Interannual mate and site fidelity is common in migratory shorebirds with monogamous mating systems. After long-distance migrations and separation during the winter, birds often relocate their former mate at their previous breeding territory. Although pairs frequently reunite, new pairs are also formed. Why birds change mates is still not completely understood. Mate change can involve active decisions, in which one or both mates actively chooses to divorce from a previous mate, but can also be related to arrival timing or mate availability at the breeding grounds. We explored possible causes of mate change in the Pacific subspecies of the migratory shorebird Dunlin (Calidris alpina pacifica) breeding at the subarctic Yukon-Kuskokwim Delta, Alaska, USA (61°36′N, 165°12′W). Interannual return rates of Dunlin to their breeding grounds were higher for males (74%) than for females (54%) and were 14% higher for birds with high previous breeding success. Mate change was rare if both birds returned to the breeding grounds in a consecutive breeding season: only 8% of all pairs divorced when previous mates were available. When former mates failed to return or returned late, however, many individuals formed new pairs (45% of males and 53% of females). Nest initiation dates were not delayed for new pairs compared to reuniting pairs, and nest survival did not differ between new and reuniting pairs; however, renesting after nest failure within a season was faster for reuniting pairs. We conclude that avoiding delayed nesting is a strong determinant of breeding decisions in Pacific Dunlin nesting in the short subarctic summer.