Dennis Paulson wrote:

>>>> As the vast majority of bird species have juveniles in which the sexes
look alike, there must be something interesting going on here. I've
entertained a hypothesis that the adults split the brood when they leave the
nest, and perhaps males take care of males, females females, but there
aren't enough data points to support or refute this hypothesis. The
hypothesis makes sense, as there are sexual differences in foraging
behavior/foraging sites in many of these birds, and it would make sense for
a young bird to learn the ropes from an adult of the same sex (of course all
this behavior may be innate, not learned, calling in question the adaptive
value of the behavior, if it exists). I'd be interested in hearing about any
instance of an adult of any of these species feeding a young away from the
nest, if the sex of both adult and juvenile is known.>>>>>>

While I do not have direct observations to add to this discussion, I am
intrigued by the whole concept of brood-division.

The following is a quote from Ken Simmons' article "Brood-division, parental
favouritism and parental desertion in the Great Crested Grebe" which
appeared in the # 24 (1997) issue of Bristol Birds:

"Perhaps males favour female young and females male young, as is likely with
the Blackbird _Turdus merula_ and Robin [ _Erithacus rubecula_ ]? Sexual
imprinting may be particularly important in the case of species like the
Great Crested Grebe which are isomorphic in behaviour as well as appearance,
making it the possible goal of in-chick selection (adults divide broods but
even within their favored chicks one [the "in-chick"] is favored over the
other(s) ["out-chick"]; MM) while posing the difficult question of just how
an adult grebe identifies the sex of its in-chicks or reveals its own sex to
it."

He then goes on to describe one instance of a female Great Crested Grebe
switching her choice of in-chick after departure of the adult male (when
chicks were 11 weeks old) and a tendency towards male-female behavior
developing, with 18 instances of tentative and usually short-lived courtship
displays between the two developing in week 18-24.

Given Dennis' hypothesis about same-sex-teaching, it would have been neat to
know if the adult female switched her in-chick choice from a female chick to
a male chick, unfortunately we don't know the sex of the first in-chick.

Also, it would be fascinating to know if there is a difference in the
function and execution of brood division between species with dimorphic or
isomorphic plumages (the grebes obviously belong in the latter category).
Ken Simmons states that a convenient summary of brood-division articles is
given in the Volumes V-IX of "The Birds of the Western Palearctic." I have
not had time to look those up yet. Have you, Dennis?

So many things to look for out there....it's great!

Martin Muller, Seattle
MartinMuller at email.msn.com