Interspecific Competition Lowers Fitness in Collared Flycatchers Ficedula Albicollis: An Experimental Demonstration

The definition of interspecific competition requires that the fitness of individuals that are exposed to such competition be reduced. However, the effects of interspecific competition on individual fitness under natural conditions has rarely, if ever, been demonstrated in rigorous experimental tests. Here I report a study on the effects of Great and Blue Tits on the fitness of Collared Flycatchers, at the natural density, or at artificially high densities. When densities of tits were experimentally reduced, the number and mass of Collared Flycatchers fledglings increased, compared to those on control plots. Collared Flycatchers breeding under reduced competition contributed more recruits to the breeding population than did those breeding in control plots. The mechanism presumably was differential access to food for the young during both the nestling and postfledging periods. The combined fitness value of recruited young and surviving adults was significantly higher in low—competition plots. Interspecific competition affected the reproductive success of the Collared Flycatchers more than did intraspecific competition.