Birds of Seabrook Island



  Double-crested Cormorants



Species Acct.
NW Warblers



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Feather Structure
  Contour Feathers
  Flight Feathers
  Feather Care
Feather Function
Brood Patch
The Skeleton
  Legs and the Ankle Joint
  Bills and Nostrils
Muscles and Bones
Internal Structures and Systems
  Control Systems
  Digestive and Respiratory Systems -
  Urogenital System - Osmoregulation



Like most animals, birds have two sexes - male and female. However, in contrast to most mammals, the male is the homogametic sex (XX or ZZ) and the female is heterogametic (XZ). [Z is used rather than Y to prevent confusion.] Thus, it is the genetics of the female that determines the sex of the offspring. Note that Eoaves (large flightless birds such as the Ostrich) lack distinct sex chromosomes but rely on sex markers on structurally indistinct sex chromosomes.
   In a number of species of birds, male and female are structurally indistinct (monomorphic) and chromatically indistinct (monochromic). Our herons and egrets, parids (titmice and chickadees), Mockingbirds and Carolina Wrens, among others, rely on behavioral interactions to identify sex. However, in many species, males and females differ - they are said to be dimorphic. In some cases, one sex is larger than the other - true dimorphism. For example, in many hawks, females are larger than males. In most cases,however, the sexes are actually (or also) dichromatic - differing in color. The male Northern Cardinal is brilliant - the female duller. In some cases, the female is so different it is difficult to recognize as belonging to the same species (e.g., Painted Buntings). A number of species are both dimorphic and dichromatic - amid Red-winged Blackbirds and Boat-tailed Grackles, males may be a third larger and are quite morphologically distinct. In some species, tail-length, plumes, wattles, crests, etc. may be well developed in the brighter sex. Plumage may also vary seasonally with a duller winter plumage replacing the attention-gathering breeding array. Loosely, both size and color differences between the sexes are considered "dimorphic."
   This reminds us that birds live in seasonal environments - in most cases day-length and temperature vary, the more so the farther the bird lives away from the equator. Even in equatorial regions, however, there are usually seasons in rainfall or other important variables. Thus birds tend to synchronize their breeding season with the time of the year when food is most available for growing young and their chances for successful reproduction are maximized. This produces an annual cycle of activities, including breeding. In many higher latitude breeders, populations migrate away from winter extremes and return the following year (see Migration).
   As the season turns, proximate factors or signals (such as changes in day length, rainfall, the availability of food, etc.) lead to the seasonal development (regrowth) of the gonads with the production of sex hormones that certainly influence behavior. Males (usually) establish territory in may species and sing and display to attract females. In some cases, the interaction of the sexes ends with copulation and the female lays the eggs and cares for the young by herself. This may lead to the formation of "leks" - displaying groups of males offering females a banquet of choices. In most species, however, the male and female form a mated pair that may last anywhere from only for one nesting attempt to a lifetime commitment. One or the other of the pair (or both) build some kind of "nest" - ranging from a patch of ground (vultures) to a woven pendent nest (parulas, orioles). The female lays a clutch of eggs that she, the male, or both incubate for a period of some days. In the case of many passerines and smaller birds, incubation may take less than 2 weeks (from 11 days or so) and blind, naked young are "brooded" until they grow an insulating down coat. The young usually fledge (leave the nest) with a juvenile coat of feathers in another 2 weeks or less and may be tended by their parents for a further 10 days or so. Larger birds such as hawks or herons have longer incubation periods (35 days or so in Bald Eagles) and their young hatch with down but remain in the nest for an extended development period. Alternatively, species like the Wood Duck, incubate for 25 - 35 days but their young are covered with down, have open eyes, and collectively leave the nest within 24 hours of hatching to follow their mother and learn to feed. The most extreme adaptation for young is found in the megapodes - mound-builders - who lay their eggs in warm volcanic soil or in compost mounds tended by the male to regulate their developmental temperature. Young hatch and are immediately independent of their parents.
   In most species, one male and one female form a monogamous breeding pair (monogamy) - other individuals are repulsed and driven away. However, various species have adapted other reproductive strategies. Polygyny involves two or more females mating with a single male. These reproductive units may be relatively distinct and successful (e.g. the Magpie Goose, Anseranas semipalmata). Looser associations may involve more females, leading to communal displays in which males mate with any and all willing partners but perform no other reproductive role, e.g., the Greater Sage-Grouse, Centrocercus urophasianus). Polyandry involves multiple males incubating clutches laid by a female. In Spotted Sandpipers, Actitis macularia, females leave 1-2 or more males with clutches they incubate and raise - up to 5 broods.
   Birds show an interesting array of adaptations to procreate. Some of these stories are developed and may be discovered as you explore this web.

  Towhee nest Great Egret chicks Canada Goose
  Altricial young of a Eastern Towhee, Pipilo erythrophthalamus. Note down growing - these chicks are probably about 3 days old. Young Great Egrets, Ardea alba - Jenkins Point Marsh. These chicks are at least j10 days old.

Canada Goose, Branta canadensis. Adult with several day-old chicks.



    Young birds of different species hatch at different stages of development and dress or undress.
   Mound builders (megapodes) incubate their eggs in decaying vegetation and the young hatch fully developed and chicks dig out of the mounds by themselves and take to the brush without ever seeing their parents. (See an Australian Brush Turkey, Alectura lathami, on its mound near Brisbane.) (See a second picture.)
   Many non-passerines hatch with a coat of down that provides early insulation. Some (like chickens) quickly leave the nest but they receive some degree of parental care as they learn to find food. Others (like robins) hatch naked and take several days to develop feathers (down) and must be warmed ("brooded") and fed in the nest... When they leave the nest or "fledge" they may still receive parental care for a period before becoming independent. (Young oystercatchers remain with their parents well into the fall and winter as they learn to feed on shellfish.)
   Young molt into their juvenile plumage from their coat (or coats) of natal down - juvenile feathers may grow from the same or different follicles - their growth may be a continuum from the down or may be a discrete plumage. Note these plumages in field guides - they may persist for a month or so. The juvenile plumage is replaced by the first basic (winter) plumage in their first prebasic (post-juvenile) molt (but they may retain the juvenile flight feathers). It may take some birds (e.g., gulls) two or three years to display typical adult alternate (breeding) plumage.

   There are some useful terms when characterizing the developmental condition of young birds (from Gill, 1990):
Condition   Down Eyes Mobility Parental feeding Parental attendance Examples
Super-precocial   megapodes
Precocial   ducks, shorebirds, quail
Subprecocial   ○/● grebes, rails, loons
Semiprecocial   gulls, terns, alcids
Semialtricial 1 ○/● herons, hawks
  2 owls
Altricial   passerines
   Young may also be characterized as
      nidifugous- young leave the nest soon after hatching
      nicicolous - young remain in the nest

   All precocial birds are, to some extent, nidifugous and all altricial birds are nidicolous.
   All passerines are altricial and nidicolous. Their hatchlings are naked (maybe a tuft or two of down), blind (eyes closed), relatively immobile (they reach upward, open their "gape" which may reveal an attractively adorned or colored mouth cavity, and call when disturbed to attract parental feeding), and require brooding for several days to maintain their body temperature. Development in passerines is extremely rapid - a helpless, naked hatchling develops into a functional juvenile capable of flight within two weeks in most passerines (and by 10 days in some).
    Note that hatchlings retain a significant portion of the yolk provided in the egg within their digestive system - the mid-gut folds to incorporate this yolk during development and at hatching young birds all possess a rotund, ovate belly due to these gut contents. They continue to provide significant nutrition during the first few days of life. (Note the young Golden Eagle chick's umbilicus and belly.)
    Brown Pelican Herring Gull chicks Brush Turkey
    Brown Pelican, Pelecanus occidentals. On a nest on Deveaux Bank. Herring Gull, Larus argentatus, chicks. Milk Island, Massachusetts. Australian Brush Turkey, Alectura lathami. Brisbane. Click to see a mound.

Brood Patch

    In preparation for incubation, adults in most species of birds (both sexes if both incubate) develop one or more bare patches of skin on the abdomen or breast. Most passerines and pigeons have a single median patch; most shorebirds, gulls, and quail usually have two lateral patches.
   As the brood patch develops, the bird looses any feathers that may have been present. The skin becomes edematous with an infiltration of white blood cells. The skin is wrinkled, allowing better contact between the skin and the egg. The epidermis thickens into a callused surface. The skin then becomes richly vascularized. Vasomotor control enable the bird to provide abundant warm blood while incubating and to conserve it when not. The adult is able to maintain a warm area against which the eggs are incubated and, later, the young brooded.
   The seasonal development of brood patches is under hormonal control.
   Some birds lack brood patches. Waterfowl, penguins, and pelecaniform birds lack them. Waterfowl line their nests with down that retains heat provided by their bare skin. Gannets and boobies incubate with their feet (they may incubate up to two eggs, one in each webbed and vascularized foot). Murres and penguins incubate eggs on the top surface of their feet. Some penguins have a pouch of skin that holds the egg on the feet.
   Finally, megapodes incubate their eggs in mounds of decaying vegetation.
    Banner - Breeding Double-crested Cormorants, Milk Island, MS