Unhatched eggs are a common phenomenon in birds and are often referred to as being ‘infertile’, which (confusingly) can mean at least two things: (1) that the ovum has not been fertilized or (2) that the embryo has died during development. These two broad categories of hatching failure can be difficult to distinguish, particularly in the early stages of embryo development. We describe methods to distinguish between infertility (due to insufficient sperm) and early embryo mortality in passerine eggs using the Zebra Finch Taeniopygia guttata as a model. We also describe how we successfully adapted these methods for use on eggs from a wild species, the Tree Sparrow Passer montanus, collected after the incubation period, and show that sperm can be visualized on the perivitelline layer of unhatched eggs even several weeks after laying.

Polygyny and extra-pair paternity are generally thought to enhance sexual selection. However, the extent to which these phenomena increase variance in male reproductive success will depend on the covariance between success at these two strategies. We analysed these patterns over four breeding seasons in facultatively polygynous blue tits Cyanistes caeruleus. We found that both polygyny and extra-pair paternity increased variance in male reproductive success and that standardised variance in annual number of genetic fledglings was 2.6 times higher than standardised variance in apparent success when assuming strict monogamy. Nevertheless, male success at securing within-pair paternity was unrelated to success at gaining extra-pair paternity and, when considering the positive effect of age on extra-pair success and attracting a second female, polygynous males were no more likely to sire extra-pair fledglings. Overall, polygynous males fledged more genetic offspring than monogamous males, but first-year polygynous males lost a greater share of within-pair paternity. A literature review suggests that this adverse effect of polygyny on within-pair paternity is frequent among birds, inconsistent with the prediction that females engage in extra-pair copulation with successful males to obtain good genes. Furthermore, a male's share of paternity was repeatable between years, and among females of polygynous males within years, such that a compatibility function of extra-pair copulations was likewise unsupported. Instead, we suggest that the observed patterns are most consistent with a fertility insurance role for extra-pair copulations, which does not exclude the greater opportunity for sexual selection through differential ability of males to gain paternity.

Summary1. Static animal colour patches may function among competitors to minimize conflict escalation over resources, by serving as a signal of resource holding potential or aggressiveness. Empirical evidence for the use of colour patches in conflict resolution is largely restricted to pigment-based colours (melanins and carotenoids) and rarely defines the context in which the signals are used. 2. Here we test whether structural-based ultraviolet (UV) crown colouration functions in conflict resolution among dyads of first-year male blue tits Cyanistes caeruleus in captivity, in a context that is known to favour aggressive individuals. 3. We found that on first encounter in a pairwise context, experimentally UV-reduced males were significantly more likely to lose to control-treated opponents than expected by chance. However, this disparity was less pronounced when conflicts were settled with physical fighting or when the opponent was considerably smaller in size. 4. When the same dyads were tested again several weeks later, but with the UV treatment reversed among opponents, none of the effects remained significant, but instead the winner was most likely to be the individual that won at their first encounter. 5. Our results suggest that structural-based UV colouration can affect the outcome of an interaction, but that size differences and the outcome of initial interactions between opponents can override the influence of this signal in conflict resolution. Whether there is a functional basis to maintain a link between aggressiveness and colouration may thus be highly dependent on the general context under which individuals compete in the wild.

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