Foam Lake Birding No. 21

No. 21
Locally, most of our warblers have flown south to the tropics for the winter. There, on their wintering grounds, they will molt into their bright spring plumages; which, in turn, will make identification much easier when they return in the spring. A few do, however, spend their winters as far north as the southern USA, and one even winters on the west coast as far north as the state of Washington.
This week’s featured bird, the Yellow-Rumped Warbler, is this most northerly migrant. It gets its name from (you guessed it) its bright yellow rump (upper tail coverts) that is usually visible to the naked eye. Until fairly recently, the Yellow-Rump was divided into two species: the Audubon’s Warbler of the Rocky Mountains to the west coast, and the Myrtle Warbler east and north of the Rockies. Where their ranges overlap, they hybridize freely. For this reason, the two were considered races and were lumped together into one species – the Yellow-Rumped Warbler. As readers may recall, exactly the same thing has happened to some other birds also. It is quite likely that they may be restored to two species. Once again, only time will tell. The local race of the Yellow-Rumped Warbler is the Myrtle Warbler, and will be referred to as such.
In spring the males, and even the females, are unmistakable. Check with your bird books for colour details. The bright yellow rump patch is clearly visible, especially when the bird has its back turned to the observer as it flits through the trees. Usually, the yellow rump patch is diagnostic, but one has to be a little cautious. Another bird, the Magnolia Warbler, also has a yellow rump patch and that can cause some confusion. The big difference is that the Magnolias have bright yellow breasts and bellies while the Myrtles are white. Keeping these differences in mind makes identification quite easy. Another thing to keep in mind is that Magnolias are highly migratory and fly south much earlier, thus are seldom seen together with the Myrtles in the fall.
In North America, the Yellow-Rumped Warbler is unique among warblers in two ways. One, as already mentioned, it winters extremely far north for a warbler; two, it is one, of only a few, warbler known to eat fruit. This flexibility in diet allows it to survive in harsher climates (farther north) where insects may not be readily available. It is also the reason why it hangs around so late in the fall when other warblers have been long gone. Around here, it will commonly stay until after the Thanksgiving weekend.
I had taken a nice picture of a Myrtle in fall plumage in our back yard; but as luck would have it, I accidentally deleted it! Just as some minor panic was starting to set in, I recalled having taken some good pictures of the Audubon’s this March on its wintering grounds in Arizona. Therefore, this week’s photo is of a male Audubon’s in full spring plumage. Keep in mind that both races can be seen in the foothills of the Rocky Mountains.
In summary, a drab, brownish grey striped bird with a bright yellow rump is almost certainly the Myrtle. The overall impression is that of a small sparrow with a yellow rump. It has been quite common in our yard this fall. See if you can identify it.