By Linda DeKort

If you ask when Yellow-rumped Warblers will be returning to our valley, Bruce Tannehill, from Flathead Audubon, will tell you immediately, “between April 25th and May 1st”. Bruce and his wife, Gail Cleveland, are avid birders who know their neighborhood birds intimately. Like the rest of us, they look forward to the return of this vivid warbler, the first to return every spring and last to leave in the fall.

The Yellow-rumped Warblers, also known by local birders as “butterbutts”, are members of the family of wood warblers. All members of this group are small and active with short straight pointed bills; the bill of the Yellow-rumped is black. Until 1973, the Yellow-rumps were divided into two separate species: Audubon’s in the west and Myrtle in the eastern U.S. These two forms were apparently separated during the last ice age and developed distinguishing characteristics. For example, the Myrtle, named for the wax myrtle berries it eats, has a white throat and Audubon’s has a yellow throat. When it was shown, however, that these two forms were able to interbreed and produce viable young, the American Ornithological Union elected to merge them into one species. Many avid birders did not like this change; what before had been two entries on their life lists was now reduced to one!

During breeding season, the males of both varieties have slate blue backs streaked with black, white wing patches, streaked breasts and conspicuous yellow patches on their flanks, crowns and rumps. The females, fall males and young are streaked gray brown and are sometimes mistaken for Pine Siskins; like the Pine Siskin, these small warblers are about 5” long and have a wing span of about 9”. But, unlike the Siskins, the “butter-butts” of all ages have yellow rumps.

Populations of this adaptable warbler are stable or even increasing in most areas. We have observed these conspicuous little birds in many locations this past year: in the Copper Canyon in November, in the Tennessee Valley in January and on the Queen Charlotte Islands in August. The population on the Queen Charlotte Islands stays there year round; in Montana, Yellow-rumps have shown up in small numbers on the Christmas Bird Counts.

The apparent widespread success of Yellow- rumped warblers might be due to the fact that they do not require specific habitat nor diet. They prefer to build their nests on horizontal branches of coniferous trees, but will use deciduous trees as well. (The nests are unique, being lined with feathers that are carefully woven and positioned to conceal the eggs; this little trick possibly fools the parasitizing Brown-headed cowbird into thinking no incubation is occurring in that nest.) During the breeding season, they reach highest densities in mature, unlogged coniferous forest. But Bruce Tannehill confirms Yellow- rumps can breed successfully in selectively logged forests with some mature trees left standing. Steve Gniadek has noted that they are quite numerous in less severely burned areas of Glacier National Park. And though they are mostly insect eaters, their ability to digest the waxy coating of berries enables them to winter farther north and in more diverse habitats than most warblers.

They winter in loose flocks from the southern US to Central America, the males wintering farther north than the females. Winter flocks of both genders are easy to spot as they are kept together by a constant emphatic “chek.” In spring, their song is similar to the trill of a Junco, described as a flat, shivering tyew-tyew-tyew-tyew of 4 to 7 syllables.

Bruce Tannehill tells that a few years ago they heard a different Yellow-rumped Warbler song in their neighborhood; it had an extra “zip” on the end. They heard the song for about two weeks, until mid-May and then it disappeared; but the warblers themselves continued to raise young in their back lot. This pattern repeated itself for the next four summers. Bruce recorded the unusual song and produced a sonogram. The resulting sonogram was truly different from the more commonly heard song of the Yellow-rumped warblers. During the fourth year, Bruce and Gail heard the same song with the unusual zip not only in May but also in the beginning of July. They watched, recorded carefully and found that the same male Yellow- rumped warbler was singing two different songs. Bruce hypothesized from these observations that the song with the zip is a truly “come hither” courtship song, while the more common song is a “keep off” territorial song.