the palm warbler knows its role.

Barking up the right tree

So does the black-and-white warbler. This early migrant lives up to its name. It is, indeed, black and white. It has a striped crown with stripes running lengthwise down its back and sides. It is sometimes called the zebra bird.

The black-and-white warbler winters from the Gulf Coast to northern South America. Compared to the palm, the black-and-white warbler is less choosy about the forests it nests in. It can be found in a variety of broadleaf and mixed-forest habitats. Given these eclectic tastes, it nests across much of southern Canada and the eastern U.S.

It specializes in feeding along the larger limbs and trunks of trees. Unlike other warblers, its bill is slightly curved, which facilitates poking through nooks and crannies in search of spiders, leaf beetles, wood borers, bugs, moths, and weevils. Clearly, the back-and-white warbler will not be upstaging the palm warblers when it comes to habitat niches.

Blackburnian beauties

The star in this three-warbler performance is the Blackburnian warbler. It is a brilliantly colored Neotropical migrant (that's college talk for flying back and forth from South America). The male has a flaming orange throat and head markings. The female's markings are similar, but paler.

Speaking of male and female, this bird's name involves a couple; Anna and Ashton Blackburne.

Anna was an 18th century English naturalist. Her brother Ashton lived in New York and collected birds for his sister during the time of the Revolutionary War.

Anna was working on her own natural history museum in England. She shared her collection with others, including Thomas Pennant, who first described this warbler species in his volume Arctic Zoology. In the preface to the book, Pennant acknowledges Anna's generosity with her collection of North American bird skins. Of course, the birds were shot by her brother. It is thought that the bird's name honors both of the Blackburns.

Let's get back to the Northwoods. Picture a mature coniferous tree, say 60 feet tall, like you might see in Canada or northern Minnesota. A palm warbler could be feeding on the ground under a lower bough while a black-and-white warbler is investigating crevices in the tree's trunk. Where is the Blackburnian? At the top.

The male Blackburnian would be feeding around the top 20 feet of that 60-foot tall conifer. Females often feed a bit lower down the tree. While these warblers will eat spiders and beetles, they are especially fond of butterfly and moth larvae.

You might wonder how a treetop warbler can specialize in caterpillars that lack wings. I can explain. Take, for example, the spruce budworm, a moth larva. You might think it eats spruce, but it prefers to dine on balsam-fir (think Christmas trees).

Budworms are capable of webbing shoots together to make a protective shelter. This silk can also be used to move around, as well. In order to efficiently move from one tree to another, the budworm crawls out to the branch tip and spins out a line of silk. A gust of wind can catch the silk strand, with caterpillar attached, and carry it to an adjacent tree top. Then the budworm starts munching on fresh needles.

Now budworms will occasionally explode in numbers and give foresters headaches. Guess who are the foresters' and park managers' allies in reducing budworm numbers? You got it, the colorful Blackburnian warblers. The Blackburnians feed where the "ballooning" caterpillars alight.

Unfortunately, pesticides used to control spruce budworm can have a negative impact on the number of Blackburnian warblers.

Back to the beginning

Before wrapping up, let's return to the species we started with and a wonderful poem titled Palm Warbler by Barry Middleton. His description of hyperactive warbler behavior is perfect. I'll forgive this talented poet for suggesting they are interested in tree seeds.

They are like feathered

over active children

the way they dart about

bobbing their dun tails

they never sit for a second

their song is high and faint

a cheeping that sounds

yellow like their breast

they dart away from me

in a naughty guilty panic

caught stealing pine nuts

ashamed and a bit scared.

• Mark Spreyer is the executive director of Stillman Nature Center in Barrington. Email him at stillmangho@gmail.com.