The decision of when to breed is an important determinant of individual fitness. However, despite a multitude of studies investigating the intraspecific relationship between timing of breeding and reproductive performance, less is known about why the strength of this relationship varies
between species. Furthermore, environmental change has the potential to alter the relationship between lay date and fitness, but there is still
a limited understanding of what mechanisms drive these differential responses to change environmental conditions. We propose that the potential effects of environmental change on the relationship between timing of breeding and fitness are dependent on 2 primary factors: (1) the
potential constraints imposed by breeding early and (2) the drivers of higher fitness of early breeders. We first summarize multiple hypotheses
proposed to explain why breeding early, either based on absolute date or relative to conspecifics, increases fitness. We then summarize the
factors that may constrain when individuals initiate breeding, including limits on the ability to advance their lay date or extend the length of their
breeding season under favorable conditions. Understanding constraints on the timing of breeding allows for the identification of obligate (singlebrooded species that do not attempt to breed after a specific date) and facultative (predominantly multi-brooding species that have long breeding
seasons) early breeding species that are likely differently affected by climate change. Finally, we propose a simple mathematical formula that
incorporates the costs and benefits associated with early breeding to quantify how climate change could influence the benefits of early breeding
and either mitigate or exacerbate the costs. Our cost-benefit approach provides a clear framework to predict how species may shift the timing
of their breeding to maximize fitness in a changing world