Dave,
If you search scholar.google.com for Red Crossbill, I think you will find links
to dozens of research articles, many of which are based on both observation and
experiment. The work from Craig Benkman and his colleagues is especially strong
and insightful.
Crossbills are difficult to study. That is why only Cassia Crossbill in North
America has been split. It is hard to demonstrate assortative mating in the
field. The other types are not known in sufficient detail to demonstrate
limited gene flow; yet. That is part of what makes crossbills a fun problem to
solve. How many cryptic species might there be? None? One more? 10 more?
I am unaware of any evidence that individual crossbills ever utter more than
one flight call type. I think we can feel good that, if it happens, it is rare,
based on thousands of hours of observation and hundreds of recordings.
It is obvious that most crossbills eat more than one species of conifer seed.
There are at least 5 types eating Sitka Spruce on the Oregon Coast now. All of
those except type 10 are thought to specialize on other species besides Sitka.
Specialization does not mean exclusivity. There is good scientific data showing
that bill morphology influences efficiency of seed extraction depending on the
match of cone morphology and bill size and shape. One can eat food from other
crops even though one is not very efficient at it. You can eat nectar from
honeysuckle if you’re in a pinch but hummingbirds are obviously far more
efficient at it.
This is a well studied group of birds yet we still have much to learn.
Doug
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