Search

We studied the shapes of eggs from 955 extant bird species across the avian phylogeny, including 39 of 40 orders and 78% of 249 families. We show that the elongation component of egg shape (length relative to width) is largely the result of constraints imposed by the female's anatomy during egg formation, whereas asymmetry (pointedness) is mainly an adaptation to conditions during the incubation period. Thus, egg elongation is associated with the size of the egg in relation to both the size of the female's oviduct and her general body conformation and mode of locomotion correlated with pelvis shape. Egg asymmetry is related mainly to clutch size and the structure of the incubation site, factors that influence thermal efficiency during incubation and the risk of breakage. Importantly, general patterns across the avian phylogeny do not always reflect the trends within lower taxonomic levels. We argue that the analysis of avian egg shape is most profitably conducted within taxa where all species share similar life histories and ecologies, as there is no single factor that influences egg shape in the same way in all bird species.

Since the late 1600s it has been assumed that the Great Auk Pinguinus impennis was similar to the Common Guillemot Uria aalge and Brünnich’s Guillemot U. lomvia in having a single, central brood patch. Through the examination of eight mounted museum specimens, we show that this is incorrect and that like its closest relative the Razorbill Alca torda, the Great Auk had two lateral brood patches. We discuss how such misinformation persisted for so long. We also review the relationship between the number of brood patches and clutch size in the Alcidae. One implication of two brood patches is that the Great Auk would have incubated in a horizontal posture like the Razorbill, rather than in a semi-upright posture like the Uria guillemots. Assuming that the Great Auk incubated like the Razorbill, it would probably have done so horizontally with its single egg pressed against one of the two lateral brood patches, positioned against the inside of one tarsus (and partially on the web of one foot), and with the wing on that side drooped to provide additional protection for the egg. Incubating in this way may have meant that the Great Auk’s pyriform egg would have enabled it to use both level and sloping terrain, as in the Uria guillemots (but unlike the Razorbill). A horizontal incubation also has implications for estimates of their breeding density, which we estimate to have been around four pairs per square meter and, hence numbers on its largest known colony, Funk Island, Newfoundland (maximum 250,000 pairs).

Nearly 70 years ago, Preston published a pioneering study in which he provided formulae for the shapes of birds’ eggs. One of these formulae is universal in that it provides an almost perfect representation for all eggs, even pyriform ones, and is better than all other formulae published since. This essentially perfect representation of egg shape is obtained by estimating the parameters in Preston’s universal formula by least squares, using hundreds of measurements of the egg’s radii along its entire length. Preston’s universal formula can also be used to obtain an equation for avian egg shape that uses just 5 measurements (the length and 4 appropriately spaced diameters). The equation based solely on these 5 measurements provides an egg shape that is virtually indistinguishable from one based on hundreds of measurements. We demonstrate the usefulness of Preston’s formulations using digital photographs of eggs. Our perspective is that, despite a number of subsequent approaches, Preston’s original one has not been bettered and should be the standard for studying avian egg shape.

Most of the approximately 75 known eggs of the extinct great auk ( Pinguinus impennis) are in public museums, with a few in private collections. A small number of these eggs has sustained damage, either at the time of collection or subsequently, and two of these eggs are known to have been repaired. The two eggs suffered rather different types of damage and were subsequently restored using different techniques. The first, known as Bourman Labrey's egg, sustained extensive damage sometime prior to the 1840s, when the shell was broken into numerous pieces. This egg was repaired by William Yarrell in the 1840s, and when it was restored again in 2018, it was discovered that Yarrell's restoration had involved the use of an elaborate cardboard armature. This egg is currently in a private collection. The second egg, known as the Scarborough egg, bequeathed to the Scarborough Museum in 1877, was damaged (by unknown causes) and repaired, probably by the then curator at Scarborough, W. J. Clarke, in 1906. This egg was damaged when one or more pieces were broken adjacent to the blowhole at the narrow end (where there was some pre-existing damage). The media reports at the time exaggerated the extent of the damage, suggesting that the egg was broken almost in two. Possible reasons for this exaggeration are discussed. Recent examination using a black light and ultraviolet (UV) revealed that the eggshell had once borne the words, “a Penguin's Egg”, that were subsequently removed by scraping.

We compared the shape and eggshell thickness of Great Auk Pinguinus impennis eggs with those of its closest relatives, the Razorbill Alca torda, Common Guillemot Uria aalge and Brunnich's Guillemot Uria lomvia, in order to gain additional insights into the breeding biology of the extinct Great Auk. The egg of the Great Auk was most similar in shape to that of Brunnich's Guillemot. The absolute thickness of the Great Auk eggshell was greater than that of the Common Guillemot and Razorbill egg, which is as expected given its greater size, but the relative shell thickness at the equator and pointed end (compared with the blunt end) was more similar to that of the Common Guillemot. On the basis of these and other results we suggest that Great Auk incubated in an upright posture in open habitat with little or no nest, where its pyriform egg shape provided stability and allowed safe manoeuvrability during incubation. On the basis of a recent phylogeny of the Alcidae, we speculate that a single brood patch, a pyriform egg and upright incubation posture, as in the Great Auk and the two Uria guillemots, is the ancestral state, and that the Razorbill – the Great Auk's closest relative – secondarily evolved two brood patches and an elliptical egg as adaptations for horizontal incubation, which provides flexibility in incubation site selection, allowing breeding in enclosed spaces such as crevices, burrows or under boulders, as well as on open ledges.

We studied the ground colors and maculations of 161 Common Murre (Uria aalge) eggs laid by 43 females in 3 small breeding groups on the cliffs of Skomer Island, Wales, in 2016–2018. Both the colors and maculations varied much more among than within females, providing quantitative evidence for the egg traits that might facilitate the parents' ability to identify their own eggs on the crowded breeding ledges where the density is typically ∼20 eggs m–2. Ground colors had a trimodal distribution of hue values (whitish to pale brown, pale blue, or vivid blue-green) and maculations ranged from none to complex squiggles and blotches. The eggs laid by each female in different years were similar to one another, and replacement eggs laid by females within years were also more similar to their first egg than to other eggs in the same breeding group. Egg appearance did not differ among the 3 breeding groups that we studied. Our findings thus support anecdotal observations that, within and between years, female Common Murres lay eggs that have similar ground colors and maculations. We do not, however, find evidence that there is much difference among the eggs laid in different parts of a colony. LAY SUMMARY For more than a century, naturalists have marveled at the diversity of colors and markings on the eggs of Common Murres (Uria aalge) breeding in dense coastal colonies, where they lay and incubate a single egg. Largely anecdotal observations have suggested that each female lays distinctive eggs, and that eggs laid on the same cliff ledge tend to be more similar than expected if pairs are distributed randomly. We measured the appearance of 161 eggs laid by 43 Common Murres over a 3-year period on Skomer Island, Wales. Each female consistently laid eggs within a narrow range of whitish to pale brown (11 females) or pale blue to vivid blue-green ground color (32 females) and similar markings both within- and between breeding seasons. Despite offspring tending to return to breed near their parents, eggs laid within a few meters of one another were no more similar to each other than they were to eggs laid on cliff ledges 200 m away.

Edward Walter Wade (1864–1937), author of The Birds of Bempton Cliffs (1903, 1907), is almost unknown. He worked as a clerk for the family timber company in Hull and in his spare time visited Bempton Cliffs, where he climbed (routinely without a rope) to observe the seabirds and to collect eggs. He was an active member of the Hull Scientific and Field Naturalists' Club and became one of their Vice-Presidents and President. Between 1902 and 1920 he was a member of the British Ornithologists' Union. Wade's published comments on guillemots and their pyriform-shaped eggs, at Bempton, were unusually insightful, in part because his observations were made at close range (a consequence of his climbing ability), and partly because he thought carefully about what he saw. Wade was among the first to point out that the spinning-like-a-top explanation for the guillemot's pyriform egg shape was incorrect based on his own observations. An appendix lists Wade's 69 articles, mostly about birds in Yorkshire, published between 1901 and 1930.

In the three decades, since Birkhead and Møller published Sperm competition in birds (1992, Academic Press) more than 1000 papers have been published on this topic, about half of these being empirical studies focused on extrapair paternity. Both technological innovations and theory have moved the field forward by facilitating the study of both the mechanisms underlying sperm competition in both sexes, and the ensuing behavioural and morphological adaptations. The proliferation of studies has been driven partly by the diversity of both behaviours and morphologies in birds that have been influenced by sperm competition, but also by the richness of the theory developed by Geoff Parker over the past 50 years. This article is part of the theme issue ‘Fifty years of sperm competition’.

Following copulation, females of many seabird species spend a prolonged period of time away from the colony, building up reserves for egg formation and incubation. Here, we report that the number of sperm associated with eggs of single-egg clutch seabirds was almost an order of magnitude greater than predicted from the relationship between ovum size and sperm numbers in multi-egg clutch non-seabirds. Sperm numbers were also several times greater than the estimated number necessary for maximal fertilization success. Our results are consistent with 3 unusual features of seabird reproduction: (1) single-egg clutches, (2) prolonged sperm storage, and (3) a lag period between the end of yolk formation and ovulation. We hypothesize that sperm release from storage is under precise temporal control in these species, with high sperm numbers acting as an insurance against infertility in single-egg clutches. If true, the lag period may have evolved to provide sufficient time for sperm to be released simultaneously from storage and accumulate at the site of fertilization prior to ovulation. LAY SUMMARY After copulation, female birds store sperm in their reproductive systems for some time before egg production and fertilization. It is thought that sperm are gradually lost during storage, meaning that later in the storage period, if there are no further inseminations, fewer sperm should be available for insemination. We studied the number of sperm reaching eggs in 6 seabird species, in which females spend a long time feeding at sea between mating and producing eggs. Surprisingly, we found that the number of sperm that reached eggs in these species was much higher than expected, based on a known relationship between egg size and sperm number. We suggest that, in these species, sperm are released simultaneously from storage just before ovulation. This may be particularly important in the species we studied, because they all lay only a single egg. Our hypothesis implies that female birds have greater control over sperm storage and use than previously thought.

The adaptive significance of avian egg shape is a long-standing problem in biology. For many years, it was widely believed that the pyriform shape of the Common Murre (Uria aalge) egg allowed it to either “spin like a top” or “roll in an arc,” thereby reducing its risk of rolling off the breeding ledge. There is no evidence in support of either mechanism. Two recent alternative hypotheses suggest that a pyriform egg confers mechanical strength and minimizes the risk of dirt contamination of the blunt end. We present a new hypothesis: that the Common Murre egg's pyriform shape confers stability on the breeding ledge, thus reducing the chance that it will begin to roll. We tested this hypothesis by measuring the stability of Common Murre and Razorbill (Alca torda) eggs of different shapes on slopes of 20°, 30°, and 40° above the horizontal. Common Murre eggs were more stable, and easier to stabilize, than the more elliptical Razorbill eggs. Within Common Murre eggs, more pyriform eggs were more stable. From a fitness perspective, the stability of the Common Murre egg on a slope seems likely to confer an advantage and thus may be a strong force of natural selection favoring the pyriform shape.

A recent broad comparative study suggested that factors during egg formation – in particular ‘flight efficiency’, which explained only 4% of the interspecific variation – are the main forces of selection on the evolution of egg shape in birds. As an alternative, we tested whether selection during the incubation period might also influence egg shape in two taxa with a wide range of egg shapes, the alcids (Alcidae) and the penguins (Spheniscidae). To do this, we analysed data from 30 species of these two distantly related but ecologically similar bird families with egg shapes ranging from nearly spherical to the most pyriform eggs found in birds. The shape of pyriform eggs, in particular, has previously proven difficult to quantify. Using three egg‐shape indices – pointedness, polar‐asymmetry and elongation – that accurately describe the shapes of all birds’ eggs, we examined the effects of egg size, chick developmental mode, clutch size and incubation site on egg shape. Linear models that include only these factors explained 70–85% of the variation in these egg‐shape indices, with incubation site consistently explaining > 60% of the variation in shape. The five species of alcids and penguins that produce the most pyriform eggs all incubate in an upright posture on flat or sloping substrates, whereas species that incubate in a cup nest have more spherical eggs. We suggest that breeding sites and incubation posture influence the ability of parents to manipulate egg position, and thus selection acting during incubation may influence egg‐shape variation across birds as a whole.

Huge numbers of Common Guillemot (Uria aalge) eggs were harvested by local men known as “climmers” (climbers) at Bempton Cliffs, Flamborough, Yorkshire, until 1954 when egg collecting became illegal. Guillemot eggs are more variable in colour and pattern than those of any other bird. Egg collectors (oologists) particularly favoured sets of unusually coloured eggs laid by the same bird. Red guillemots eggs were extremely rare and eagerly sought. An example of one such egg known as the “Bempton Belle” was found in the collection of J. W. Goodall (active between about 1896 and 1909), and was celebrated in a poem. What was probably another red egg, known as the “Metland Egg”, was collected each year at Bempton between 1912 and 1938. The current whereabouts of the Metland eggs is unknown. We estimate that females laying red eggs occur less than once in a thousand (or ten thousand). We also speculate about the factors responsible for red eggs and why such eggs are so rare.

Sperm competition is an important selective force in many organisms. As a result, sperm have evolved to be among the most diverse cells in the animal kingdom. However, the relationship between sperm morphology, sperm motility and fertilization success is only partially understood. The extent to which between-male variation is heritable is largely unknown, and remarkably few studies have investigated the genetic architecture of sperm traits, especially sperm morphology. Here we use high-density genotyping and gene expression profiling to explore the considerable sperm trait variation that exists in the zebra finch Taeniopygia guttata. We show that nearly all of the genetic variation in sperm morphology is caused by an inversion polymorphism on the Z chromosome acting as a ‘supergene’. These results provide a striking example of two evolutionary genetic predictions. First, that in species where females are the heterogametic sex, genetic variation affecting sexually dimorphic traits will accumulate on the Z chromosome. Second, recombination suppression at the inversion allows beneficial dominant alleles to become fixed on whichever haplotype they first arise, without being exchanged onto other haplotypes. Finally, we show that the inversion polymorphism will be stably maintained by heterozygote advantage, because heterozygous males have the fastest and most successful sperm. Sperm morphology is remarkably variable yet its genetic architecture is poorly known. Now, sperm morphology variation in zebra finches is shown to be largely explained by an inversion polymorphism on the Z chromosome acting as a supergene.

In the kitchen record books of the L'Estrange family in the sixteenth and seventeenth centuries, there are references to a bird, widely shot on the Norfolk coast, called a Spowe. On the basis of the similarity to the Icelandic name, J. H. Gurney (sen.) and Fisher (in their “An account of birds found in Norfolk” published in 1846) assumed this to be the Whimbrel (Numenius phaeopus) as have all ornithological texts ever since. Internal evidence from the kitchen records strongly suggest that the Spowe was a winter visitor, not a passage migrant, thus throwing considerable doubt on Gurney and Fisher's ascription. We suggest that it is much more likely that the Spowe was the Bar-tailed Godwit (Limosa lapponica).

The adaptive significance of avian egg shape in birds is poorly understood. The pyriform (pear-like) shape of the Common Guillemot's Uria aalge egg has long been considered to be an adaptation to prevent eggs rolling off the bare cliff ledges on which this species breeds. Rolling was thought to be prevented either by the egg spinning like a top, which is not the case, or by rolling in an arc, which it does but with little influence on whether the egg will fall from a ledge. We therefore sought alternative explanations for the pyriform shape of the Common Guillemot's egg. This species breeds in extremely dense colonies, which makes their eggs vulnerable to mechanical damage from conspecifics, and to contamination by debris such as faeces and soil. We present evidence consistent with both these possible explanations. First, the pyriform shape of Common Guillemot eggs means that a higher proportion of the eggshell lies in contact with the substrate and this may minimize the effect of impacts. Resistance to impacts may be further enhanced because their eggshells are especially thick where they are in contact with the substrate. Secondly, Common Guillemot eggs are often heavily contaminated with faecal material and other debris during incubation. Most contamination is on the pointed end of the egg where it is in contact with the substrate; the pyriform shape thus keeps the blunt end of the egg, which has the highest porosity, relatively free of contamination, which in turn may facilitate both gas exchange during incubation and the hatching process, because the chick emerges from the blunt end of the egg.

During the 1830s, Charles Thorold Wood jun. and his younger brother Neville Wood, published, separately, three books and a series of articles dealing with two ornithological subjects: the common and scientific names of birds, and the cataloguing of publications. Probably following William Swainson's lead, the Woods were enthusiastic about standardizing the common (English) bird names and making them logical and meaningful. They also each published an annotated bibliography of ornithological publications, notable for being among the first of such compilations, but also for the vitriol with which they criticized those – James Rennie and Hugh Strickland, in particular – whose work they did not like. In contrast, the praise they heaped on those they did approve of – William Swainson, John Latham, Robert Mudie, Prideaux John Selby, Francis Willughby and each other – was excessive. Possibly because of the tenor of their comments about other ornithologists, and the strangeness of their proposed English bird names, the Woods’ work has rarely been cited, and their new names for birds were virtually ignored from the start.

Hatching failure is widespread in birds, and is usually the result of embryo death rather than infertility. Embryo death can result from both intrinsic and extrinsic factors, some of which may vary across the developmental period. Determining the point at which an embryo died during development may therefore help us to understand the underlying cause of death. Here we describe simple criteria that can be used by field ornithologists to establish the developmental stage of dead embryos found in unhatched passerine eggs, and explain how this can be used to estimate the date of embryo death. We compared the pattern of embryo development over the incubation period for three species, the Zebra Finch Taeniopygia guttata, Blue Tit Cyanistes caeruleus and Great Tit Parus major. We also compared rates of Zebra Finch embryo development under artificial and standard (parental) incubation. Embryo development rates were remarkably similar across the three species and between Zebra Finch embryos under artificial and natural incubation conditions. We therefore suggest that the pattern of embryo development in the Zebra Finch may provide a model for other small passerines with similar incubation periods, but acknowledge that further interspecific comparisons are required before this model is considered more widely applicable. By estimating embryo death dates using our approach, ornithologists will be able to determine temporal patterns of embryo mortality in relation to extrinsic environmental conditions. This approach may shed light on how extrinsic factors such as climate and parental behaviour influence embryo survival in wild birds.

Birds are arguably the most evolutionarily successful extant vertebrate taxon, in part because of their ability to reproduce in virtually all terrestrial habitats. Common guillemots, Uria aalge, incubate their single egg in an unusual and harsh environment; on exposed cliff ledges, without a nest, and in close proximity to conspecifics. As a consequence, the surface of guillemot eggshells is frequently contaminated with faeces, dirt, water and other detritus, which may impede gas exchange or facilitate microbial infection of the developing embryo. Despite this, guillemot chicks survive incubation and hatch from eggs heavily covered with debris. To establish how guillemot eggs cope with external debris, we tested three hypotheses: (1) contamination by debris does not reduce gas exchange efficacy of the eggshell to a degree that may impede normal embryo development; (2) the guillemot eggshell surface is self-cleaning; and, (3) shell accessory material (SAM) prevents debris from blocking pores, allowing relatively unrestricted gas diffusion across the eggshell. We show that (1) natural debris reduces the conductance of gases across the guillemot eggshell by blocking gas exchange pores. Despite this problem, we find (2) no evidence that guillemot eggshells are self-cleaning, but instead show that (3) the presence of SAM on the eggshell surface largely prevents pore blockages from occurring. Our results demonstrate that SAM is a crucial feature of the eggshell surface in a species whose eggs are frequently in contact with debris, acting to minimise pore blockages and thus ensure a sufficient rate of gas diffusion for embryo development.

Describing the range of avian egg shapes quantitatively has long been recognized as difficult. A variety of approaches has been adopted, some of which aim to capture the shape accurately and some to provide intelligible indices of shape. The objectives here are to show that a (four‐parameter) method proposed by Preston (1953, The Auk, 70, 160) is the best option for quantifying egg shape, to provide and document an R program for applying this method to suitable photographs of eggs, to illustrate that intelligible shape indices can be derived from the summary this method provides, to review shape indices that have been proposed, and to report on the errors introduced using photographs of eggs at rest rather than horizontal.

The European Honey-buzzard (Pernis apivorus) was first accurately described and clearly distinguished from the Common Buzzard (Buteo buteo) by Francis Willughby and John Ray in their Ornithology, originally published in Latin in 1676. Alfred Newton's statement that Pierre Belon had described the species over a century earlier is not entirely correct, as Belon confused this honey-buzzard's features with those of the common buzzard and even appeared uncertain whether it was a separate species. One of Willughby's important contributions to ornithology was the identification and use of “characteristic marks” to distinguish and identify species, including those that distinguish the European Honey-buzzard from the Common Buzzard. Because Willughby provided the first accurate description of Pernis apivorus – and because his contribution to ornithology has never been formally recognized – we propose that the common name of the European Honey-buzzard be changed to Willughby's Buzzard.

Oskar Heinroth was a pioneering German ornithologist of the early-mid 20th century whose three major contributions to ornithology have been largely overlooked. First, he conducted a systematic comparative study of the avian social signals and other behaviours (in waterfowl, including hybrids) and was the first to recognise that their instinctive, ritualized displays could serve as taxonomic criteria. Konrad Lorenz later developed the idea and also gained most of the credit for it. Second, Heinroth, together with his first wife Magdalena, hand-reared about 1,000 individuals of 286 species of birds in their apartment over 28 years, documenting and photographing their development. The results, published in four substantial volumes (1924–1933) were well received, but the deteriorating financial and political situation in Germany before WWII, together with the lack of an English translation, meant that the project received less attention than it warranted. Third, Heinroth provided a framework for studying and understanding the behaviour of birds that included detailed observations throughout the animals’ life; making use of tame individuals; and comparisons between captive and wild individuals. The value of this approach to the study of behaviour is exemplified by the Heinroths’ results from rearing European Nightjars Caprimulgus europaeus in their apartment. We propose that these contributions as well as numerous other anecdotes and observations in Heinroth’s publications merit further examination.

When females mate promiscuously, female sperm storage provides scope to bias the fertilization success towards particular males via the non-random acceptance and utilization of sperm. The difficulties observing post-copulatory processes within the female reproductive tract mean that the mechanisms underlying cryptic female choice remain poorly understood. Here, we use zebra finches Taeniopygia guttata , selected for divergent sperm lengths, combined with a novel technique for isolating and extracting sperm from avian sperm storage tubules (SSTs), to test the hypothesis that sperm from separate ejaculates are stored differentially by female birds. We show that sperm from different inseminations enter different SSTs in the female reproductive tract, resulting in almost complete segregation of the sperm of competing males. We propose that non-random acceptance of sperm into SSTs, reflected in this case by sperm phenotype, provides a mechanism by which long sperm enjoy enhanced fertilization success in zebra finches.

We identify the little known, anonymous author of a treatise on nightingales, Traité du rossignol, as Nicolas Venette (1633–1698), a physician based in La Rochelle, France. As well as writing about nightingales, Venette had a wide range of scientific interests, and produced several other books, including an extraordinarily popular sex manual. In his nightingale monograph Venette made observations and wrote in a logical, “scientific” manner, critically assessing previous speculations about the causes of migration in birds. In addition, he was the first to describe the phenomenon of migratory restlessness and to accurately identify its biological significance. Venette is therefore one of a small number of individuals who began the development of scientific ornithology in the seventeenth century.

Honeyguides (Indicatoridae, Piciformes) are unique among birds in several respects. All subsist primarily on wax, are obligatory brood parasites and one species engages in ‘guiding' behavior in which it leads human honey hunters to bees' nests. This unique life history has likely shaped the evolution of their brain size and morphology. Here, we test that hypothesis using comparative data on relative brain and brain region size of honeyguides and their relatives: woodpeckers, barbets and toucans. Honeyguides have significantly smaller relative brain volumes than all other piciform taxa. Volumetric measurements of the brain indicate that honeyguides have a significantly larger cerebellum and hippocampal formation (HF) than woodpeckers, the sister clade of the honeyguides, although the HF enlargement was not significant across all of our analyses. Cluster analyses also revealed that the overall composition of the brain and telencephalon differs greatly between honeyguides and woodpeckers. The relatively smaller brains of the honeyguides may be a consequence of brood parasitism and cerophagy (‘wax eating'), both of which place energetic constraints on brain development and maintenance. The inconclusive results of our analyses of relative HF volume highlight some of the problems associated with comparative studies of the HF that require further study.

The relationship between sperm energetics and sperm function is poorly known, but is central to our understanding of the evolution of sperm traits. The aim of this study was to examine how sperm morphology and ATP content affect sperm swimming velocity in the zebra finch Taeniopygia guttata . We exploited the high inter-male variation in this species and created extra experimental power by increasing the number of individuals with very long or short sperm through artificial selection. We found a pronounced quadratic relationship between total sperm length and swimming velocity, with velocity increasing with length up to a point, but declining in the very longest sperm. We also found an unexpected negative association between midpiece length and ATP content: sperm with a short midpiece generally contained the highest concentration of ATP. Low intracellular ATP is therefore unlikely to explain reduced swimming velocity among the very longest sperm (which tend to have a shorter midpiece).

The female reproductive tract is where competition between the sperm of different males takes place, aided and abetted by the female herself. Intense postcopulatory sexual selection fosters inter-sexual conflict and drives rapid evolutionary change to generate a startling diversity of morphological, behavioural and physiological adaptations. We identify three main issues that should be resolved to advance our understanding of postcopulatory sexual selection. We need to determine the genetic basis of different male fertility traits and female traits that mediate sperm selection; identify the genes or genomic regions that control these traits; and establish the coevolutionary trajectory of sexes.

When females are inseminated by more than one male (polyandry) sexual selection continues after insemination in the form of sperm competition and cryptic female choice. The sexually-selected sperm hypothesis proposes that, under the risk of sperm competition, additive variation in male traits determining fertilising efficiency will select for female propensity to be polyandrous in order to increase the probability of producing sons with superior fertilising efficiency. Two factors complicate this prediction: sex-biased transmission of male fertilising efficiency traits and sexual antagonism of sex-limited traits, fostered by sex-biased inheritance. Here, we (i) review the evidence that male traits contributing towards fertilising efficiency are heritable through sex-biased mechanisms, and (ii) explore the evolutionary implications for male and female reproductive strategies caused by both sex-biased transmission and sexual antagonism of fertilising efficiency traits. Many male fertilising efficiency traits are heritable through sex-biased mechanisms and may not necessarily increase female fitness. The predictions of the sexually-selected sperm hypothesis change dramatically under these different mechanisms of inheritance of fertilising efficiency traits, and different fitness pay-offs derived by females from the expression of such traits. Both sex-biased control of fertilising efficiency and sexual antagonism may also be important in explaining the maintenance of the genetic variance and selection potential of fertilising efficiency. We propose that a useful approach to test the sexually-selected sperm hypothesis is to combine studies which identify behavioural and physiological mechanisms explaining variation in reproductive success with artificial selection experiments to infer the underlying evolutionary patterns.

When a female is sexually promiscuous, the ejaculates of different males compete for the fertilization of her eggs; the more sperm a male inseminates into a female, the more likely he is to fertilize her eggs. Because sperm production is limited and costly, theory predicts that males will strategically allocate sperm (1) according to female promiscuity, (2) saving some for copulations with new females, and (3) to females producing more and/or better offspring. Whether males allocate sperm in all of these ways is not known, particularly in birds where the collection of natural ejaculates only recently became possible. Here we demonstrate male sperm allocation of unprecedented sophistication in the fowl Gallus gallus. Males show status-dependent sperm investment in females according to the level of female promiscuity; they progressively reduce sperm investment in a particular female but, on encountering a new female, instantaneously increase their sperm investment; and they preferentially allocate sperm to females with large sexual ornaments signalling superior maternal investment. Our results indicate that female promiscuity leads to the evolution of sophisticated male sexual behaviour.\ud \ud

Although much attention has been recently directed to sexual selection arising after insemination from sexual promiscuity, little is known about the mechanisms determining reproductive success after insemination, and the way these mechanisms interact with each other and with selective mechanisms occurring before insemination: mate choice and mate acquisition. Here, we briefly review the findings of an on-going study investigating the mechanisms generating variation in reproductive success at both a pre- and a post-insemination stage in the domestic fowl. Female preference consistently favours socially dominant males before and after insemination. However, although social status mediates the number of sperm that a male inseminates into a female, dominant males may inseminate sperm of lower fertilising quality than their subordinates. We argue that mitochondrial genes may contribute to determine sperm quality, and speculate that the maternal control of mitochondrial genes may prevent sexual selection from operating on males, thus explaining both the lack of a positive correlation between social dominance and sperm quality and the maintenance of variation in male quality in the fowl.

James A. Mills et al.-- Letter.

The population of common guillemots Uria aalge on Skomer Island, Wales has been monitored since 1963, and in the last 30 yr has increased at an almost constant rate of 5% yr−1. A previous attempt to model the population based on intrinsic demographic parameters estimated over just five years failed to explain the observed population increase, probably because the estimate of juvenile survival was too low. This raised the possibility that immigration fuelled the population increase. Here we use > 30 yr of detailed field observations to re-estimate key population parameters (productivity, adult survival and juvenile survival) in order to model the population. We show that the observed rate of increase can be explained by these intrinsic parameters, and we therefore conclude that immigration is not necessary to generate the observed population growth.

Many reproductive traits, including ejaculate characteristics, usually show remarkable seasonal variation, but the potential for such dynamics in sperm morphology has been overlooked. Several studies have revealed high within-male repeatability in sperm morphology, but samples have typically been collected within a few hours or days, and the consistency of sperm morphology over longer periods remains unexplored. Here, we tested whether ejaculate traits, including sperm morphology, sperm number, and sperm velocity, exhibit seasonal phenotypic plasticity in a long-lived seasonal breeder, the red-winged blackbird (Agelaius phoeniceus). We found absolute and/or relative flagellum length and sperm velocity to increase across the season, whereas sperm numbers within ejaculates declined. Sperm morphological traits were further positively associated with harem size or the total number of offspring that fledged in each male’s territory, suggesting that sperm morphology is likely to be linked to male reproductive quality. The underlying mechanisms of these patterns of seasonal variation remain unresolved, but we discuss our results in the context of dynamics of reproductive hormones, testicular structures and function, and reproductive behavior.

Alfred Newton (1829–1907) was a founding member of the British Ornithologists’ Union, served as editor of The Ibis, and became one of the best-known British ornithologists of the nineteenth-century. Between 1855 and 1864 he travelled in Europe and North America, making a trip to Iceland in 1858 with John Wolley in search of information about the Great Auk Pinguinus impennis. In 1866 Newton obtained the chair of Zoology and Comparative Zoology at the University of Cambridge, where he remained for rest of his career. An unenthusiastic teacher, Newton nevertheless encouraged successive generations of young ornithologists through the soirees he held in his college rooms. Newton published extensively, but his most significant publication was the Dictionary of Birds (1896). Newton's death marked the beginning of the end of a long period in which scientific ornithology comprised little more than taxonomy and nomenclature, although Newton was also instrumental in initiating conservation legislation. Extremely conservative in most aspects of his life, Newton was nevertheless the first ornithologist to appreciate the significance of natural selection. He therefore constitutes an important figure in an era of ornithology that immediately precedes the current interest in field ornithology.

The Zebra Finch Taeniopygia guttata is a model bird species for the experimental study of behavioural and evolutionary concepts in captivity and especially sexual selection. The validity of sexual selection studies of domesticated birds is of long-standing concern as little is known about the influence of domestication on sexually selected traits. Most domesticated Zebra Finch populations are maintained under a strict breeding regime to avoid potential inbreeding. However, these breeding regimes may interfere with the processes of sexual selection and influence the evolution of sexually selected traits because they may limit or prohibit active mate choice. Here, we investigated the potential impact of a monogamous breeding scheme in a domesticated population in which active mate choice is largely inhibited, on the evolution of sperm morphometry as a sexually selected trait. We compared sperm morphometric traits (total sperm length and length of sperm head, midpiece and flagellum), and the variance thereof, between a domesticated and two wild Zebra Finch populations. Although we found significant differences between the three populations for certain sperm traits (head length, midpiece length), which may be of importance in postcopulatory sexual selection, overall, variance in sperm morphometry did not differ between the domesticated and the wild Zebra Finch populations. Our results validate the use of domesticated Zebra Finches for further studies of postcopulatory sexual selection and sperm competition.

background: Sperm motility is regulated by mitochondrial enzymes that are partially encoded by mitochondrial DNA (mtDNA). MtDNA has therefore been suggested as a putative genetic marker of male fertility. However, recent studies in different populations have identified both significant and non-significant associations between mtDNA variation and sperm motility. Here, we tested whether mtDNA variation was associated with sperm motility in a large cohort of men from the UK, to test the robustness of previous studies and the reliability of mtDNA as a marker of poor sperm motility. methods: A total of 463 men attending for semen analysis as part of infertility investigations were recruited from a UK laboratory. Sperm motility was measured using both computer-assisted sperm analysis and traditional manual measurements. MtDNA haplogroup and haplotype were determined in 357 and 298 men, respectively, using single nucleotide polymorphism (SNP) markers throughout the mtDNA genome, and compared with sperm motility data. The linkage between the SNP markers, and possible associations between individual SNPs and motility, were also investigated. results: We found no statistical association between haplogroup or haplotype and sperm motility, regardless of how it was measured (P . 0.05 in all cases). Moreover, individual SNPs which were in linkage disequilibrium and dispersed across the mitochondrial genome, and therefore sensitive to mtDNA variation, were not predictive of sperm motility. conclusions: Mitochondrial haplotype is unlikely to be a reliable genetic marker of male factor infertility.

Spermatozoa are amongst the most variable cells, and three factors are thought to account for this variation in design: fertilization mode, phylogeny, and postcopulatory sexual selection. In addition, it has long been assumed that a tradeoff exists between sperm size and number, and although postcopulatory sexual selection affects both traits, empirical evidence for a tradeoff has so far been elusive. Our recent theoretical model predicts that the nature of a direct tradeoff between sperm size and number varies with sperm competition mechanism and sperm competition risk. We test these predictions using a comparative approach in two very different taxa with different sperm competition mechanisms: passerine birds (mechanism: simple raffle) and Drosophila fruit flies (sperm displacement). We show that in both groups, males increase their total ejaculate investment with increasing sperm competition risk, but whereas passerine birds allocate disproportionately to sperm number, drosophilids allocate disproportionately to sperm size. This striking difference between the two groups can be at least partly explained by sperm competition mechanisms depending on sperm size relative to the size of the female reproductive tract: in large animals (passerines), sperm numbers are advantageous in sperm competition owing to dilution inside the female tract, whereas in small animals (drosophilids), large sperm are advantageous for physical competition (sperm displacement). Our study provides two important results. First, we provide convincing evidence for the existence of a sperm size–number tradeoff. Second, we show that by considering both sperm competition mechanism and dilution, can we account for variation in sperm size between different taxa.

Many species differ genetically, physiologically, and morphologically between geographically distinct populations, typically in response to variation in ecological and climatic variables. Little is known, however, about geographical variation in sperm morphology. Sperm morphology is under strong sexual selection, has been shown to evolve rapidly, and often co-varies with other reproductive traits (e.g., testis size or mating system) that differ between populations in some species. The aim of this study was to establish whether sperm morphology varies between populations of the red-winged blackbird (Agelaius phoeniceus), a species with an enormous breeding range and marked inter-population variation in both body size and mating system. We found (1) highly significant variation in sperm morphology among study sites, (2) a gradual increase in sperm length from the southwest to the northeast of the breeding range, and (3) a strong negative association between sperm length and body size. However, the relationship with the mating system remains unclear. Several hypotheses to explain these patterns are proposed.