"William Morse"  wrote in message
news:cd3jr0$2ks5$1{at}darwin.ediacara.org...
> jimmenegay{at}sbcglobal.net (Jim Menegay) wrote in
> news:ccd7e9$96i$1{at}darwin.ediacara.org:
> > What this thread lacks is some good, old fashioned, unpronounceable
> > scientific terminology:
> >
> > Aposematism (adj.: aposematic): "warning coloration" of
an inedible
> > insect, like the monarch, to "advertise" this fact to potential
> > predators; after the first unpleasant experience (see
"Emetic Unit")
> > the predator learns to avoid this and other similar appearing prey
> > (see "mimicry").
[snip]
> > Sequestration: the internal process whereby a caterpillar feeding
> > on a toxic plant is able to shunt the poisons out of the normal
> > metabolic pathways, and "sequester," or store them in
the body (as
> > opposed to excreting the poisons or converting the poisons to
> > non-toxic forms). Monarch larvae sequester cardenolides, which
> > are subsequently incorporated into the body of an adult monarch.
[snip]
> This leads to the question of why there are no toxic birds. Again
> contingency is one answer. Anybody have any others?

One possibility is that birds have not developed sequestration.
Of course, that invites the followup question of "why not?"  One
reason why herbivorous birds may not have developed sequestration
would be that the more innovative insects get there first - hence
there are no unexploited toxic plants that an adventuresome bird
could learn to be the first to tolerate.

A bird that eats a lot of DDT containing insects, and "sequesters"
the toxin in its fatty tissues, may well be toxic to raptors.  But
the raptors have not yet developed the emetic response - the raptor
doesn't know it has been poisoned until its eggs fail to hatch.
Hence, it doesn't yet pay for the insect eater to advertise his
high load of DDT.  The raptor is still too dumb to notice.
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