Peter O. Dunn

Helpers liberate female fairy-wrens from constraints on extra-pair mate choice

In cooperatively breeding superb fairy-wrens (Malurus cyaneus), all males contribute to the feeding and defence of young. Despite the expectation that such paternal care should be directed only to relatives, DNA fingerprinting revealed that most offspring (76%, 138 out of 181) were sired by extra-group males that contributed no care, and that almost all broods (95%, 38 out of 40) contained young sired by extra-group fathers. This is the highest known incidence of cuckoldry. This remarkable mating system is produced by female control of fertilization and consistent preference for certain extra-group male genotypes. This choice leads to the production of sons that also gain extra-group fertilizations. One constraint on the extra-pair mate choice of females is the level of parental assistance received from males. Males living in pairs contribute relatively more parental care and are more likely to gain paternity in one of their broods (85%, 11 out of 13) than dominant males in multi-male cooperative groups (30%, 8 out of 27). In groups helpers compensate for the lower parental assistance of dominant males, so the total feeding rate is similar between pairs and groups. This suggests that females allow males in pairs more paternity to ensure their assistance with parental care. Helpers provide an alternative source of paternal investment, and allow females to express unrestricted mate choice. Mating options for females in other species with female control of fertilization may also reflect a trade-off between acquiring the genes of high-quality males for their offspring and parental care of those offspring.

Climate change has affected the breeding date of tree swallows throughout North America

Increasing evidence suggests that climate change has affected the breeding and distribution of wildlife. If such changes are due to global warming, then we should expect to see large‐scale effects. To explore for such effects on avian reproduction, we examined 3450 nest records of tree swallows from across North America. The egg‐laying date in tree swallows advanced by up to nine days during 1959 to 1991. This advance in phenology was associated with increasing surface air temperatures at the time of breeding. Our analysis controlled for several potentially confounding variables such as latitude, longitude, breeding density and elevation.We conclude that tree swallows across North America are breeding earlier and that the most likely cause is a long‐term increase in spring temperature

Breeding Dates and Reproductive Performance

Publisher Summary This chapter examines how temperature and climate change affect reproductive performance in different species and locations. There are several ways in which the correlation between laying date and temperature may arise. First, there may be a direct effect of temperature on the energetic demands of females that influences their timing of laying. Second, temperature may influence the growth of gonads, which could indirectly affect the timing of laying. Precipitation could have a similar effect, as the growth of spotted antbird testes and follicles in Panama is slower in a dry year. In any case, there are likely several direct and indirect relationships between food, temperature and gonadal development. Temperature may also influence food availability, particularly insects, which, in turn, may limit the ability of females to produce eggs. Warmer temperature is expected to lead to greater production of young, because laying earlier is often associated with larger clutch size and more young fledged. However, recent evidence from great tits (Parus major) suggests that warmer spring temperature can lead to a mismatch in the timing of egg-laying relative to the availability of food for nestlings, and, as a consequence, earlier laying females may produce fewer surviving young birds. Thus, it is important to examine the effects of climate change on all aspects of reproduction, not just laying date. In particular, changes in clutch size are likely to have a major impact on fitness, as it places an upper limit on total reproductive success for a given brood.

Predicting the effects of climate change on avian life-history traits

Across North America, tree swallows have advanced their mean date of clutch initiation (lay date) by ≈9 days over the past 30 years, apparently in response to climate change. In a sample of 2,881 nest records collected by the lay public from 1959 to 1991, we examined whether clutch size has also responded to climate change. We found that clutch size is strongly related to lay date, both within and among years, and there has been no significant temporal variation in the slopes or intercepts of the clutch-size/lay-date regressions. As a consequence, we expected increases in clutch size with advancement in lay date; however, we detected no such trend over time. The distributions of egg-laying dates were more constricted in the warmest (and earliest) years, suggesting that changes in mean clutch size might be constrained by changes in the distribution of laying dates. If spring temperatures continue to increase, we predict further reductions of variance in laying dates and relatively small increases in clutch size. Such constraints on life-history variation probably are common and need to be considered when modeling the effects of climate change on reproduction in natural populations. Predicting the long-term effects of constraints and interpreting changes in life-history traits require a better understanding of both adaptive and demographic effects of climate change.

Colour polymorphism in birds: causes and functions.

We studied polymorphism in all species of birds that are presently known to show intraspecific variation in plumage colour. At least three main mechanisms have been put forward to explain the maintenance of polymorphism: apostatic, disruptive and sexual selection. All of them make partly different predictions. Our aims were to investigate evolutionary causes and adaptive functions of colour polymorphism by taking into account a number of ecological and morphological features of polymorphic species. Overall, we found 334 species showing colour polymorphism, which is 3.5% of all bird species. The occurrence of colour polymorphism was very high in Strigiformes, Ciconiiformes, Cuculiformes and Galliformes. Phylogenetically corrected analysis using independent contrasts revealed that colour polymorphism was maximally expressed in species showing a daily activity rhythm extended to day/night, living in both open and closed habitats. All these findings support the hypothesis that colour polymorphism probably evolved under selective pressures linked to bird detectability as affected by variable light conditions during activity period. Thus, we conclude that selective agents may be prey, predators and competitors, and that colour polymorphism in birds may be maintained by disruptive selection.

Offspring sex ratios in tree swallows: females in better condition produce more sons

Organisms are expected to adjust the sex ratio of their offspring in relation to the relative fitness benefits of sons and daughters. We used a molecular sexing technique that amplifies an intron of the CHD1 gene in birds to examine the sex ratio at egg‐laying in socially monogamous tree swallows (Tachycineta bicolor). We examined all individuals in 40 broods (210 young), including all unhatched eggs and nestlings. Thus, the sex ratio we measured was the same as the sex ratio at laying. Overall, the mean sex ratio per brood (± SD) was biased significantly towards males (57 ± 2% male). Within broods, male‐biased sex ratios were associated with females in better body condition, and these females were more likely to produce sons in better condition. Tree swallows have one of the highest known levels of extra‐pair paternity in birds (38–76% extra‐pair young), and, as a consequence, variance in male reproductive success is greater than that of females. Thus, in tree swallows, investment in sons has the potential for higher fitness returns than investment in daughters, assuming that sons in better condition have greater reproductive success.

Sperm competition and the evolution of testes size in birds.

Comparative analyses suggest that a variety of ecological and behavioural factors contribute to the tremendous variability in extrapair mating among birds. In an analysis of 1010 species of birds, we examined several ecological and behavioural factors in relation to testes size; an index of sperm competition and the extent of extrapair mating. In univariate and multivariate analyses, testes size was significantly larger in species that breed colonially than in species that breed solitarily, suggesting that higher breeding density is associated with greater sperm competition. After controlling for phylogenetic effects and other ecological variables, testes size was also larger in taxa that did not participate in feeding their offspring. In analyses of both the raw species data and phylogenetically independent contrasts, monogamous taxa had smaller testes than taxa with multiple social mates, and testes size tended to increase with clutch size, which suggests that sperm depletion may play a role in the evolution of testes size. Our results suggest that traditional ecological and behavioural variables, such as social mating system, breeding density and male parental care can account for a significant portion of the variation in sperm competition in birds.

Fairy-Wren Helpers often Care for Young to Which they are Unrelated

In contrast to most cooperatively breeding birds, helpers in groups of superb fairy-wrens (Malurus cyaneus) are often not related to the young they provision because frequently their m others either die or move, and because most fertilizations are gained by males which are outside the group that rear the young. All helpers provision broods, but this care does not enhance productivity when com pared to unassisted pairs. Helpers provide care regardless of the degree of relatedness to the young they provision. Therefore, it is unlikely that helping behaviour in fairy-wrens is maintained sim ply by indirect benefits gained through the production of non-descendant kin. Alternative explanations of care: ‘unselected’ hypothesis; enhanced future probability of breeding; and direct increases in reproductive success can also be rejected. Our data support the view that helping is a payment to breeders which allows helpers to stay in the group.

EXTRAPAIR MATE CHOICE AND HONEST SIGNALING IN COOPERATIVELY BREEDING SUPERB FAIRY-WRENS

In many species of monogamous birds females copulate with males other than their social mates, resulting in extrapair fertilizations. Little is known about how females choose extrapair mates and whether the traits used to choose them are reliable indicators of male quality. Here we identify a novel male trait associated with extra‐group mating success in the superb fairy‐wren (Malurus cyaneus), a cooperatively breeding bird with one of the highest known frequencies of extra‐group mating. Female fairy‐wrens chose extra‐group mates that molted earlier into breeding plumage. Males molted up to five months before the breeding season began, and only males that molted at least one month prior to its onset gained any extra‐group fertilizations. This conclusion held after controlling statistically for the effect of age and social status on molt date. Once males acquired breeding plumage, they began courtship display to females on other territories. Thus, some males were displaying to females for several months before the breeding season began. This extraordinarily long period of advertisement by males may be facilitated by the long‐term ownership of territories. We suggest that early acquisition of breeding plumage or the subsequent display behavior can be reliable cues for mate choice because they are costly to acquire or maintain.

Contrasting patterns of mitochondrial and microsatellite population structure in fragmented populations of greater prairie-chickens.

Greater prairie‐chickens (Tympanuchus cupido pinnatus) were once found throughout the tallgrass prairie of midwestern North America but over the last century these prairies have been lost or fragmented by human land use. As a consequence, many current populations of prairie‐chickens have become isolated and small. This fragmentation of populations is expected to lead to reductions in genetic variation as a result of random genetic drift and a decrease in gene flow. As expected, we found that genetic variation at both microsatellite DNA and mitochondrial DNA (mtDNA) markers was reduced in smaller populations, particularly in Wisconsin. There was relatively little range‐wide geographical structure (FST) when we examined mtDNA haplotypes but there was a significant positive relationship between genetic (FST) and geographical distance (isolation by distance). In contrast, microsatellite DNA loci revealed significant geographical structure (FST) and a weak effect of isolation by distance throughout the range. These patterns were much stronger when populations with reduced levels of genetic variability (Wisconsin) were removed from the analyses. This suggests that the effects of genetic drift were stronger than gene flow at microsatellite loci, whereas these forces were in range‐wide equilibrium at mtDNA markers. These differences between the two molecular markers may be explained by a larger effective population size (Ne) for mtDNA, which is expected in species such as prairie‐chickens that have female‐biased dispersal and high levels of polygyny. Our results suggest that historic populations of prairie‐chickens were once interconnected by gene flow but current populations are now isolated. Thus, maintaining gene flow may be important for the long‐term persistence of prairie‐chicken populations.