Ultimate Explanation of Bird Breeding Seasons: Food Availability

1. 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

3. 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

4. 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 • Eleonora’s Falcon in Europe

6. 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

8. 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

9. 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

10. 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

11. 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

13. 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)

14. 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)

15. 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