For birds, moulting is an energetically costly endeavour (Murphy & King 1991), the timing and location of which may be flexible and governed by local ecological factors (Pyle et al. 2009, Rohwer et al. 2005). Some species or individuals may pause during long-distance migration, or migrate specifically to moult (Greenberg et al. 1974, Pyle et al. 2009, Rohwer et al. 2005, 2008). This strategy may be most common when food abundance reaches a nadir at the end of the breeding period, promoting movement to areas where food is more plentiful and the energetic and nutritive demands of moult may be met more suitably (Rohwer et al. 2005). This pattern is exemplified by insectivorous songbirds breeding in temperate, western North America that pause on southward migration to moult amidst the food flush that occurs following heavy rains in the Mexican monsoon region (Pyle et al. 2009, Rohwer et al. 2005, 2009), or more rarely, migrate upslope after breeding to moult in more moist, productive areas at higher elevation (Butler et al. 2002, Greenberg et al. 1974, Rohwer et al. 2008, Steele & McCormick 1995). Such altitudinal migration may be much more common in the Neotropics where many species engage in seasonal shifts in elevation. Hypotheses proposed to explain this behaviour, however, have focused on links made between migration and a principally frugivorous or nectarivorous diet (Levey & Stiles 1992, Stiles 1985, 1988) and not on moult requirements or phenology. Fruit and nectar availability may vary seasonally over an elevational gradient, and birds may migrate in order to track peak abundances (Levey & Stiles 1992, Loiselle & Blake 1991). Hummingbirds may also track arthropod (particularly spider) abundance, but this possibility remains little explored (Cotton 2007, Stiles 1980).