Asymmetric competition between two tit species: a reciprocal removal experiment

1. In a 5-year field experiment, competition for food was tested between gr eat tit (Parus major L.)and blue tit (P. caeruleus L.), two common hole-nes ters in Central Europe. Experimental ('allopatric') populations of both spe cies were created during the breeding seasons by preventing the nesting or egg laying of one of the competing species. 2. An asymmetric relationship was found between the two tits; blue tits wer e more successful in the competitive interaction. Detectable effects were f ound only in nestling condition. Great tits raised lighter nestlings when b reeding sympatrically with blue tits. 3. A possible mechanism is suggested that could be responsible for the diff erent competitive abilities of the two species; blue tits are more effectiv e in utilizing the most abundant size categories of caterpillar food supply than great tits.