> my comments:
> 1. Kettlewell and Tinbergen's film was not "natural" predation.
> My understanding is that the moths were disoriented - having been
> released in the day time in a sluggish state. (If this unnatural
> situation has been the trigger for an increasing perception that bird
> predation is important, it would be wise for lepidopterists and
> ornithologists to check that the mistake has not been repeated in
> later "observations and experiments").
>
The claim of "disorientation" is special pleading; there is no evidence that
the moths were disoriented. Instead this claim is based on an
anthropomorphic reaction, specifically the "How would you feel if you were in
the moth's place?" reaction. Even if that were a valid basis for comparison,
it is not scientific evidence, just a subjective emotional response.
The film itself may have been made under "unnatural" conditions, but it was
meant to document and illustrate what Kettlewell and Tinbergen were seeing on
a regular basis. As such, despite its "unnaturalness", other people found it
convincing because it backed up Kettlewell's and Tinbergen's observations in
a way that was indesputable. The reason is very simple: birds do not waste
time going after unfamiliar objects in the hopes that it might be food; they
concentrate instead on what they believe to be food. If birds did not eat
moths, the mere presence of moths on tree trunks would not incite birds to
eat them, because the birds would not recognize the moths as food. It was
because birds love to eat moths that they attacked those that had settled on
the tree trunks. The ornithologists and entomologists recognized this from
the film, hence its "unnaturalness" was irrelevant to them. And this new
perception was reinforced by their own experiments and observations which
they made to test this new idea. Many such experiments and observations
probably were "unnatural" as Dave would define it, but since the question
being tested was, "Are birds a major predator of moths," and not, "Is bird
predation of moths important for understanding the cause of industrial
melanism", the "unnaturalness" of the experiments and the observations is
irrelevant.
>
> 2. I am entirely comfortable with the thought that birds are the
> major predators of many species of adult moth. Perhaps we ought to
> be enquiring about the relative importance of other predators - such
> as bats.
>
Bats are indeed major predators of moths (probably more so than birds, but
not by much), but because they locate and capture prey using sound rather
than sight, their "relative importance" with regard to industrial melanism
would be very slight. The same is true for mammalian insectivores like
shrews, except that they use smell instead of sight or sound. Reptiles and
amphibians would probably follow close behind bats and birds as major moth
predators, but there are no major tree-dwelling species in Great Britain,
Europe or North America, so their "relative importance" is likely to be very
slight as well. That leaves birds, which are the most aubundant moth
predators after bats, and the easiest to study in a "natural" setting. So
while bats probably eat more moths than birds do, they would eat both color
phases indescriminantly; with the evidence we have, birds are the most likely
predator to cause the industrial melanism affect.
>
> 3. We have almost no information about the peppered moth
> predators in the natural state, nor which is the most important.
>
We have no *direct* evidence, but the indirect evidence by itself is
sufficiently convincing to the experts that, until contradictory direct
evidence is discovered, they will use what they have, plus informed
speculation, to establish an explanation. Besides, assuming that the
"important" predation with regard to industrial melanism is predation of
resting moths rather than of active moths, the most important predators
(based on general observational history) are likely to be those predators who
go hunting for moths in their resting places. In Great Britain, Europe and
North America, this is predominantly the small "creeper" birds that "crawl"
around tree truncks and branches looking for insects, birds like titmouses,
nuthatches, chickadees, wrens and certain species of warblers.
>
> 4. Majerus's view is that significant bird predation levels are
> "highly probable" - but this is based on the consensus that bird
> predation is important in many species. Majerus points to the lack
> of data relating to the Peppered Moth.
>
But this lack of specific evidence in the case of the peppered moth is not
enough to cause Majerus to doubt the overwhelming evidence establishing the
general trend that birds are the most important predator as far as industrial
melanism is concerned. That is what we need to keep in mind. The question
is, what causes industrial melanism in moths? The evidence we have so far
indicates that it is selective predation by visually-oriented predators. Of
the three major groups of vertebrate predators of moths -- bats and other
mammalian insectivores, birds and reptiles/amphibians -- only the latter two
hunt predominatly by sight, so only the latter two are likely to cause
industrial melanism. And since birds are far more numerous and far more
active hunters than reptiles/amphibians, birds are the best candidates to be
the predator that causes industrial melanism. As such, only if birds took a
tiny fraction of all moths eaten by all predators would bird selective
predation have no significant affect, and this is very much not the case;
they rival bats as far as total intake is concerned.
>
> It MAY BE that the story about bird predation will turn out to be
> true. But our present state of knowledge is inadequate for this to
> be the textbook example of natural selection changing gene
> frequencies in a population of organisms.
>
Only if we concentrate on the gaps in our knowledge. In point of fact, the
evidence we do have paints a very strong picture in favor of selective
predation by birds as the cause of industrial melanism.
Kevin L. O'Brien