Data from: Host response to conspecific brood parasitism in colonial Red-breasted Mergansers (Mergus serrator): Positioning of parasitic eggs within the clutch

Brood parasitism can be costly to host fitness, which in turn may favour
host strategies that decrease these costs. Duck (Anatinae) nests are often
parasitized by eggs of other ducks, and one way that hosts can respond to
potentially costly brood parasitism is to move parasitic eggs to the
periphery of clutches where egg incubation temperatures can be suboptimal
relative to the central region of clutches. We explored whether
Red-breasted Mergansers (Mergus serrator) use discriminatory egg
incubation against parasitic eggs laid by conspecifics in a population
where conspecific brood parasitism (CBP) is common. We used isoelectric
focusing electrophoresis of egg albumen from entire clutches of 12
parasitized nests to identify parasitic eggs. A randomization test pooling
identified parasitic eggs (n=50) across nests revealed that hosts did not
position parasitic eggs along the periphery of clutches or out of the
central region more than that expected by chance, and this was the case
for parasitic eggs laid prior to and after the onset of incubation.
Similarly, nest-level analyses showed that parasitic eggs were random in
all but the smallest clutch, which contained one identified parasitic egg.
Thus, parasitic eggs were not moved to the periphery of heavily
parasitized clutches, where egg temperature gradients between central and
peripheral regions of nests are expected to be greatest. Only four eggs
(<0.5 % of 1,276 eggs) were found buried within nest bowls. Eggs
that were removed from nests consisted of parasite and host eggs and were
more likely along the periphery of clutches prior to their removal than
that expected by chance. Our results indicate that discriminatory egg
incubation of parasitic eggs is not a well-developed tactic for defending
against CBP in Red-breasted Mergansers, though hosts may rely on certain
cues to decide which eggs are to be removed from nests (e.g., addled
eggs).