Extra-pair paternity in two passerine birds breeding in a gradient of urbanisation

Urbanisation has been increasing worldwide in recent decades, driving environmental change and exerting novel selective pressures on wildlife. Phenotypic differences between urban and rural individuals have been widely documented in several taxa. However, the extent to which urbanisation impacts mating strategies is less known. Here, we investigated extra-pair paternity variation in great tits (Parus major) and blue tits (Cyanistes caeruleus) breeding in nestboxes set in a gradient of urbanisation in Warsaw, Poland, over three breeding seasons. Urbanisation was quantified as the amount of light pollution, noise pollution, impervious surface area (ISA) and tree cover within a 100-m radius around each nestbox. We obtained genotypes for 1213 great tits at 7344 SNP markers and for 1299 blue tits at 9366 SNP markers with a genotyping-by-sequencing method, and inferred extra-pair paternity by computing a genomewide relatedness matrix. We report higher extra-pair paternity in blue tits breeding in more urbanised areas, for example, with higher light pollution and ISA, and lower tree cover. However, no such trend was found in great tits. Late-stage survival of individual nestlings in both species was not associated with paternity or urbanisation proxies, thus we were not able to detect fitness benefits or drawbacks of being an extra-pair offspring in relation to urbanisation. Our results contribute to the growing body of knowledge reporting on the effects of urbanisation on avian ecology and behaviour, and confirm species-specific and population-specific patterns of extra-pair paternity variation.

Experimental extra-pair copulations provide proof of concept for fertility insurance in a socially monogamous bird

Extra-pair paternity is common among socially monogamous birds, but whether females benefit from having extra-pair copulations remains unresolved. In this study, I staged single extra-pair copulations in captive pairs of socially monogamous Japanese quail (Coturnix japonica) and used extra-pair males of a different colour to identify extra-pair fertilizations. This eliminated among-female variation in extra-pair copulations as a source of variation in extra-pair fertilizations and tested whether a single extra-pair copulation would insure against infertility of the social male. Overall, the probability of extra-pair fertilization was 0.46 for the first egg that was fertilized after extra-pair copulation, but this rapidly declined over consecutive eggs in the laying sequence. However, a single extra-pair copulation was effective in ensuring fertilization of the majority of a typical clutch in the few cases where the social male was completely infertile. Hence, my results show that variation in extra-pair paternity can be independent from variation in extra-pair copulation behaviour and that a single, strategically timed, extra-pair copulation can largely insure against social male infertility. Among-female variation in extra-pair fertilizations, and their declining probability over the laying sequence, as typical for many bird species, can thus, in principle, be parsimoniously explained by a uniform female fertility insurance strategy.

Temporal dynamics of competitive fertilization in social groups of red junglefowl (Gallus gallus) shed new light on avian sperm competition

Studies of birds have made a fundamental contribution to elucidating sperm competition processes, experimentally demonstrating the role of individual mechanisms in competitive fertilization. However, the relative importance of these mechanisms and the way in which they interact under natural conditions remain largely unexplored. Here, we conduct a detailed behavioural study of freely mating replicate groups of red junglefowl, Gallus gallus, to predict the probability that competing males fertilize individual eggs over the course of 10-day trials. Remating frequently with a female and mating last increased a male's probability of fertilization, but only for eggs ovulated in the last days of a trial. Conversely, older males, and those mating with more polyandrous females, had consistently lower fertilization success. Similarly, resistance to a male's mating attempts, particularly by younger females, reduced fertilization probability. After considering these factors, male social status, partner relatedness and the estimated state of male extragonadal sperm reserves did not predict sperm competition outcomes. These results shed new light on sperm competition dynamics in taxa such as birds, with prolonged female sperm storage and staggered fertilizations. This article is part of the theme issue 'Fifty years of sperm competition'.

Exposure to predator models during the fertile period leads to higher levels of extra-pair paternity in blue tits

The perceived risk of predation can affect breeding behaviour and reduce reproductive success in prey species. Individuals exposed to predators may also adopt different mating tactics with potential consequences for the distribution of paternity in socially monogamous species that engage in extra-pair copulations. We experimentally increased perceived predation risk during the fertile period in blue tits Cyanistes caeruleus. Every morning between nest completion and the onset of egg laying, we presented a model of either a predator or a non-predator (control) near active nestboxes. Broods from pairs exposed to predators had higher levels of extra-pair paternity than control broods. This mainly resulted from a higher proportion of extra-pair offspring in broods with at least one extra-pair young. Females exposed to predators first emerged from the nestbox later in the morning, stayed away from the nestbox for longer and were less likely to be visited at the nest by their social mate, but we detected no behavioural differences once the model was removed. Our results suggest that the higher rates of extra-pair paternity resulted from the disruption of morning routines, which may have inhibited within-pair copulations or increased opportunities for females to engage in extra-pair copulations. We conclude that the perceived risk of predation can have substantial effects on levels of extra-pair paternity.

No relationship between female emergence time from the roosting place and extrapair paternity

In several bird species, the period around dawn seems important for extrapair behavior. For example, a study on great tits (Parus major) showed that females that emerged earlier from their roosting place during the peak of their fertile period were more likely to have extrapair young in their brood. We investigated the potential effect of female emergence times on extrapair behavior in the blue tit (Cyanistes caeruleus). First, we tested the relationship between natural female emergence times from the nest-box and the presence or frequency of extrapair offspring in the brood, using 4 years of data. Females progressively emerged earlier from the nest-box as egg laying approached, with the earliest emergence 2 days before the start of laying. However, we found no relationship between female emergence time and the occurrence of extrapair young in the brood. Secondly, in 2 breeding seasons, we experimentally advanced female emergence times by supplying the roosting females with additional light in the early morning. Although the experiment had inconsistent effects on the occurrence of extrapair young in the brood, we found no evidence that female emergence time during peak fertility is directly linked to extrapair paternity. Interestingly, females exposed to artificial light were more likely to return to breed in the next year.

Covariance of paternity and sex with laying order explains male bias in extra-pair offspring in a wild bird population

It has been hypothesized that parents increase their fitness by biasing the sex ratio of extra-pair offspring (EPO) towards males. Here, we report a male bias among EPO in a wild population of blue tits (Cyanistes caeruleus). This resulted from a decline in both the proportion of males and EPO over the laying order of eggs in the clutch. However, previous studies suggest that, unlike the decline in EPO with laying order, the relationship between offspring sex ratio and laying order is not consistent between years and populations in this species. Hence, we caution against treating the decline in proportion of males with laying order, and the resulting male bias among EPO, as support for the above hypothesis. Variable patterns of offspring sex and paternity over the laying order may explain inconsistent associations between offspring sex and paternity, between and within species.

Declining extra-pair paternity with laying order associated with initial incubation behavior, but independent of final clutch size in the blue tit

Although functional explanations for female engagement in extra-pair copulation have been studied extensively in birds, little is known about how extra-pair paternity is linked to other fundamental aspects of avian reproduction. However, recent studies indicate that the occurrence of extra-pair offspring may generally decline with laying order, possibly because stimulation by eggs induces incubation, which may suppress female motivation to acquire extra-pair paternity. Here we tested whether experimental inhibition of incubation during the laying phase, induced by the temporary removal of eggs, resulted in increased extra-pair paternity, in concert with a later cessation of laying, in blue tits (Cyanistes caeruleus). As expected, experimental females showed a more gradual increase in nocturnal incubation duration over the laying phase and produced larger clutches than controls. Moreover, incubation duration on the night after the first egg was laid predicted how extra-pair paternity declined with laying order, with less incubation being associated with more extra-pair offspring among the earliest eggs in the clutch. However, incubation duration on this first night was unrelated to our experimental treatment and independent of final clutch size. Consequently, the observed decline in extra-pair paternity with laying order was unaffected by our manipulation and larger clutches included proportionally fewer extra-pair offspring. We suggest that female physiological state prior to laying, associated with incubation at the onset of laying, determines motivation to acquire extra-pair paternity independent of final clutch size. This decline in proportion of extra-pair offspring with clutch size may be a general pattern within bird species.

Superiority of extra-pair offspring: maternal but not genetic effects as revealed by a mixed cross-fostering design

Extra-pair copulations (EPC) are the rule rather than an exception in socially monogamous birds, but despite widespread occurrences, the benefits of female infidelity remain elusive. Most attention has been paid to the possibility that females gain genetic benefits from EPC, and fitness comparisons between maternal half-siblings are considered to be a defining test of this hypothesis. Recently, it was shown that these comparisons may be confounded by within-brood maternal effects where one such effect may be the distribution of half-siblings in the laying order. However, this possibility is difficult to study as it would be necessary to detect the egg from which each chick hatched. In this study, we used a new approach for egg-chick assignment and cross-fostered eggs on an individual basis among a set of nests of the collared flycatcher Ficedula albicollis. After hatching, chicks were ascribed to mothers and therefore to individual eggs by molecular genetic methods. Extra-pair young predominated early in the laying order. Under natural conditions, this should give them a competitive advantage over their half-siblings, mediated by hatching asynchrony. However, we experimentally synchronized hatching, and after this treatment, extra-pair young did not outperform within-pair young in any studied trait including survival up to recruitment and several indicators of reproductive success and attractiveness. We obtained only modest sample sizes for the last two traits and did not test for extra-pair success of male offspring. Thus, we cannot exclude the possibility of advantages of extra-pair young during the adult phase of life. However, our data tentatively suggest that the more likely reason for females' EPCs is the insurance against the infertility of a social mate.

Polygyny and extra-pair paternity enhance the opportunity for sexual selection in blue tits

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.