Indirect evidence of conspecific nest parasitism in the colonial whiskered tern (Chlidonias hybrida).

Conspecific nest parasitism (CNP) is widespread among birds and is particularly common in colonial species. Nevertheless, this reproductive tactic is remarkably little described in terns. In this study we use egg morphology as indirect evidence of CNP in the whiskered tern (Chlidonias hybrida) in 17 colonies from two French populations. We study CNP with respect to the duration of egg laying (i.e. clutch size), and also present some behavioural aspects that could facilitate CNP. From the inspection of 121 two-egg, 279 three-egg and 11 more egg-clutches, we detected a minimum %CNP of 9.2% for the two study populations, combined with large differences between the two populations (4.0% vs. 14.5% in three-egg clutches) and between colonies (0 up to 30%). %CNP was significantly higher in three-egg (9.0%) than in two-egg clutches (1.7%), suggesting that duration of egg laying is a determining factor of parasitism. The rather high %CNP detected in this species might be facilitated by (1) the observed long unattendance of nest by any of the pair members, mainly during the pre-incubation period, and (2) high synchrony in laying dates within colonies. A possible link between nest density and CNP rate was suggested. Finally, we encourage new investigations of CNP in other tern species.