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Abstract Co‐parasitism is ubiquitous and has important consequences for the ecology and evolution of wild host populations. Studies of parasite co‐infections remain limited in scope, with few experimental tests of the fitness consequences of multiple parasites, especially in natural populations.We measured the separate and combined effects ofPhilornis seguyinest flies and shiny cowbirdsMolothrus bonariensison the fitness of a shared host, the chalk‐browed mockingbird (Mimus saturninus) in Argentina.Using a two‐factor experimental approach, we manipulated the presence of nest flies and cowbirds in mockingbird nests and assessed their effects on mockingbird haemoglobin levels, begging and provisioning rates, body size, and fledging success. We also monitored rates of nest predation in relation to parasitism by flies and cowbirds.Nest flies reduced the haemoglobin concentration, body size, and fledging success of mockingbirds, likely because mockingbirds did not compensate for parasitism by begging more or feeding their nestlings more. Cowbirds also reduced the fledging success of mockingbirds, even though they had no detectable effect on haemoglobin or body size. Nests with cowbirds, which beg more than mockingbirds, attracted more nest predators. There was no significant interaction between the effects of flies and cowbirds on any component of mockingbird fitness. The combined effects of nest flies and cowbirds were strictly additive.In summary, we show that nest flies and cowbirds both reduce host fitness, but do not have interactive effects in co‐parasitized nests. Our results further suggest that predators exacerbate the effects of nest flies and cowbirds on their hosts. Our study shows that the fitness consequences of co‐parasitism are complex, especially in the context of community‐level interactions.