Yellow and Wilson’s warblers
Flashes of yellow zip through the willows outside my window. It is a reminder to write about warblers before they soon depart for their winter homes in the tropics.
Seven warbler species are found regularly in parts of our region. I will highlight two of the most common and widespread Seward Peninsula warblers that also happen to be the most eye-catching: The yellow warbler and the Wilson’s warbler.
Warblers are tiny songbirds, about the size of a chickadee. In May, when yellow and Wilson’s warblers first arrive, they are in bright yellow breeding plumage. The male yellow warbler has reddish streaks down his flashy yellow breast and the Wilson’s warbler is distinguished by a sporty black cap.
The small birds stand out as they flit constantly through bare branches of leafless willows. Males perch conspicuously and sing loudly to establish and defend their territories and to attract mates. During this time, before leaf out, they are hard to miss.
After the leaves open up, even colorful birds can be hard to see in the dense foliage they prefer, but true to their “warbler” name, they are vocal birds. Once you learn their songs and calls, you realize how common these species are in willow thicket habitats.
Yellow warblers are one of the most widespread and abundant warblers in North America. On the Seward Peninsula they are found where tall or medium-height willow thickets grow.
Yellow warblers arrive in our region in late May. They typically return to the same nesting territories year after year, sometimes pairing with the same mate.
Nesting begins in mid-June. The female builds a deep nest cup of grasses in a willow crotch, typically three to eight feet off the ground. Nest building takes about four days and egg laying begins as soon as the nest is complete. She lays an average of five eggs, one egg per day. Incubation begins just before the last egg or two is laid.
Sometimes nests contain more eggs than normal. Interestingly, yellow warblers are prone to a practice called “egg dumping,” where a female will invade a neighboring territory and lay an egg in another pair’s nest.
Incubation averages 11 days. Only the female incubates and broods the nestlings while the male defends their territory, helps feed the young and sometimes brings food to his incubating mate.
The young are born naked with eyes closed. The female broods the young as needed during the nestling period. If it is cold or rainy, she fluffs her wings out to the rim of the nest and lowers them to seal the nest. If it’s hot, she may spread her wings to shade the nest but allow air circulation, or in extreme heat she will flap her wings to fan the young.
The parents feed and protect the nestlings for nine to ten days in the nest and remain with them for another 17-21 days after they fledge.
Wilson’s warblers are also widespread on the Seward Peninsula in habitats with dense, low to medium height shrubs. In the western parts of the Peninsula, where shrub heights are lower, Wilson’s warblers are more commonly seen than yellow warblers.
Both species have similar breeding and nesting habits with some notable differences. Wilson’s warblers arrive earlier in May than yellow warblers and nesting typically begins a week or so earlier. The Wilson’s warbler female builds a nest cup of vegetation in a depression on the ground rather than in a shrub.
Warblers are insectivorous, so they do not come to bird feeders. They are in constant fast motion as they flit through dense shrubbery feeding on abundant insects while protected from predators.
They pluck spiders and insects of all sorts — including caterpillars, beetles and bees — from leaves and branches and make short aerial forays to snatch flying insects from the air. They occasionally eat berries.
This time of the summer, birds in general become harder to see and warblers are no exception. Sometimes one wonders “Where did all the birds go?” But they are here, keeping a low profile because they are molting.
Feathers wear out from abrasion, breakage and sun damage. Once a year, all birds go through a molt, systematically losing their old plumage and growing an entirely new set of feathers. For many birds, including warblers, this happens during the later part of the nesting season and before migration.
During molt, birds are particularly vulnerable to predation. As wing feathers are shed, temporary gaps form between feathers, making birds less maneuverable or strong on the wing. They minimize activity and remain quietly hidden in the brush during this vulnerable time. If you do see them, often they look scruffy.
The yellow and Wilson’s warblers feeding in the willows outside my window are a combination of adults, mostly in their new, drabber, winter plumage and slightly bedraggledlooking juveniles, still growing their first-year plumage. Yellow warblers have acquired an olive hue to their back feathers, which Wilson’s warblers already had.
Green-backed, yellow-breasted plumage is common for warblers that winter in tropical forests because such coloration is good camouflage. From above their backs blend into the leaves and from below the yellow breast matches sun-dappled leaves.
Both species are tropical birds for most of the year, coming north for only a few months to breed where insects abound. Our yellow warblers come all the way from Central and South America and Wilson’s warblers winter in Mexico and Central America.
These long-distance migrants travel mostly in the dark of night. Because they take a month or more for the journey and feed en route, they do not need to build up significant fat reserves or increase body mass before migration.
Both species leave when the young are independent and strong on the wing and molt is mostly complete. Yellow warbler migration typically begins during the first three weeks of August and Wilson’s warblers tend to leave a bit later, during the last half of August.
Soon the only yellow in the willows will be autumn leaves.