Food availability, risk of predation, timing of reproduction, etc. If the species breeds multiple times, decision must be made with respect to whether food and other resources devoted to current young will impact on future broods. In short lived species with less hope of producing more offspring in the future, parents should invest more heavily in present young. A nice study on this was done on birds. Leach's Storm-Petrel lives a long time and produces only a single chick a year.
Adults journey for days to get food for young.
Wing feathers of parents were experimentally shortened increasing the amount of work they had to do to fly. When parents were handicapped this way, they passed the increased reproductive costs to offspring and maintained their own nutritional condition. Better to give up on it and do what is necessary to live to reproduce another day. Starlings produce several young in a brood and only a few broods. Older females should be less likely to have additional offspring, so they should not hold back resources or care.