The hawksbill sea turtle (Eretmochelys imbricata) is a critically endangered keystone species that forages in vulnerable reef habitats. Although research has focused on the hawksbill’s reproductive life stage, little is known about the hawksbill’s long-term foraging strategies or how their diet influences fecundity. Here, I combined stable isotope analysis of keratin tissue from 50 adult female hawksbills with long-term reproductive metrics from a hawksbill rookery beach in Antigua, West Indies to study this knowledge gap. I observed probable ontogenetic niche shifts in four individuals, and the population had high variability in isotopic composition. Niche width was not a good predictor of fecundity, but reproductive age was significantly related to niche width, with younger turtles showing higher variability. These findings provide evidence of ontogenetic dietary shifts in these hawksbills and emphasizes the necessity of protection of multiple marine habitats to aid in conservation of this critically endangered species.