[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

RE: Altitude effects on hummingbird flight?

> Hummingbirds are the largest observed species using
> hovering, a
> size-constrained locomotion; if some "un-related"
> group of similar size
> evolved hovering, it seems to me that a high degree
> of convergence
> would be expected. In either case, I wonder what
> could have knocked
> them out? Given that hummers occur in Alaska, it
> astonishes me that
> they never got a foothold in Asia. Anybody know got
> a clue?

Several passerines have hovering flight, but only
forwards or on the spot. It is rather energy-intensive
to them and cannot be maintained for long AFAIK.
Basically any small arboreal passerine can do it at
least to some extent. It is a good way to identify
vagrant Pallas's Warbler in Britain for example (they
use it to pick off insects).

It could be meaningful that by the Late Oligocene,
passeriforms (which today include specialized
nectarivores) can be inferred from the biogeography of
the fossil record to have been in the process of
colonizing tropical Africa (Cape "honeyeaters" appear
a very distinct lineage not related to
Austro-Wallacean honeyeaters) and were settled down in
tropical Asia. Likewise, by that time, crown-group
parrots (which include specialized nectarivores today)
can be expected to have occurred in SE Asia at least
(where most of their lineages which mainly feed on
nectar have evolved). Biogeography of sphingid moths
also deserves investigation.

Altogether, I'd look at the possibility of competitive
exclusion regarding a limited trophic resource.
Eurotrochilus was not yet a specialized nectarivore
nor a hummer-hoverer, but both specializations were
already incipient. Presence of similar specializations
as regards nectar-feeding may have prevented spread
south/southeastwards. Diversity of non-specialized
aerial insectivores of the apodiform lineage was
markedly higher at the time hummingbirds diverged than
it is today, and such birds were present in the
Americas too (as opposed to - as far as is known -
scores of specialized or semi-specialized
nectarivored; many S.Am. passeriforms take nectar, but
only opportunistically).



Telefonate ohne weitere Kosten vom PC zum PC: http://messenger.yahoo.de