Setophaga coronata auduboni
March 13, 2023
Out on the north end of the Mitigation Lands there is a beautiful wetland. This time of the year it functions as a small lake where ducks and geese congregate. The Tules and cattails change color with the seasons and grow lush and thick in some areas. Walking out into this area is always a delight because it is full of surprises. Recently the thing that has caught my attention has been the many Yellow-rumped Warblers on the water’s edge hunting insects. In the past I have observed small groups of Yellow-rumped Warblers following Western Bluebirds and eating insects with them. During the last few visits to these wetlands on the Watson parcel, there have been flocks of twenty or thirty of these small warblers hunting together at the edge of and in the water.
The Yellow-rumped Warbler, Setophaga coronata, I first learned as a “Butter butt” warbler because it has a bright yellow rump patch that makes for a great diagnostic tool. This is a difficult bird to photograph because it never stops flitting around looking for insects to glean off leaves or flycatching in the air. There are four subspecies of Yellow-rumped warblers, and two are found in our region so we will focus on them. Before 1973, both of these subspecies were considered separate species. After more study, there was a decision to lump them together as the same species but divide them into subspecies. In the winter both subspecies, the Myrtle Warbler, Setophaga oronate oronate, and the Audubon’s Warbler, Setophaga oronate auduboni, are found here together. Referring back to my photographs I can see that we have high numbers of both subspecies wintering in Little Lake Valley. Here are the obvious differences:
The more common Audubon’s Warbler male has a bright yellow throat that does not extend past the auricular and white eye-rings that stand out against a dark background. The Myrtle subspecies male has a white throat that extends into the auriculars, and they have a white supraloral spot and supercilium line near their white eye-lines. Both subspecies have a yellow rump patch as I said before.
Female Yellow-rumped Warblers are overall paler than the males. The males also appear drabber when not in breeding plumage. Toward the end of their stay here, which is around the end of April, the males display brilliant yellow, black, white, and bluish grey coloring and they begin to sing a beautiful warbling song. However, neither subspecies has been found to nest in our area. The Myrtle Warblers head north into British Columbia and across North America into Ontario and the eastern seaboard. The Audubon’s Warblers nest on the west coast up into Canada and are resident along our northern coast and southern coast but not along the central coast. They don’t nest regularly in inland Mendocino County but are found nesting in the surrounding mountains at a higher elevation. There is intergradation between the two subspecies. Birds are found to share characteristics of both subspecies where the populations overlap.
This time of year, you can watch them catching insects in the air like a flycatcher, even ‘kiting’ like an American Kestrel, flitting around water edges picking out insects. Besides the gleaning that they are so good at, Yellow-rumped Warblers also hunt on the ground like a shorebird. Their sharp small bills and keen eyesight help them find aphids, spiders, and other small insects.
According to the Cornell Bird Library, “On migration and in winter they eat great numbers of fruits, particularly bayberry and wax myrtle, which their digestive systems are uniquely suited among warblers to digest. The habit is one reason why Yellow-rumped Warblers winter so much farther north than other warbler species. Other commonly eaten fruits include juniper berries, poison ivy, poison oak, greenbrier, grapes, Virginia creeper, and dogwood. They eat wild seeds such as from beach grasses and goldenrod, and they may come to feeders, where they’ll take sunflower seeds, raisins, peanut butter, and suet. On their wintering grounds in Mexico, they’ve been seen sipping the sweet honeydew liquid excreted by aphids.”
These adaptations have helped the Yellow-rumped Warbler keep its populations higher than other warblers. Keep a look out for this small and elegant winter migrant in our backyards.
