Parental regulation of the probability and timing of avian brood reduc
tion is expected to exist and evolve because natural selection should
favor parents that match brood size to food availability most efficien
tly. Since food availability varies among species, interspecific varia
tion in this regulation is expected. Comparative study of species with
different brood reduction systems is one way to test the hypothesis t
hat these mechanisms evolve as adaptations. Previous work on obligatel
y siblicidal Masked Boobies (Sula dactylatra) and facultatively siblic
idal Blue-footed Boobies (S. nebouxii) has shown that their different
hatching asynchronies cannot fully explain the qualitative difference
in their brood reduction systems. In this study, I report interspecifi
c differences in nest shape that appear to contribute to early siblici
de in Masked Boobies, but that suppress early siblicide in Blue-footed
Boobies. Blue-footed Booby nest shape is closely regulated by parents
. Differences in egg sizes of nest mates do not appear to contribute t
o the difference in social systems.