Birds of Seabrook Island

COAST BIRDS
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WORLD BIRDS
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ANECDOTES

  Brown Pelicans
 
 

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Species Acct.
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Migration
 
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Annual Cycles Breeding Migration Migration in Captive birds
  Activity Cages
  Operations Recorder (Activity)
  Actual Count Data
  Zugunruhe
Energetics and the Annual Cycle
Estimates
A Model for Control
References
 
 

Breeding

  As the breeding season approaches, gonads grow (recrudesce) - in male White-throated Sparrows testes may increase from 1-2 mg to 1+ g (each) in mass (1 mm to 10 mm in size) and in females the left ovary grows from 4-5 mg to > 1 g as ova mature and are released.
   In migrant species, males may arrive at the breeding ground before females. They establish territories, advertising with song and display to discourage other males and attract females. Through a variety of behavioral interactions, females select a mate and pair formation occurs in most species. The couple (or one of the pair) build a nest (not all birds build nests, e.g., Turkey Vultures). In most birds, copulation consists of cloacal apposition (most birds do not possess copulatory organs) leading to the release of sperm into the female urogenital system. Most female birds (exceptions are found in some of the diurnal birds of prey) have only a left ovary (the right gonad remains an undifferentiated ovo-testis) - this probably prevents problems with more than one egg being produced at a time. Once ready to lay, most females ovulate once a day (usual intervals are > 24 hr. apart so later eggs are laid later in the day). The ovulated ovum is surrounded by albumin, egg membranes, and a shell as it passes down the oviduct, taking about 24 hours to reach the exterior. Larger birds may lay only one egg - smaller birds may lay a clutch of 4 or more (larger birds with precocial young  like ducks also lay large clutches but invest little effort in their care).
   In many birds, the incubating sex (female, male, or both in some pattern) lose down feathers (if they have them) in the ventral apteria (unfeathered areas) and develop a richly vascularized brood patch that facilitates the transfer of body heat to eggs and young.
   In clutches with multiple eggs, females (or males) may begin to incubate with the first or second egg meaning that the earliest eggs develop ahead of later eggs and hatching is asynchronous. In this case, the oldest egg/chick has the advantage - it hatches first, receives food before its nest-mates, and usually receives most of the food provided. Later chicks may starve unless food is unusually abundant.
   If females wait to begin incubation until all eggs are laid, they usually hatch synchronously and have relatively equal opportunity to be fed and survive.
   Upon hatching, young may leave the nest (they are nidifugous) and follow their parents to gather food. However, in passerines and many other birds, young hatch at an early stage of development with little down and their eyes closed and must remain in the nest during their early development (they are nicicolous). Parents brood (warm) them for several days and provide food (actually at hatching all young contain significant food reserves in their yolk sacs but this must be supplemented by external food as development proceeds). Nidifugous young hatch with a downy coat providing insulation. As they grow, contour and flight feathers develop. Nidicolous young also grow feathers - contour feathers to conserve heat and flight feathers to facilitate flight.
   In small passerines, incubation usually lasts 11-12 days and early feather growth leads to active thermoregulation by the 5th to 6th day. Flight feathers begin to erupt (emerge from their quills) at day 7 or so and young may leave the nest by the 11th - 12th day but probably can't fly until they are two weeks old (or older). Parents continue to feed the young after fledging (leaving the nest) until they are three weeks or more of age. In some cases parental care may be extended as young learn to feed themselves (oystercatchers may take several months to learn appropriate feeding techniques).
   Among smaller birds, multiple broods are possible. Sometimes the male takes over care of the first group of young while the female starts a second clutch. If a nest, clutch of eggs, or young are lost, many species renest - often extending the breeding period beyond its normal end. Alternatively, if the season is short (as for many shorebirds nesting in higher latitudes), parents may abandon further efforts and return south.
   In a few species (e.g., Cackling Geese), families remain together through fall migration. In most, the young are abandoned by their parents who may depart ahead of their offspring who still manage to find appropriate winter quarters.
   Following breeding, gonads regress to their non-breeding size and reproductive behavior ceases.
   Some species are polygynous - males may display to attract multiple females but take little or no role in raising the young (turkeys). In some polygynous species, males defend a territory that may include the territory of several females with whom they mate. In this case, they may assume some role in caring for their offspring (e.g., Red-winged Blackbirds). Other species may be polyandrous - females may mate with multiple males, often leaving each with a clutch to incubate and raise (e.g., Spotted Sandpipers). In the majority of cases, however, one male and one female form the reproductive unit.
   Some species that form active pair bonds may change mates between clutches but, if both parents survive, they usually remain together for the season. Pairing with the same individual in subsequent years may be due to the tendency of birds to return to the same area (particularly if successful the year before) or may be due to individual recognition. In some species, individuals pair for life.
  Bald Eagle nest
Great Egret
Laughing Gull
  Bald Eagle, Haliaeetus leucocephalus. Young, nest along golf course. Lauging Gull, Larus atricilla. On nest on Deveaux Bank.
      Great Egret, Ardea alba, on nest along golf course.  
       
    Banner - Brown Pelican, Pelecanus occidentalis, on nests, Deveaux Bank