Annual Molt Pattern in a Malaysian Population of Fantail Warblers (Cisticola juncidis)

During a two-year study in Malaysia, I recorded molt in 117 adult Fantail Warblers (Cisticola juncidis) to examine the annual molt pattern in relation to the population's breeding seasons. Adult male warblers that were molting primaries were recorded only during January, September, and October, while females in primary molt were recorded in every month except July. Molt in the secondaries, rectrices, and head and body followed similar schedules. There appears to be just one molt annually, unlike the pattern in most other parts of the species' range. Adult male warblers begin to molt synchronously at the end of the summer breeding season; most complete their primary molt before territory establishment and courtship activity in the winter breeding season. The females' postbreeding molt is less synchronous because many females in this polygynous population are caring for fledglings weeks after males stop breeding. Thus, many female birds enter the winter breeding season still in molt. Rather than risk losing a breeding opportunity, these birds apparently delay, or interrupt, their molt to begin breeding, and then complete molting after the winter breeding season. The flexible molt schedule of birds in this population is a facultative response to the seasonal availability of nesting cover and food, and enables the warblers to exploit valuable, brief opportunities for breeding.