Birds of Seabrook Island

COAST BIRDS
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ANECDOTES

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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
 
 

Annual Cycles

  Most plants and animals in the world live in seasonal environments. Away from the equator, day length increases and it becomes warmer in summer and day length decreases and it becomes colder in winter. Even near the equator there are regular variations in precipitation that may affect survival and reproductive activity. Only in a few relatively arid areas (like the Australian interior) are environmental changes less than regular.
   Plants flower and animals reproduce at the time in their seasonal cycle when their offspring (seeds, young) have the greatest chance to establish themselves and grow. In this context, the environmental factors selecting the timing (by natural selection) are known as ultimate factors. However, organisms must prepare their systems for reproductive activity well before conditions become optimal - hence proximate factors in the environment interact with and set internal cycles. In birds like our small sparrows (emberizines), the male gonads are about the size of pin-heads and weigh 1-2 mg in the winter but during breeding they are oval spheres measuring up to 1 cm in length and weighing 1 g each (2 g, or 10% of the body weight collectively). The important timing factor for gonadal recrudescence (regrowth) and other seasonal changes in temperate regions is often photoperiod (the length of the day).
   The keystone event in the annual cycle is reproduction - selection is intense to insure survival of the species. Birds in temperate regions mate, build a nest, lay eggs, incubate those eggs, and care for one or more sets of young at the appropriate time (spring, early summer - or later if they feed their young seeds). Among species that provide extended care for their young, larger birds lay fewer eggs which take longer to develop and usually only attempt one nesting - smaller birds lay more eggs and may attempt to raise multiple broods. Birds that migrate away from the equator gain day length in the summer - north of the Arctic Circle the sun does not set. This gives them much more time to gather food to support rapidly growing young. However, the window of favorable conditions may be short in the Arctic (if nesting attempts fail, adults may not renest and will migrate south quite early)...
   After breeding, most birds undergo a complete molt - they replace all of their contour and flight feathers and they do it in a regular sequence so they retain the power of flight (with exceptions) and some insulation. Unless you have a bird in your hand, they only signs may be a shed flight feather on the beach or a bird with a gap in the wing or tail of one to one and a half feathers (primaries molt from the wrist outward, secondaries molt from the wrist inward and the body outward - in most cases). Birds are usually quiet (they don't sing) and are relatively inactive during molt. Many birds undergo a partial molt of some of the contour feathers in the spring, usually involving those feathers around the head and throat - feathers most worn in daily activities. Note that the spring molt is called the pre-alternate (fancy) or prenuptial (older terminology) molt. The summer-fall molt after breeding is called the pre-basic or post-nuptial molt.
   The rest of the year, individuals face the task of surviving. They must find enough food to live, avoid predators and other hazards, and stay healthy. In increasingly seasonal environments, abundant food supplies produced in the summer may be depleted or buried under snow in winter. Because birds maintain a constant body temperature, increasing metabolic expenditures with colder weather, shorter days in which to feed, and difficulty finding food all may strongly impact winter survival.
   Some mammals solve this problem and survive adverse periods by hibernating (small mammals may fatten or store food in a sheltered burrow and become torpid, dropping their body temperature to reduce metabolism - larger mammals may also hibernate but usually do not drop their body temperature precipitously).
   Birds, however, do not hibernate (one exception). Their common solution to this problem is migration. It has been adapted by many birds living in seasonal environments. (Note that mammals also migrate.) In migration, individuals are move from breeding grounds to wintering grounds - this may be just a shift southward with a retreat from more northern areas or it may be a massive movement of all of the population between discrete areas, sometimes crossing the Equator. Spring migration may overlap molt and precedes breeding. Spring migrants arrive as soon reliable food supplies become available - the earliest arrival is most likely to get the best territory but is also most likely to be caught in a late ice storm - this timing is closely tuned by natural selection. Fall migration can be more leisurely - some birds, especially young of the year, may wander farther north (reverse migration) before heading in the appropriate direction. (Young Bald Eagles born in our area are fledged by March and may move north to New England and the Great Lakes during the summer.) Fall migration is more loosely timed by the availability of food and follows post-nuptial molt (a few species, especially long-distance migrants, wait to molt until they complete migration)...
   Thus, a typical annual cycle consists of breeding, molt, (migration), winter survival, molt (migration), and breeding. These events can be represented as follows:
  Annual Cycle This is the annual cycle for the White-throated Sparrow, Zonotrichia albicollis. Click to see this diagram in its larger version.
   Months are indicated around the outer circle and the cycle rotates clockwise. Major environmental events are the winter and summer solstices which precede by 6 weeks or so minimum and maximum temperatures but represent the switch between decreasing and increasing daylength in winter and the reverse in summer. This species is primarily a seed eater but, like most birds, feeds its young soft invertebrate prey so insects must be abundant during the breeding period. Available seeds increase in the fall.
  The middle circle defines the phenology of seasonal events in this species - migration, molt, breeding, and over-winter survival.
   The inner circle goes a bit farther suggesting changes in activity, energetic reserves (fat), and gonadal development.  
       One of my research interests in the annual cycle has been the role migration plays in seasonal adaptation in smaller birds. But first, let us briefly consider breeding activities.
       
    Banner - annual cycle (above).