Ultimate Explanation of Bird Breeding Seasons: Food Availability

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Ultimate Explanation of Bird Breeding Seasons
Food Availability

• Breeding requires 2-3x energy sustain self
• Due to growth pattern of birds complete growth
  while under care parents
• Need for young to survive becoming independent
  also a factor

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Birds may migrate to take advantage of a flush of
food to breed, or wait breed food supply peaks
where they are resident

• Insects (and other animal prey) are the key food
  due to protein requirements of growing young

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Timing of Breeding Seasons

• Temperate zones spring (later farther north)
• Tropics wet season (Kenya example)
• May be irregular in drier areas where rainfall is
  erratic (central Australia)
• May differ among species in same location
• Water birds breed in dry season
• Vultures in Kenya
• Eleonoras Falcon in Europe

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Breeding in Aseasonal Areas

• Some tropical areas are relatively aseasonal
• Long annual breeding seasons (up to 9 months)
• Low level of reproduction (1-2 young)
• Pelagic areas are aseasonal
• Low level of reproduction (often 1 young)
• No annual cycle, no regular breeding season,
  length of cycle depends on time required to
  recover from previous attempt

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Birds Avoid Overlap Between Breeding, Migration
and Molt

• Timing of breeding set by food, timing of
  migration set by breeding (arrival) and food
  (departure), timing of molt most flexible
• Advantages to molting when warm (insulation),
  lots food (energy demand)
• Late summer ideal for local birds due to
  temperature and food, timing of migration

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Unusual Molt Schedules

• Interrupted molt in some species
• In some, due to energy and time constraints
• In others, due to opportunistic breeding
• Rapid molt in divers, ground-dwellers (low flight
  costs)
• Slow molt in seabirds (high flight costs due to
  dependence on dynamic soaring)
• Special cases hornbill example
• Overlap with breeding in a very few

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Proximate Control of the Timing of Breeding The
Problem

• Must anticipate timing of peak food, time
  egg-laying so young in nest at time of peak
• Major changes must be brought about
• Gonads must grow from regressed non-breeding
  state to become functional
• Structural changes in other body systems may be
  required
• Breeding cycle must start well in advance of
  egg-laying

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Component 1 Internal Circannual Rhythm

• Internal, periodic circadian rhythm (neural)
  located in pineal gland, entrained to light
  dark cycle
• Circannual rhythm entrained to seasonal changes
  in daylength
• Photoreceptors in hypothalamus have period of
  sensitivity controlled by circadian rhythm
• Daylight overlaps period of sensitivity as days
  grow longer

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Component 2 Triggers

• Inhibit or accelerate processes driven by
  internal rhythm
• Enable fine-tuning in timing of breeding, precise
  matching to peak of resources in a particular
  year
• Stimuli used vary among species temperature,
  rainfall, vegetative growth, stimulation from
  males (females)
• Food itself rarely used (conifer crops)

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Free-running Circannual Rhythm

• Period usually less than 12 months
• Unentrained rhythm determines breeding cycle in
  some species in aseasonal environments (9-month
  cycle pelagic terns)
• In lab, birds with unentrained rhythms show
  similar breeding intervals
• Refractory period exhibited temperate, but not
  tropical, birds (short days terminate)

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Proximate control of the timing of breeding
illustrates the operation of the avian control
system well

• Nervous system directs hormonal changes based on
  sensory input