birding-aus Are birds affected by daylength?

Subject: birding-aus Are birds affected by daylength?
Date: Thu, 11 Feb 1999 08:09:03 -0500

G'day all

I've been mulling over a theory for a while now and thought I would ask
for other opinions.

Riding to work along the same rural roads south of Hamilton each morning I
get to know the local birds quite well.  Last spring I was used to seeing
and hearing a range of species (Stubble Quail, Shining and Horsefield's
Bronze-Cuckoos, Golden-headed Cisticolas, Richard's Pipit, Skylark, Brown
Songlark, Clamorous Reed-warbler).  This was the case until 21 Dec when I
went to Melbourne for Christmas.  Back on the bike again on the 29th the
scene was completely different.  None of the listed species were calling
and very few can still be seen.  Looking back over past years' bird lists
seems to support my theory that all calling and display flight activity
ceases around Christmas.

An exception to this was our Koel which continued calling for a couple of
weeks into the new year.  This bird is obviously not reliable as it
shouldn't be here at all. 

I don't think Christmas is an important event for birds (geese, turkeys
excepted) but it occured to me that Dec 22nd is the summer solstice - the
day when daylength is greatest and the beginning of shortening days.  Is
this the trigger for many species to stop breeding activity (calling and
display flights)?  I know daylength (or nightlength) is important for many
plants and I'm sure that I've read where it is a probable trigger for
migrating birds but the daily change around the solstice is so small
(seconds) that I find it hard to believe birds could detect it as
precisely as my theory suggests.

I welcome any discussion on this but please - not too technical. 



Stephen Clark
Agriculture Victoria, Pastoral & Veterinary Institute
Private Bag 105  HAMILTON  3300  Australia
Phone 0355 730 977   Fax 0355 711 523


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