Body conditions of nestling birds at prefledging stage have been suggested to affect future survival. Thus, even distribution of resources between nestlings may have negative effects on parents' reproductive success when resources are insufficient for the entire brood. In order to manage unpredictable brood-rearing conditions, parent birds are supposed to adopt brood reduction (partial brood loss) by distributing food unevenly. Despite the fact that access to food resources is often disturbed by adverse weather in many bird species, few previous studies have examined the effects of variations in weather conditions on parental provisioning rules. We here examined the allocation of food to barn swallow (Hirundo rustica) nestlings in relation to weather conditions. Weather conditions were related to 1) feeding frequency, 2) within-brood variations in body size before fledging, which indicate how evenly food was distributed throughout the nestling period, and 3) parental provisioning rules. These results suggested that barn swallow parents may skew allocation within their brood under weather conditions that are likely to decrease food availability.