The question of why Arizona birds are more quiet has another aspect,
that of the drought. As I traveled to Organ Pipe National Monument last
year and found miles of eerily silent but otherwise beautiful and mostly
intact desert I thought more on the subject. The resident biologist
told me that his bird counts were as much as 90% lower than past years.
He attributed this to the drought. I suspect he was mostly correct
although there may be a factor of loss of wintering habitat as well.
As the drought decreases plant and arthropod food, the birds starve or
move on. I suspect (and would be interested in data if anyone knows of
any) that reducing the breeding male population of a species by 1/2 in
an area will reduce the amount of song by much more than 1/2. I suspect
a logarithmic relationship here. Birds sing if they have to, but
singing carries the risk of predation and the cost of lost energy and
foraging time.
I am currently doing a small project of counting western meadowlark song
sequences in 2 areas for 10 min. each week all year. During the winter
the birds crowd into a rich lowland farm area with bare ground for
feeding. The song intensity increases dramatically due to the crowding.
As other areas opened up in the spring and many birds moved out, the
song intensity calmed considerably. I suspect that below a certain
critical mass of breeding males there will be a dramatic drop in song
frequency.
Birds and nature are resilient and will bounce back to some degree as
the drought resolves assuming the natural infrastructure is not lost.
(This assumes the drought is not a permanent new feature of global
warming.) Of course in the long term scheme of things a bounce back
could take 10 million years.
Kevin Colver
"Things are always more complicated than they seem."
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