Annual Cycles of Birds. A time to break down. To every thing, There is a season, And a time to every purpose under heaven

Transcription

1 Annual Cycles of Birds A time to be born To every thing, There is a season, And a time to every purpose under heaven Pete Seeger Ecclesiastics 3:1 And a time to die A time to break down 1

2 And a time to build up A time for peace And a time for war A time to mourn 2

3 And a time to dance A time to keep silent And a time to speak Annual cycles Bird s annual cycles represent adaptations to synchronize the demands of reproduction, molt, and (sometimes) migration with seasonal differences in the availability of resources Variability in resources explains many of the patterns in the annual cycle of birds 3

4 Food is the limiting resource that explains most annual cycles David Lack The natural regulation of animal numbers I = food needed for body maintenance II = food needed for egg formation X = time needed for egg formation and incubation Y = time needed for raising young Non migratory birds show similar patterns in Borneo (Fogden 1972) and North America African Storks are adapted to breed during the dry season when food for other birds is scarce Annual cycles of migratory birds are complex Annual cycles of waterfowl are even more complex 4

5 Annual cycles can vary within a species Biological clocks regulate cycles Circadian cycles 24 hour daily cycles of light and darkness Circannual cycles Endogenous rhythms that control annual cycles Cycles synchronized with external clues zeitgebers The role of photoperiod Day length or photoperiod plays a key role in synchronizing a bird s physiology to their environment Receptor control system Very sensitive photo receptors lie in the hypothalamus of the the midbrain Light penetrates brain Nerves stimulated Neurons release hormones Hormones stimulate pituitary Pituitary releases hormones that influence molt, reproduction, etc. 5

6 Proximate factors trigger cycles Mechanisms that trigger a behavior or physiological response Habitat cues Green vegetation Social stimulation Displays or aggression by neighbors Rainfall Temperature Food availability David Ligon found that Pinyon Jays in the west begin breeding when exposed to green pine cones Captured birds after breeding season when testes were shrinking Group 1 control Group 2 fed green pine cones Results: testes of Group 2 stopped regressing and increased in size; Group 1 testes continued to regress Conclusion: green pine cones are a proximate cue used to regulate the annual gonadal cycle Habitat cues Rainfall synchronizes nesting Alexander Skutch (1976) found close association between number of species nesting and monthly rainfall in Costa Rica 1357 nests of 140 species Food synchronizes nesting cycles Crossbills will nest in any month if food is available Lack of seasonality in food at low latitudes decreases nesting synchrony 6

7 Nesting cycle terminology Restricted cycles Single clutch with replacements, Downy Woodpecker Double clutch with replacements, Robin Seasonal breeders, e.g. rainfall, Crossbill Non-annual cycles Continuous, tropical seabirds Irregular, penguins Reproduction is energetically expensive Reproduction can increase energy demands by 50% Courtship Territoriality Nest building Egg production Feeding nestlings Eggs can amount to 50% of the mass of some female birds Molt is energetically expensive too Feathers = 20-49% lean dry weight of birds Scrub Jay metabolism up 15% when molting Two basic strategies Molt quickly when food is plentiful, most passerines Molt gradually, many seabirds, raptors Strategies balance constraints of reproduction, migration, etc. Changes in annual cycles may reflect climate change House Martins in Oxfordshire arrive, on average, three weeks earlier today than in Average Tree Swallow laying dates in North America declined by 9 days from Many birds are expanding the northern limits of their breeding and wintering ranges 7