Red-footed and masked boobies stable isotope data

Animals that co-occur in a region (sympatry) may share the same
environment (syntopy), and niche differentiation is expected among closely
related species competing for resources. The masked booby (Sula
dactylatra) and smaller congeneric red-footed booby (Sula sula) share
breeding grounds. In addition to the inter-specific size difference,
females of both species are also larger than the respective males
(reversed sexual size dimorphism). Although both boobies consume similar
prey, sometimes in mixed-species flocks, each species and sex may
specialize in terms of their diet or foraging habitats. We examined inter-
and intra-specific differences in isotopic values  (δ13C and
δ15N) in these pelagically feeding booby species during the incubation
period at Clarion Island, Mexico, to quantify the degrees of inter- and
intra-specific niche partitioning throughout the annual cycle. During
incubation, both species preyed mainly on flyingfish and squid, but masked
boobies had heavier food loads than red-footed boobies. There was no
overlap in isotopic niches between masked and red-footed boobies during
breeding (determined from whole blood), but there was a slight overlap
during the non-breeding period (determined from body feathers). Female
masked boobies had a higher trophic position than conspecific males during
breeding; however, no such pattern was detected in red-footed boobies.
These results provide evidence of inter- and intra-specific niche
partitioning in these tropical seabird species, particularly during the
breeding period and in the more dimorphic species. Our results suggest
that these closely related species use different strategies to cope with
the same tropical marine environment.