Parental care assists fledglings in achieving adult‐like plumage and foraging behaviours in the Golden‐winged Warbler

In migratory passerines, the post‐fledging period is characterized by numerous physiological/life history milestones, supported by parental care. Although some aspects of the avian post‐fledging period have received adequate attention in recent decades (e.g. movement, habitat use, survival), major life history benchmarks (e.g. plumage (i.e. pre‐formative moult) and foraging development) remain poorly studied. We examined the extent to which plumage and foraging development occurred within the period of parental care in a forest‐dependent migratory passerine, the Golden‐winged Warbler Vermivora chrysoptera . We hypothesized that fledgling Golden‐winged Warblers would be under selective pressure to achieve adult‐like foraging behaviours (i.e. use of specialized foraging manoeuvres (probe‐and‐gape) at rates comparable to that in adults) before independence but that adult‐like plumage would be achieved post‐independence. We monitored warbler nests within regenerating timber harvests and, as nests approached anticipated fledging, one or two nestlings were marked with radio transmitters and tracked daily. Each daily tracking occasion included a 6‐min behavioural observation and an assessment of plumage development. Young fledglings wore greenish juvenile plumage and lacked the bold facial markings of adults; however, by independence (c.30 days post‐fledging), juveniles largely resembled adults. Plumage development began with flight feathers (rectrices/remiges) which were fully grown by 18 days post‐fledging. Head/body moult began on day 13 post‐fledging and were both complete within about 10 days. Fledglings began foraging soon after departing the nest with gleaning observed first (day 3) then probe‐and‐gape (day 4). Although the rate of both foraging manoeuvres increased linearly over the post‐fledging period, probe‐and‐gape rates increased faster, which yielded adult‐like probe‐and‐gape rates (70% of manoeuvres) by day 32 post‐fledging. These results collectively provide support for the hypothesis that fledglings would develop adult‐like foraging behaviours while still under the aid of parental care but reject the hypothesis that pre‐formative moult would be postponed. Given that these benchmarks occur during the post‐fledging period (before day 30 post‐fledging), independent juveniles may spend much of the post‐breeding period honing foraging skills, prospecting for future nesting territories and preparing for their first migration.