Facultative nest-parasitism among Iberian shrubsteppe passerines

This paper analyses intra- and interspecific facultative nest parasitism of shrubsteppe passerines on the Iberian peninsula through 4 seasons, from 1991-94. Daily frequency of nest parasitism teas very low (f = 3.9 x 10(-3) during laying and f = 2.8 x 10(-4) during incubation). Observation of the 3 examples found in the field of 2 intraspecific and one interspecific instances revealed that the eggs were accepted and, in the latter case, that the parasitic chick was viable. The experimental tests in the field of placing I additional egg that was similar to (n = 57) or clearly different from the host (n = 50), in order to simulate both intra and interspecific parasitism revealed: (i) higher rejection rates in areas with higher risk of predation and (ii) similar rejection rates for both types of eggs. The experimental addition of nestlings of other species (n = 6) revealed that in 67% of the cases studied, the parasitic chick had a normal growth rate. It is suggested that nest parasitism among these species is an occasional behaviour pattern, caused by high nest predation rates in Iberian shrubsteppe habitats.