20 December 2012:

Tomorrow is the shortest day of the year, the winter solstice, when Pittsburgh will have only 9 hours 17 minutes of daylight compared to 15 h 4 m during the summer solstice.

This annual ebb and flow of daylight is an important cue for organisms that live in the temperate zone.

Birds, animals, plants, fungi and even blue-green algae all have internal clocks approximately 24 hours long.  Humans have long clocks: 24 hours and 11 minutes plus or minus 16 minutes.  Some birds have short clocks that run less than 24 hours.

The discrepancy doesn’t matter because our internal clocks reset every day in response to daylight.  In constant dim light we have no cues.  Experiments with common chaffinches show that their circadian clocks drift until their “days” are only 23 hours long in the absence of sunrise and sunset.

Birds also have circannual clocks that trigger their annual cycles of molt, migration and reproduction.  These clocks respond to the shorter days of fall and winter and the lengthening days of spring and summer.

After the breeding season birds’ reproductive organs shrink, an adaptation for flight that lightens their load during most of the year.  The shrinking is triggered by the decreasing light of fall and winter days.  After the winter solstice, the increasing photoperiod triggers their organs to grow in preparation for breeding.

Experiments with juncos show that they require a winter solstice for this to happen.  If the photoperiod increases without first decreasing, their reproductive organs don’t grow.

Our birds need the solstice to set their clocks.

(Inspiration for this Tenth Page is from page 250 of Ornithology by Frank B. Gill.    Photo from Wikimedia Commons; click on the image to see the original)