Egg arrangement in avian clutches covaries with the rejection of foreign eggs.

In birds, the colour, maculation, shape, and size of their eggs play critical roles in discrimination of foreign eggs in the clutch. So far, however, no study has examined the role of egg arrangement within a clutch on host rejection responses. We predicted that individual females which maintain consistent egg arrangements within their clutch would be better able to detect and reject foreign eggs than females without a consistent egg arrangement (i.e. whose eggs change positions more often across incubation). We tested this "egg arrangement hypothesis" in blackbirds (Turdus merula) and song thrush (T. philomelos). Both species are suitable candidates for research on egg rejection, because they show high inter-individual variation and individual repeatability in egg rejection responses. As predicted, using our custom-defined metrics of egg arrangement, rejecter females' clutches showed significantly more consistent patterns in egg arrangement than acceptor females' clutches. Only parameters related to blunt pole showed consistent differences between rejecters and acceptors. This finding makes biological sense because it is already known that song thrush use blunt pole cues to reject foreign eggs. We propose that a disturbance of the original egg arrangement pattern by the laying parasite may alert host females that maintain a consistent egg arrangement to the risk of having been parasitized. Once alerted, these hosts may shift their discrimination thresholds to be more restrictive so as to reject a foreign egg with higher probability. Future studies will benefit from experimentally testing whether these two and other parasitized rejecter host species may rely on the use of consistent egg arrangements as a component of their anti-parasitic defence mechanisms.

Experimental reduction of ultraviolet wavelengths reflected from parasitic eggs affects rejection behaviour in the blackcap Sylvia atricapilla.

Hosts of brood parasites use a variety of cues relating to eggshell appearance of parasitic eggs, which facilitate their recognition and rejection. It has been documented that host visual perception of different colours including UV wavelengths plays an important role in this respect. In the study reported here, we aimed to test whether artificial reduction of the UV wavelengths reflected from parasitic eggshell affects rejection behaviour in the blackcap Sylvia atricapilla. To achieve this goal, we experimentally manipulated, with UV, part of spectra reflected from conspecific eggs and reduced it into the range of 320-350 nm. We used an ultraviolet light blocker, which did not influence the reflectance shape of other spectrum parts. We also used a group of experimental eggs coated in Vaseline, which had no effect on the spectral shape in the UV or visible ranges of the light spectrum. Additionally, we used a third experimental group of unmanipulated eggs as a control. The experimental eggs coated in the UV blocker were rejected at a higher rate than those coated in the Vaseline. Moreover, a binary logistic regression revealed that an artificial reduction of the UV wavelengths reflected from the parasitic egg significantly affected the probability of being rejected by the hosts. To our knowledge, this is the first experimental study to reveal that manipulation with UV wavelengths affects the recognition of parasitic eggs, indicating that the UV part of the spectrum has an important role in host recognition behaviour.