Bohemian waxwing, Bombycilla garrulus (formerly, Bombycilla garrula and Ampelis garrulus; protonym, Lanius Garrulus), also known as the Bohemian chatterer or the Bohemian wax-chatterer, the greater waxwing, the Lapland waxwing, northern waxwing, silktail, or simply as the waxwing, photographed at Over, Cambridgeshire, England.
Image: Richard Thomas/TRAFFIC, December 2010 (with permission) [velociraptorize].
Question: This lovely English mystery bird species engages in what is technically known as "irruption". If I am not misspelling the word that describes one activity that volcanoes are commonly known for, what am I talking about? I also am going to ask you the obvious question of all about this species, a question that is as plain as the red tips on its wing coverts -- what question is that? Can you answer that question?
Response: This is an adult Bohemian waxwing, Bombycilla garrulus, a passerine with a circumpolar distribution. These distinctive songbirds are named for their wandering lifestyle, which is exemplified by their propensity towards irruptive behaviour -- a spontaneous mass movement of these birds from their northern range into more southern areas, usually due to the lack of food.
The "obvious question" I was going to ask was so obvious that I thought all of you would think of it right away: What are those red feather tips made of and what is their function?
The red feather tips, which resemble drops of sealing wax (and hence, the origin of the bird's common name), are flattened extensions of the feather shafts themselves. The red color comes from the carotenoid pigment, astaxanthin, which is derived from the bird's diet.
Many hypotheses have been proposed as to the function of these red feather tips that look like sealing wax. The Eskimos thought the red tips were the clotted blood of the waxwing's victim. Considering that waxwings are frugivores and are strongly attracted to red berries, this idea has some merit. Another idea was that these knobs protect the feathers from being broken or worn away. But there is no support for this hypothesis.
The most widely accepted hypothesis is that the red "sealing wax" tips on the secondaries are badges that provide visual cues to potential mates about each bird's age. In waxwings, egg size, clutch size, ability to find and deliver food all increase as the bird ages, as do the number of these red knobs on the bird's secondaries -- indicating each individual's likely reproductive success. Thus, waxwings show "associative mating" based on age, where younger birds and older individuals tend to bond and mate with individuals of their own age class. Additionally, the side hopping observed in courtship supports this hypothesis, since this behavior allows individuals to assess the relative number of red knobs on a potential mate's wings.
Here's a lovely video that captures Bohemian waxwings chattering whilst eating crabapples in Maine (USA):
You are invited to review all of the daily mystery birds by going to their dedicated graphic index page.
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