Kevin J. Colver wrote:
> Many bird species do this layering or alternating of song as well,
> especially if two species have songs with similar frequencies or
> patterns that could be confused or that would obscure each other if sang
> at the same time. When I first heard it 12 years ago I thought the 2
> species were dueting or countersinging but I have now realized that
> something else is going on here.
>
> Generally one species dominates the other. An article was published
> documenting that Bewick's Wren dominates Wrentit in CA. I have a long
> recording wherein a Song Sparrow seems to dominate the Fox Sparrow. The
> Fox Sparrow consistently waits until the Song sparrow has finished for a
> few seconds before inserting his song in the empty slot. This spring I
> noticed the same thing with a Western Meadowlark dominating a Vesper
> Sparrow. It seems the louder and more vigorous songster tends to
> dominate the sound space and the secondary species fits in his sounds
> when the coast is clear.
I'm not so sure it should be interpreted as deliberate dominance. In the
end for each species this has to do with reproductive success and
allowing another species to determine that would be counter productive.
So each species will sort out their own calling. Certainly in the
original example of the Virginia Rail calling at a unusual time vs the
American Toads and Spring Peepers calling at a normal time for them, I
strongly doubt that either was dominant.
In many cases the calls of the different species can be simultaneous and
still get through because they are of different frequencies, or even
just a different pattern. Time spacing is only one way that different
species coexist. The more interesting ones are those that interleave
their call components with other species. Calling at the same time but
using the silence between components of the other's call. That is also
common within a single species, where each individual uses a offset to
make their call carry. In fact this is probably more common within a
species than between species.
In the case of American Toads, they appear to carry on a shouting match.
The long calls evolved as a test. When two toads are calling their long
call simultaneously it's hard to locate them due to the beat frequencies
created. But a single call is easy to locate. So, the part that counts
is when your call is alone. Coming in on your neighbor's call a little
way in means you can outlast him if evenly matched. Your clear call will
be at the end of the sequence. American Toads have the longest call of
any frog or toad here. Though Southern Toads are close.
In the case of the Spring Peepers, they don't appear to react to any
other species, simply doing their thing regardless. But, within a group
calling you can generally find by sonogram that none of the individual
call elements from the group are exactly simultaneous. Though the larger
the group the more difficult this becomes. In all the din each Peeper is
getting his message out, timing it between the others. And you can
fairly easily locate calling individuals. Obviously female choice is by
some other method than being able to locate the male.
Walt
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