08 December 2009

Proximate Controls

Two general types, each sharpens birds' physiology more and more:

1) Endogenous rhythms--birds in labs still show internal rhythmic cycles of breeding and molting, but cycles won't continue forever, they must be readjusted by:

2) Zeitgeber, time-giver, resets endogenous rhythms. This is usually photoperiod but can be rainfall. For example, daylight increases, hormones are released by hypothalamus, these affect the anterior pituitary, which in turn releases hormones that affect the gonads. The eyes not needed needed in this process. The pineal gland may play a role.

You can artificially induce breeding, except during what's called REFRACTORY PERIOD. You can't induce breeding from end of season until midwinter. Is this adaptive (Indian Summer would be a foolish time to begin breeding) or the result of physiological drain?

Birds can respond to minor variations in environment--temperature (hot or cold), insect abundance (rains in tropics) --all can delay breeding time. In the Arctic, jaegers and Snowy Owls feed on lemmings that vary greatly in numbers. If the visual presence of lemmings is absent, both won't breed! Appearance of habitat may also affect breeding. Ruffs want short, light green fields, otherwise fields will be too high by breeding time.

A number of seabirds are acyclical or have a shorter than 12 month cycle--Sooty Terns breed every 9.6 months. This allows birds to breed and molt and breed again in less than a year, resulting in 4 broods every 3 years rather and 3 on an annual cycle.


Sooty Terns

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