The threatened population of Atlantic Coast piping plovers (Charadrius melodus) has increased under intensive management of predation and disturbance. However, the relative importance of habitat quality, nest predation, and chick predation in population dynamics and reproductive success of this species are poorly understood. We examined effects of breeding-habitat alterations, predation, and breeding phenology on population size, habitat use, and reproductive output of piping plovers from 1993 to 2004. We studied piping plovers at a newly colonized site (West Hampton Dunes [WHD]) on a New York, USA, barrier island, and an adjacent reference site (REF) with a long-standing population. We monitored population size and reproductive success; determined chick habitat use and behavior; and monitored changes in habitat availability, prey abundance, and predator presence. Resource agencies managed predation by mammal trapping and by fencing nests with predator exclosures in some years. Following storm- and human-related increases in nesting and foraging habitat, the population at WHD grew from 5 pairs in 1993 to 39 pairs in 2000. The WHD population then declined to 18 pairs by 2004 concurrent with habitat losses to human development. In contrast, the population size at REF was not correlated with nesting habitat area. Population growth rate decreased with density at WHD but not at REF, which was likely close to equilibrium when the study began. Neither reproductive output nor any of its components were correlated with population density, and reproductive output was correlated between the sites despite their different population trajectories, suggesting that the population was primarily regulated by adult survival, emigration, or immigration. The latter 2 factors should be especially sensitive to local habitat quality, and the main differences between our sites was that bayside intertidal flats were available adjacent to nesting habitat at WHD but not at REF, and that a village construction project took place at WHD. Clutch size and renest rate decreased over the breeding season. Predator exclosures improved nest daily survival, and mammal trapping improved chick daily survival. Chick foraging rate was highest in bayside intertidal flats and in ocean- and bayside fresh wrack. Chicks used the bay side more than expected from percentage habitat area, and survived better on the bay side before village construction and the initiation of predator trapping, but not after. At both sites, number of chicks fledged per pair was lowest for pairs that nested late and lost a nest late in the season and increased with the annual number of cats (Felis catus) and foxes (Vulpes vulpes) trapped. Restoring nesting habitat adjacent to bayside intertidal flats may increase the carrying capacity (nesting pairs) at piping plover breeding sites. However, without predation management, restored sites may not contribute many recruits to the regional population.