Molting - Molt

This refers to the change in feathers of birds, either partially or completely, depending on the season and growth. In the case of captive hawks for falconry and egg-laying chickens, they are sometimes called "toriya". When all birds reach adulthood, they molt their entire bodies, including the wings and tails, after the breeding season. These new feathers are called basal feathers. However, the flight feathers of large eagles and cranes may only be molted once every two years. Geese also molt from August to February of the following year, and hawks also molt gradually from spring to autumn, regardless of breeding. Many adult birds molt partially, excluding the wings and tails, before the breeding season, to become their adult feathers. Molting to adult feathers varies by region, even for the same species. In some cases, such as in many northern hemisphere ducks, males have both basal and adult feathers, while females only have basal feathers, as in the case of mallards and other ducks. When there are two seasonal types, the winter plumage is generally called winter plumage, the summer plumage summer plumage, the breeding season plumage reproductive plumage, and the non-breeding season plumage non-reproductive plumage. Usually, summer plumage, reproductive plumage, and surrogate plumage, and winter plumage, non-reproductive plumage, and basal plumage all refer to the same thing, but there are also many birds that become reproductive plumage in winter, and many birds that change from winter plumage to summer plumage due to changes in pigmentation or wear and tear of the feather edges without molting, so they do not necessarily match.

Birds that have just hatched from eggs are either completely naked like passerines, or covered with cotton-like feathers with only the tips of the feathers that will eventually grow out like many birds of other orders. Eventually feathers grow and grow back to cover the whole body. Birds that reach this stage are called juveniles, and those at the previous stage are called chicks. Once they become juveniles, they have almost the same athletic ability as adults. Juvenile feathers are generally faint, making them difficult for predators to spot. The juvenile feather period is short, and they undergo their first molt within three weeks to three months, becoming the first basal feathers. In small birds such as the great tit, all of the feathers on their body, including the tail and flight feathers, are molted and become the adult feathers, but in many birds, the flight feathers, tail feathers, and primary coverts are not molted, and the molted feathers are different from those of adult birds, so birds at this stage are called young birds. Many young birds acquire their adult plumage by partial molting in the spring of the second year after surviving the winter, but larger birds only acquire their adult plumage after several molts. Reproductive ability is essentially complete once they have attained their adult plumage; in the slow-growing species of albatrosses, it takes several years for them to reach maturity. When molting, the old feathers are pushed out by the new feathers and fall out. Flight feathers are replaced symmetrically on each wing, in small numbers, so they usually do not lose their ability to fly when molting, but ducks molt all at once, making them unable to fly. Penguins shed all the feathers on their bodies at once, so they cannot go into the water for two to three weeks while their new feathers grow in.

[Nobuo Takeshita]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

鳥類の羽毛が、季節によって、また成長に応じて、その一部または全部が抜け替わることをいう。飼育下にある鷹狩(たかがり)用のタカ、産卵用のニワトリなどの場合は、鳥屋(とや)ということがある。すべての鳥は成鳥になると、繁殖期のあとに、翼と尾も含む全身の換羽を行う。この新しい羽衣(うい)を基羽(きう)という。ただし、大形のワシとツルの風切羽(かざきりばね)は2年に1回しか換羽しないことがある。また、ガンの換羽は8月から翌年の2月ごろまでかけて行われ、タカ目でも、繁殖に関係なく春から秋まで徐々に行われる。多くの種類の成鳥は、繁殖期に先だって翼と尾を除く部分的な換羽を行い、代羽(だいう)となる。代羽への換羽は、同じ種でも地方によって有無があり、マガモなど北半球産の多くのカモのように、雄は基羽と代羽をもつが、雌は基羽しかもたない例がある。季節による2型がある場合、一般には、冬の羽衣を冬羽、夏の羽衣を夏羽といい、また繁殖期の羽衣を生殖羽、非繁殖期の羽衣を非生殖羽という。普通、夏羽、生殖羽および代羽、そして冬羽、非生殖羽および基羽はそれぞれ同じものをさすことが多いが、冬に生殖羽となるもの、換羽によらず基羽のままで色素の変化または羽縁の擦り切れによって冬羽から夏羽にかわる鳥も多く、かならずしも一致するわけではない。

　卵からかえったばかりの鳥は、スズメ目のようにまったく裸か、ほかの目の多くの鳥のようにやがて伸びてくる羽の先端だけが現れている綿状の羽に覆われている。やがて羽が生え、また伸びて全身を覆う。この段階に達したものを幼鳥といい、それまでの段階のものを雛(ひな)という。幼鳥になると、成鳥とほぼ同じ運動能力がある。幼鳥の羽（幼羽）は一般にじみで、外敵にみつかりにくい。幼羽の時期は短く、3週間から3か月の間に最初の換羽があり、第1回目の基羽となる。体の小さなシジュウカラなどでは、尾羽や風切羽も含む全身の羽が抜け替わり成鳥羽になるが、多くの鳥では風切羽、尾羽、初列雨覆(あまおおい)は換羽せず、また換羽した部分も成鳥の羽とは異なり、この段階にあるものを若鳥という。若鳥は、冬を越して第2年の春の部分的な換羽によって成鳥羽になるものが多いが、大形の鳥では、数度の換羽を経て初めて成鳥羽になる。繁殖能力は成鳥羽になった段階で完成するといってよく、成長の遅いアホウドリ科の鳥では成熟するまでに数年かかる。換羽の際、古い羽は、新しく生えてくる羽に押し出されて抜け落ちる。風切羽は左右の翼が対称的に少数ずつ抜け替わるので、換羽に際して飛翔(ひしょう)能力を失わないのが普通であるが、カモは一斉に換羽するので、飛べなくなる。ペンギンは全身の羽毛が一斉に抜け落ちるので、新しい羽が伸びる2、3週間は水に入らない。

［竹下信雄］

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