Charles Deeming a member of our group has published an excellent paper
(see below).

Anybody fancy a PDF copy?

Xavier



Why were extinct gigantic birds so small?
D. Charles Deeminga* and Geoffrey F. Birchardb
aDepartment of Biological Sciences, University of Lincoln, Riseholme
Park, Lincoln LN2 2LG, UK
bDepartment of Environmental Science and Policy, MSN 5F2, George Mason
University, Fairfax,
Virginia 22030, USA
*E-mail: cdeeming@...
ABSTRACT
This review details the six lineages of large flightless birds that
evolved in the Late Cretaceous,
Tertiary and Quaternary periods of geological time. Estimates of mass
for each type of bird suggest
that maximal mass is no greater than 500 kg with most species
attaining only 250–300 kg or less. By
contrast, non-avian Archosaurs of the Mesozoic, and many mammal
species of the Tertiary, attained
great size with many species reaching several tonnes. Size has been
limited in flightless birds because
of the strength of the eggshell and in the largest species
reproduction was only possible if the smaller
males incubated. That reproductive characteristics limit mass in
flightless birds suggests that truly
gigantic non-avian theropods could not contact incubate their eggs and
had to rely on environmental
sources of heat energy to drive embryonic development. If fossil
evidence ever arises to support
proper contact incubation in a non-avian theropod then it is predicted
that it will only be from a
small (5250 kg) species.