Protecting the Declining Cerulean Warbler

A cerulean warbler being handled by a refuge researcher. Photo by Emma DeLeon
A cerulean warbler being handled by a refuge researcher. Photo by Emma DeLeon

It’s late May and the forest is fresh with new leaves. Deep in the woods everything takes on a pale, spring-green tint. The forest echoes with a cacophony of birdsong.  Amid the jumble, biologists at Iroquois National Refuge are searching for the sound of the elusive cerulean warbler. The song is high-pitched and burry. It starts with three buzzy notes, then four softer fast-warbling calls, and ends with a buzzy trill. Some people say it sounds like “chyoo,-chyoo,-chyoo -TSEEE” ( or if you prefer “BEER-BEER-littlebit- PLEASE”!)

Cerulean warblers are notorious for staying high in the canopy of mature forests, and can be difficult to find even for an experienced birder.  Your best chance at tracking one down is to learn the call, then spend a while squinting into the treetops.  Ceruleans are one of our smallest warbler species and they are always in motion. As soon as you catch a flash of movement, they will invariably disappear directly into the sun or behind a branch. Males have bright blue backs with a dark blue necklace, and streaks on their sides.  Females are a subtle greenish-blue, and lurk silently and invisibly in the treetops. Despite the frustration and the neck crunching perspective, ceruleans are worth seeing!  

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Like many species of warbler, ceruleans are declining across their range. Since these birds are so difficult to observe – especially the females – less is known about their behavior than most other species of warbler. Iroquois National Wildlife Refuge is participating in an international project of large scope that aims to learn more about the full life cycle of these birds, including their behavior and habitat on their wintering grounds in South America, and their breeding grounds in eastern North America and along their migratory route.

Researchers in the United States, Canada, and Columbia are catching cerulean warblers and attaching tiny transmitters to track the birds’ movements. Each transmitter weighs less than a gram, and is made up of a battery, radio components, and antenna attached to an elastic harness. The harness slips over the birds’ legs and sits flat against its back like a tiny backpack.  After a few months the elastic will wear out, and the transmitter will drop off the bird. The project aims to catch mostly female warblers, but some males receive transmitters as well.

We use two types of transmitters at the refuge. One type emits quick signal pulses every 3-7 seconds, and is used to track bird movements within their breeding territory. These transmitters can be tracked using a handheld receiver antenna, so researchers can follow individual birds to look at territory size and see which trees and habitats are used most consistently.  A second type of transmitter emits a pulse only every 29 seconds, but has a longer battery life.  Birds with the long pulse transmitters can be tracked on migration by a series of receiver towers scattered through the US, Mexico, and South America.  If a bird happens to fly close enough to a tower for a signal to transmit, researchers will know what migratory route the bird is taking, and may have a better idea of what challenges a bird is facing or what migratory stops are most important.   

So how do you catch a nearly invisible tree-top dwelling bird?  The trick is to get the birds to come down out of the canopy.  Biologists use call recordings and painted decoys to lure curious birds down lower where they can be caught in the mesh of a special net so fine, it’s called a mist net. Researchers carefully untangle the bird, take a series of measurements to determine age, sex, and body condition, and band the bird with a uniquely numbered federally issued aluminum leg band.  If they are healthy, the proper weight, and the desired age and sex, they also attach a transmitter.  After only a few minutes, the bird is released safely back into its territory. 

The biologists who catch and handle the birds are all highly trained. Disturbance to the birds is limited as much as possible while still collecting information that will directly help the long-term survival of the species.  Cerulean warblers and other migrants at Iroquois National Wildlife Refuge travel thousands of miles to reach a safe and appropriate habitat for raising their young.  The breeding season is short, and birds must work hard to attract a mate, build a nest, produce eggs, and feed nestlings. It is important for both researchers and visitors to the refuge to limit the amount of contact and disturbance in warbler habitat.  For this reason, it is not recommended that people leave the trails or use recorded calls to get a good look at ceruleans or any species of bird.  However, if you are interested in seeing a cerulean warbler, they can be seen and heard from several of the trails throughout the summer. Listening to calls at home before coming to the refuge, or using a silent non-intrusive phone app like Merlin, can help alert you to the presence of a warbler. Preparation, a good pair of binoculars, and a little patience can go a long way toward getting a rewarding view of an amazing bird. 

By Emma DeLeon