Abstract:
Despite our intuition, birds are no smaller than mammals when the constraints of a
flying body plan are taken into account. Nevertheless, the largest mammals are ten
times the mass of the largest birds.
Allometric equations generated for anseriforms and ratites suggest mid-shaft femur
circumference is the best measure to use in estimating avian body mass. The small
sample size of extant ratites makes mass estimate extrapolation to larger extinct
species inaccurate. The division of ratites into cursorial and graviportal groups is
supported. Aepyornithids do not show atypical femoral shaft asymmetry.
New and more accurate estimates of egg masses, and separate male and female body
masses for sexually-dimorphic ratites are generated. Egg mass scaling exponents for
individual bird orders differ from that Aves as a whole, probably due to between-taxa
effects. Ratite egg mass does not scale with the same exponent as other avian orders,
whether kiwi are included or excluded. Total clutch mass in ratites, however, scales
similarly to egg mass in other birds, perhaps as a consequence of the extreme variation
in ratite clutch size.
Kiwi and elephant bird eggs are consistent with the allometric trend for ratites as a
whole, taking clutch size into account. Thus kiwi egg mass is probably an adaptation
for a precocial life history, not a side effect of their being a dwarfed descendant of a
moa-sized ancestor.
Relatively small body size in ancestral kiwis is consistent with a trans-oceanic
dispersal to New Zealand in the Tertiary, as suggested by recent molecular trees. This
implies multiple loss of flight in Tertiary ratite lineages, which is supported by
biogeographic, molecular, paleontological, and osteological evidence, but which is not
the currently prevailing hypothesis.
