Lecture 18: Annual Cycles

I.                     Annual Cycles (Chapter 11)

a.     Competing demands

                                                              i.      Breeding

                                                            ii.      Molting

                                                          iii.      Migration (for some)

b.     selection to minimize overlap (review overhead)

c.      timing?

                                                              i.      Hormones, proximately

                                                            ii.      Daylength, ultimately, in most cases

                                                          iii.      Wet/dry season

                                                           iv.      Also food supply (Barn owls)

d.     Circadian vs. Circannual rhythms

                                                              i.      inherent as shown by exposing birds to constant light

                                                            ii.      but drift - cycle needs constant resetting - Why?

                                                          iii.      some “annual” cycles 6-9 months in the tropics

e.     Tropical species wet/dry seasons

                                                              i.      Breed

                                                            ii.      Molt

                                                          iii.      survive to next cycle

f.        Temperate/arctic species - temp and daylength

                                                              i.      Breed

                                                            ii.      Molt

                                                          iii.      Migrate (in some species)

                                                           iv.      survive to next cycle

g.     Timing is everything

                                                              i.      avoid famine

1.      peak food availability

2.      usually spring

                                                            ii.      breed (molt) when food is abundant

1.      but Eleonora’s Falcon in fall to catch migration

2.      Barn Owls breed whenever there’s enough food

                                                          iii.      breed when mates are available

                                                           iv.      avoid overlap of molt and breeding as much as possible

1.      raptors female molts first

2.      hornbills - female does complete molt in nest cavity

                                                             v.      Normal sequence is

1.      Breed

2.      Molt

3.      Migrate

4.      Lots of variations

5.      some species do not complete molt before migration e.g. Redtails on MVY

h.      How does timing work?

                                                              i.      first, it’s remarkably sensitive - 5-10 minute change in daylength stimulated gonadal activity in Rowan’s 1929 study of Dark-eyed Juncos

                                                            ii.      Pineal gland important, but not photoreceptor as shown by removal of Pineal

                                                          iii.      Mammals “measure” daylength visually, birds via receptor DEEP in brain!- Hypothalamus

1.      neurohormones released in hypothalamus stimulate anterior pituitary to release gonadotropic hormones