Friday, September 18, 2009

Warbler Guy: To help me with my warbler identification skills, can you tell me which warblers look the same year-round and don’t seem to become drab?

Among the 52 wood-warbler species typically seen annually in North America (north of Mexico), only 11 wear their flashy, bright, high-definition (alternate) plumage year-round. The vast majority — the other 41 species — undergo a molt before they migrate, so the males turn drab and less colorful after the breeding season.

Which11 warbler species look similar year-round?:
According to two sources I carefully checked — “Field Guide to Warblers,” (Jon Dunn & Kimball Garrett) and “Identification Guide to North American Birds (Part 1)” (Peter Pyle) — the answer is Golden-Winged, Yellow-Throated, Pine, Prothonotary, Worm-Eating, Swainson’s, Ovenbird, Louisiana Waterthrush, Red-Faced, Painted, and Yellow-Breasted Chat.

Expanding upon the basic facts presented above, most North American birds replace all their feathers during a complete molt that occurs in late summer or fall. The new feathers that develop before the birds leave on migration create an appearance that will be present throughout the fall and into winter. The technical word for this appearance is “basic plumage” because it usually persists longer throughout the year than the breeding (or alternate) plumage stage.

As indicated above, the majority of wood-warblers undergo a second molt that may occur progressively throughout the winter on the non-breeding grounds or as a more rapid molt before they leave to migrate north. This molt is called the peralternate molt and results in alternate plumage (or breeding plumage). In simple terms, think of this molt as providing birds the ability to “alter” their appearance to be colorfully attractive for the breeding season.

Of course, it is the males that most benefit from shedding their basic, non-breeding, drab (basic) plumage and transform into a brighter façade. That’s because the females pick their male partners when nesting occurs on the breeding grounds, a process that begins after migrating males arrive and preceded by the prealternate molt that occurred for them in their non-breeding grounds.

If you’re lucky enough to see warblers performing breeding and courtship displays, then you’ll see first-hand how the pretty males attempt to catch the eye of their female suitors (that are typically slightly more drab in appearance). Courtship poses include various wing gyrations and diving/hovering displays, among other behaviors.

As a closing note, it’s a good idea to remember that not all prealternate molts result in a prettier, more colorful appearance among male birds. Consider three ptarmigan species in the western USA and Alaska that molt from a majestic snow white basic plumage appearance to a more mottled, camouflaged expression that matches their tundra (and other habitat) surroundings and is, thus, more suitable for the ptarmigans’ survival in their summer environs.


Kevin said...

Thanks for the interesting information. I am bit confused as to whether the 11 mentioned wood warblers appear similar year-round because they lack a pre-alternate molt or lack a pre-basic molt. Do these 11 birds molt only once per year and if so, when?

Anonymous said...

Answer from Warbler Guy in relation to above comment by Kevin on 9/27:


Good question.

It may be more obtuse than I intended, but my prose in the 9/18/09 article states that the 11 all-year-round "pretty" warbler spp. have only one prebasic molt per year.

In other words, they lack a pre-alternate molt as ADULTS (but not necessarily as hatch year individuals when they are 3-6 months old and experience some molting of feathers).

The 11 spp. molting once per year usually do so BEFORE they migrate south and (if they are residents and do not migrate as part of their life cycle behavior) after the breeding season, say, from July-October-ish (for most Parulidae spp. in N. Am. above Mexico).

Please note my comment here relates to the 52 Parulidae spp. in N. Am. north of Mexico and not necessarily to the balance of other wood-warbler species that live south of the continental USA.

All of the S., Central, and N. Am. wood-warblers together total 114 warblers within the Parulidae family -- and, thus, ONLY the New World hosts wood-warblers/Parulidae family members.


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