Fledging is a critical stage for avian development with impacts on survival and fitness, yet it remains one of the most understudied components of bird breeding biology. The timing of fledging, what traits predict fledging age, and how brood composition affects fledging age are virtually unknown in most species. The prothonotary warbler (Protonotaria citrea) is a cavity-nesting, neotropical migrant that is commonly parasitized by the larger, non-evictor brown-headed cowbird (Molothrus ater). The negative effects of cowbird parasitism on warbler nestling growth, physiology, and survival are well-characterized, but it remains unassessed whether the parasite changes the fledging phenology of their host nestmates. Nests were experimentally parasitized with a cowbird egg (n = 19) or warbler egg (n = 14) and compared to non-manipulated control nests (n = 30). RFID (radio-frequency identification) readers were placed on each nestbox and all nestlings were banded with PIT (passive integrated transponder) tags. In 75% of cowbird nests, the parasite fledged first. Whereas parasitism treatment did not affect fledging latency between the first and last fledged chicks, warbler nestlings in the heterospecific but not in the conspecific parasitism nests fledged at significantly older ages (11 vs. 10 days posthatch) compared to controls. Warbler size (a principal component metric from mass, tarsus, and wing length) was smaller in cowbird parasitism nests and predicted the age at which nestlings fledged, with larger nestlings fledging earlier.