The spectacle of migration is well underway.\u00a0 Warblers, vireos and thrushes are streaming south in good numbers now through Maine.\u00a0 The sparrow migration will pick up speed in October.<\/p>\n
Migration, even for short distances, is an arduous task that demands a high expenditure of energy.\u00a0 Two other events in a bird\u2019s life demand equally high energy costs: reproduction and molting.\u00a0 The costs of each activity are so high that no bird can do two of them at once.<\/p>\n
There are about 10,000 species of birds in the world so exceptions always arise when one tries to make universally true comments about some aspects of birds.\u00a0 That is certainly true of molting; one size does not fit all.\u00a0 Nevertheless, we can make some general observations on molting and then look at some of the interesting variations on the theme.<\/p>\n
Generally, birds undergo one complete molt every year.\u00a0 During that molt, every contour feather on their body is replaced.\u00a0 For a bird like a Song Sparrow, that entails replacing about 2,000 feathers.\u00a0 A Tundra Swan will have to replace over 25,000 feathers.\u00a0 No wonder molting is so expensive.<\/p>\n
The usual pattern is for the complete molt after nesting has been completed but before any migration is begun.\u00a0 The bird then enters its basic plumage (sometimes called winter plumage or non-breeding plumage).<\/p>\n
This sequence of nesting to molting to migration has obvious advantages.\u00a0 After the breeding season, food is generally abundant enough to allow a post-breeding bird to find enough energy to fuel its molt.\u00a0 Then, it is ready to migrate on fresh, efficient flight feathers.<\/p>\n
A typical bird will have a second, partial molt near the end of the winter.\u00a0 Some of the body feathers will be replaced, transforming for example a drab, greenish male American Goldfinch into a stunning yellow bird.\u00a0 However, the flight feathers on the wing and tail are generally not replaced.\u00a0 So, the northward migration must be done on worn feathers.<\/p>\n
Some birds do undergo two complete molts a year.\u00a0\u00a0 Extremely long distance migrants and species that live in abrasive habits (thorn scrub or coarse grass) replace all their feathers twice a year.\u00a0 In Europe, the Short-toed Lark only has one complete molt per year in the summer but an Asian race of this species that lives in sand-blown deserts has a second molt in the spring.<\/p>\n
Molting can improve a bird\u2019s physiological condition.\u00a0 The Salt Marsh Sparrow undergoes two complete molts each year.\u00a0 Seaside Sparrows, nesting in the same marshes, have a complete molt in the fall and a partial molt in the spring.\u00a0 Seaside Sparrows have more bird lice than co-occurring Saltmarsh Sparrows.<\/p>\n
It\u2019s easy to see evidence of molting in the flight feathers of a flying.\u00a0 The flight feathers are usually replaced in a sequence so that only a few feathers are missing at any time.\u00a0 The innermost primary feathers and the outermost secondary feathers are molted first. In a molting bird in flight, you can see gaps or shorter feathers showing the current stage of wing molt.<\/p>\n
Geese, swans and ducks as well as loons opt for the fast track during their flight feather molt.\u00a0 All of the primary feathers are shed at once.\u00a0 Until the feathers regrow, these birds are flightless.\u00a0 The birds find sheltered wetlands with enough food to allow them to hide and feed as their primaries grow.<\/p>\n
To get to such a favorable habitat, many waterfowl stage a molt migration.\u00a0 After the breeding season, lakes with lots of vegetation may be populated with thousands of flightless waterfowl that flew there for the express purpose of molting.\u00a0 Canvasbacks that nest in the northern United States stage a molt migration north(!) to the Prairie Provinces of Canada after breeding.\u00a0 After their molt, the Canvasbacks then migrate south for the winter.<\/p>\n
Anna\u2019s Hummingbirds have a molt migration as well.\u00a0 These birds nest in the chaparral of coastal California in the spring, move to the summer in the high mountains to take advantage of the abundant nectar where they molt, and then migrate to Arizona or Mexico for the winter.<\/p>\n
In 1983, the AOU Check-list Committee lumped the Baltimore Oriole and Bullock\u2019s Oriole into a single species, the Northern Oriole because these two forms hybridize where they overlap in the Great Plains.\u00a0\u00a0 These forms have now been re-split into two species.\u00a0 Part of this decision stems from the finding that Bullock\u2019s Orioles stage a fall molt migration to the southwestern United States and northern Mexico before molting and then continuing their fall migration.\u00a0 Baltimore Oriole\u2019s, even in the same habitat, molt before beginning their fall migration.<\/p>\n
[First published on September 25, 2010]<\/p>\n\n
The spectacle of migration is well underway.\u00a0 Warblers, vireos and thrushes are streaming south in good numbers now through Maine.\u00a0 The sparrow migration will pick up speed in October. Migration, even for short distances, is an arduous task that demands a high expenditure of energy.\u00a0 Two other events in a bird\u2019s life demand equally high […]<\/p>\n","protected":false},"author":146,"featured_media":0,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_s2mail":"","ngg_post_thumbnail":0,"footnotes":""},"categories":[429,430],"tags":[],"builder_content":"","_links":{"self":[{"href":"https:\/\/web.colby.edu\/mainebirds\/wp-json\/wp\/v2\/posts\/335"}],"collection":[{"href":"https:\/\/web.colby.edu\/mainebirds\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/web.colby.edu\/mainebirds\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/web.colby.edu\/mainebirds\/wp-json\/wp\/v2\/users\/146"}],"replies":[{"embeddable":true,"href":"https:\/\/web.colby.edu\/mainebirds\/wp-json\/wp\/v2\/comments?post=335"}],"version-history":[{"count":1,"href":"https:\/\/web.colby.edu\/mainebirds\/wp-json\/wp\/v2\/posts\/335\/revisions"}],"predecessor-version":[{"id":336,"href":"https:\/\/web.colby.edu\/mainebirds\/wp-json\/wp\/v2\/posts\/335\/revisions\/336"}],"wp:attachment":[{"href":"https:\/\/web.colby.edu\/mainebirds\/wp-json\/wp\/v2\/media?parent=335"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/web.colby.edu\/mainebirds\/wp-json\/wp\/v2\/categories?post=335"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/web.colby.edu\/mainebirds\/wp-json\/wp\/v2\/tags?post=335"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}