Sperm competition, in which the ejaculates of multiple males compete to fertilize a female's ova, results in strong selection on sperm traits. Although sperm size and swimming velocity are known to independently affect fertilization success in certain species, exploring the relationship between sperm length, swimming velocity and fertilization success still remains a challenge. Here, we use the zebra finch (Taeniopygia guttata), where sperm size influences sperm swimming velocity, to determine the effect of sperm total length on fertilization success. Sperm competition experiments, in which pairs of males whose sperm differed only in length and swimming speed, revealed that males producing long sperm were more successful in terms of (i) the number of sperm reaching the ova and (ii) fertilizing those ova. Our results reveal that although sperm length is the main factor determining the outcome of sperm competition, complex interactions between male and female reproductive traits may also be important. The mechanisms underlying these interactions are poorly understood, but we suggest that differences in sperm storage and utilization by females may contribute to the outcome of sperm competition.

https://royalsocietypublishing.org/doi/pdf/10.1098/rspb.2014.1897

Long sperm fertilize more eggs in a bird

Over the past two decades, studies of songbird populations have detected decreases in the reproductive success of individuals living in urban areas. Anthropogenic noise is considered to be particularly detrimental, however the exact relationship between noise and reproductive success is still unclear because noise is often correlated with many other detrimental factors (e.g., predation, reduced territory quality). We used an experiment to specifically test the effects of urban noise on reproduction of captive zebra finches (Taeniopygia guttata). We found that latency to breed and the size of successfully fledged clutches were consistent between groups, however success of initial nesting attempts was reduced by traffic noise. Further, this reduced success leading to increased numbers of nesting attempts by birds in the noise condition was due to higher levels of embryo mortality in the traffic noise treatment group, which also suffered a lag in nestling growth rates during the first two weeks post-hatch. While parental baseline circulating corticosterone was not chronically affected by noise treatment, we identified some interaction effects whereby certain reproductive measures (laying latency and clutch size) were most strongly affected by treatment when mothers had higher levels of baseline corticosterone. These results indicate that traffic noise may reduce reproductive success through changes in parental behaviour, and that traffic noise may disproportionately affect chronically stressed individuals during reproduction. J. Exp. Zool. 323A: 722-730, 2015. © 2015 Wiley Periodicals, Inc.

Traffic noise affects embryo mortality and nestling growth rates in captive zebra finches

Polyspermy is a major puzzle in reproductive biology. In some taxa, multiple sperm enter the ovum as part of the normal fertilization process, whereas in others, penetration of the ovum by more than one sperm is lethal. In birds, several sperm typically enter the germinal disc, yet only one fuses with the female pronucleus. It is unclear whether supernumerary sperm play an essential role in the avian fertilization process and, if they do, how females regulate the progression of sperm through the oviduct to ensure an appropriate number reach the ovum. Here, we show that when very few sperm penetrate the avian ovum, embryos are unlikely to survive beyond the earliest stages of development. We also show that when the number of inseminated sperm is limited, a greater proportion than expected reach and penetrate the ovum, indicating that females compensate for low sperm numbers in the oviduct. Our results suggest a functional role for supernumerary sperm in the processes of fertilization and early embryogenesis, providing an exciting expansion of our understanding of sperm function in birds.

https://royalsocietypublishing.org/doi/pdf/10.1098/rspb.2015.1682

Polyspermy in birds: sperm numbers and embryo survival.

Postzygotic isolation may be important for maintaining species boundaries, particularly when premating barriers are incomplete. Little is known about the course of events leading from minor environmental mismatches affecting hybrid fitness to severe genetic incompatibilities causing sterility or inviability. We investigated whether reduced reproductive success of hybrid males was caused by suboptimal sperm traits or by more severe genetic incompatibilities in a hybrid zone of pied (Ficedula hypoleuca) and collared flycatchers (F. albicollis) on the island of Oland, Sweden. About 4 per cent hybridization is observed in this population and all female hybrids are sterile. We found no sperm in the ejaculates of most sampled hybrid males, and sperm with abnormal morphology in two hybrids. Furthermore, none of the hybrids sired any offspring because of high levels of hatching failure and extra-pair paternity in their nests. These results from a natural hybrid zone suggest that the spermatogenesis of hybrid males may become disrupted despite little genetic divergence between the parental species.

https://royalsocietypublishing.org/doi/pdf/10.1098/rsbl.2013.0169

Low fertility of wild hybrid male flycatchers despite recent divergence.

About 10 per cent of birds' eggs fail to hatch, but the incidence of failure can be much higher in endangered species. Most studies fail to distinguish between infertility (due to a lack of sperm) and embryo mortality as the cause of hatching failure, yet doing so is crucial in order to understand the underlying problem. Using newly validated techniques to visualize sperm and embryonic tissue, we assessed the fertility status of unhatched eggs of five endangered species, including both wild and captive birds. All eggs were classified as ‘infertile’ when collected, but most were actually fertile with numerous sperm on the ovum. Eggs of captive birds had fewer sperm and were more likely to be infertile than those of wild birds. Our findings raise important questions regarding the management of captive breeding programmes.

https://royalsocietypublishing.org/doi/pdf/10.1098/rsbl.2012.0655

Causes of hatching failure in endangered birds.

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.

Unhatched eggs: methods for discriminating between infertility and early embryo mortality

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Unhatched eggs: methods for discriminating between infertility and early embryo mortality