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Re: Mesozoic birds made insects shrink

Henrique Niza <drdinopt@gmail.com> wrote:

> I'm sorry if someone already brought this up through the discussion
> but aren't insect-eating birds in mid-air uncommon? And those that
> venture in mid-air predation are actually small birds that go for
> small insects? As an exception I can think of the Saw-whet Owl
> (Aegolius acadicus) that predate moths in mid-air. My knowledge in
> Mesozoic birds isn't exquisite (nor in extant birds actually) but if
> the same pattern applies to early birds shouldn't have the insects
> continue to endure large size?

Yes, some of those Late Jurassic and Early Cretaceous insects were
very large indeed, especially members of the Aeschnidiidae.  This
includes dragonflies that typically had a wingspan of at least 10 cm,
with some up to 20 cm.  So if there was aerial predation of these big
winged insects by pterosaurs or birds, then it was probably quite
different to the way modern birds catch small insects on the wing.

As I understand it, there are two main strategies for catching insects
on the wing (hawking).  First, scooping insects out of mid-air using a
wide beak and a large gape, as in today's nightjars and frogmouths,
which hunt in dim light.  Second, plucking insects out of the air
using a narrower beak, as in bee-eaters and flycatchers.  The latter
requires excellent maneuverability in the air (although this is
certainly an asset for the former too).  Anurognathids, with their
wide jaws, appear to conform to the first strategy (_Batrachognathus_
effectively means "frog mouth").  But AFAIK, based on functional
morphology, no Mesozoic bird obviously conforms to either strategy.
Then again, considering the size of some of these insects, and the
inference that some of the biggest palaeopterous insects were gliders
incapable of flapping flight, some Mesozoic aerial predators might
have used a different strategy altogether.

Big-winged palaeopterous insects really took a hit around the
mid-Cretaceous.  The rise of birds may have had something to do with
this - but perhaps it was not directly related to their flight
abilities.  Rather, by this time birds radiated into aquatic habitats,
with many forms living in or close to water.  Like modern dragonflies,
the Aeschnidiidae had aquatic larvae that (as with the adults) were
very large - especially the last instar.  These larvae might have been
vulnerable to predation by aquatic birds.  Giant aeschnidiid dragonfly
larvae have been found in the Yixian Formation.