First Records of Conspecific Brood Parasitism in Two Species of Small Passerines: Lesser Whitethroat and Common Linnet

ABSTRACT We provide the first records of possible conspecific brood parasitism in the lesser whitethroat Sylvia curruca and the common linnet Linaria cannabina, two species of small passerines. In 2018 we found two unusually large clutches, one for each species (8 and 9 eggs, respectively), both containing two distinct egg morphs differing in coloration. Further detailed analysis of egg morphology and brood phenology (in the whitethroat) suggested that the eggs in each nest had been laid by different females. Surprisingly, two seemingly full sized clutches were laid in both nests, a pattern that is unusual in conspecific brood parasitism in passerines, whose nests are typically parasitized with single eggs. Alternatives to conspecific brood parasitism are therefore discussed. We argue that traditional field-based methods, when carefully used, may be sufficient to document brood parasitism recorded during occasional observations. We finally conclude that enlarged broods, resulting from conspecific brood parasitism, are probably more frequent than expected, but they may be often overlooked.