Selection and evolutionary potential of spring arrival phenology in males and females of a migratory songbird

The timing of annual life‐history events affects survival and reproduction of all organisms. A changing environment can perturb phenological adaptations and an important question is if populations can evolve fast enough to track the environmental changes. Yet, little is known about selection and evolutionary potential of traits determining the timing of crucial annual events. Migratory species, which travel between different climatic regions, are particularly affected by global environmental changes. To increase our understanding of evolutionary potential and selection of timing traits, we investigated the quantitative genetics of arrival date at the breeding ground using a multigenerational pedigree of a natural great reed warbler (Acrocephalus arundinaceus) population. We found significant heritability of 16.4% for arrival date and directional selection for earlier arrival in both sexes acting through reproductive success, but not through lifespan. Mean arrival date advanced with 6 days over 20 years, which is in exact accordance with our predicted evolutionary response based on the breeder's equation. However, this phenotypic change is unlikely to be caused by microevolution, because selection seems mainly to act on the nongenetic component of the trait. Furthermore, demographical changes could also not account for the advancing arrival date. Instead, a strong correlation between spring temperatures and population mean arrival date suggests that phenotypic plasticity best explains the advancement of arrival date in our study population. Our study dissects the evolutionary and environmental forces that shape timing traits and thereby increases knowledge of how populations cope with rapidly changing environments.     

Repertoire size, sexual selection, and offspring viability in the great reed warbler: changing patterns in space and time

Only a few studies have focussed on the consistency of sexualselection patterns in space and time. One such case is the greatreed warbler (Acrocephalus arundinaceus), for which studiesin Germany in 1981–1982 and Sweden in 1987–1991strongly suggested that the size of a male's song repertoirewas the target of mate choice and sexual selection. Studyingthe same German population once again in 1994–2000, weinvestigated the consistency of these patterns over time aswell as between populations. Our reanalysis of the data from1981–1982 shows that male repertoire size was positivelycorrelated with male pairing success (harem size) and with clutchsize (adjusted for seasonal effects), whereas no such correlationswere found during 1994–2000 in the same population. Wesuggest that the earlier correlations were probably caused indirectlyby covariation with territory quality, and that a decline inpopulation size has changed the role of territory quality. Inthe Swedish population, an earlier study found a striking correlationbetween the size of a male's repertoire and the viability ofits offspring, suggesting good-gene effects. In contrast, wefound no such correlation in the German population, neitherin 1981–1982 nor in 1994–2000. We conclude thatrepertoire size does not seem to be a very reliable indicatorof variation in male quality. Interestingly, the analysis ofdata from 1994–2000 showed that male pairing success wasstrongly correlated with measures of strophe length and immediateversatility, traits that have been found to reflect male longevity.Future studies will have to show whether these performance-relatedtraits are more powerful indicators of male quality than isrepertoire size.     

A matter of timing: changes in the first date of arrival and last date of departure of Australian migratory birds

Although there is substantial evidence that Northern Hemisphere species have responded to climatic change over the last few decades, there is little documented evidence that Southern Hemisphere species have responded in the same way. Here, we report that Australian migratory birds have undergone changes in the first arrival date (FAD) and last date of departure (LDD) of a similar magnitude as species from the Northern Hemisphere. We compiled data on arrival and departure of migratory birds in south‐east Australia since 1960 from the published literature, Bird Observer Reports, and personal observations from bird watchers. Data on the FAD for 24 species and the LDD for 12 species were analyzed. Sixteen species were short‐ to middle‐distance species arriving at their breeding grounds, seven were long‐distance migrants arriving at their nonbreeding grounds, and one was a middle‐distance migrant also arriving at its nonbreeding ground. For 12 species, we gathered data from more than one location, enabling us to assess the consistency of intraspecific trends at different locations. Regressions of climate variables against year show that across south‐east Australia average annual maximum and minimum temperatures have increased by 0.17°C and 0.13°C decade^?1 since 1960, respectively. Over this period there has been an average advance in arrival of 3.5 days decade^?1; 16 of the 45 time‐series (representing 12 of the 24 species studied) showed a significant trend toward earlier arrival, while only one time‐series showed a significant delay. Conversely, there has been an average delay in departure of 5.1 days decade^?1; four of the 21 departure time‐series (four species) showed a significant trend toward later departure, while one species showed a significant trend toward earlier departure. However, differences emerge between the arrival and departure of short‐ to middle‐distance species visiting south‐east Australia to breed compared with long‐distance species that spend their nonbreeding period here. On average, short‐ to middle‐distance migrants have arrived at their breeding grounds 3.1 days decade^?1 earlier and delayed departure by 8.1 days decade^?1, thus extending the time spent in their breeding grounds by ～11 days decade^?1. The average advance in arrival at the nonbreeding grounds of long‐distance migrants is 6.8 days decade^?1. These species, however, have also advanced departure by an average of 6.9 days decade^?1. Hence, the length of stay has not changed but rather, the timing of events has advanced. The patterns of change in FAD and LDD of Australian migratory birds are of a similar magnitude to changes undergone by Northern Hemisphere species, and add further evidence that the modest warming experienced over the past few decades has already had significant biological impacts on a global scale.     

Temperature during egg formation and the effect of climate warming on egg size in a small songbird

The predicted effects of recent climate warming on egg size in birds are controversial, as only two long-term studies have been reported, with contrasting results. Long-term data on egg size variation are analyzed in relation to ambient temperatures in a southern European population of pied flycatchers where breeding phenology has not matched the spring advancement in the last decades. Cross-sectional, population analyses indicated that egg breadth, but not egg length, has decreased significantly along the 16-year period, leading to marginally non-significant decreases in egg volume. Longitudinal, individual analyses revealed that despite females consistently laying larger eggs when they experienced warmer temperatures during the prelaying and laying periods, there was an overall negative response – i.e. decreasing egg volume and breadth with increasing spring (May) average temperatures – across individuals. This trend is hypothesised to be caused by the mismatched breeding phenology, in relation to climate warming, of this population. Except in the unlikely cases of populations capable of perfectly synchronising their phenology to changes in their environment, maladjustments are likely for traits such as egg size, which depend strongly on female condition. Slight changes or absence thereof in breeding dates may be followed by mismatched dates, in terms of food abundance, for optimal egg formation, which would be reflected in smaller average egg size, contrary to early predictions on the effects of climate warming on bird egg size.     

Mate selection in Litoria chloris and Litoria xanthomera: Females prefer smaller males

It is generally accepted that high quality males are those that succeed in male–male competition: in either aggression or rivalry to attract and be selected by females. Previous studies of amphibians have suggested that the main characters influencing male mating success include variation in call characteristics (e.g. call rate, call intensity), calling behaviour, body condition, age and chorus tenure. In the present paper, several of the characters influencing female mate choice (male body size, body condition, call rate, call frequency and chorus tenure) are investigated in two closely related, explosive breeding frog species Litoria chloris and Litoria xanthomera. Smaller males of both species are shown to be more successful than larger males and this success is attributed to the increased chorus tenure of smaller males in L. xanthomera. This increased chorus tenure was attributed to the lower total energy used per call by a small male calling at a higher frequency. Whether increased chorus tenure explains female mate choice in L. chloris is uncertain but is highly probable given the strong similarity between the two species in both ecology and call characteristics.     

The potential for selection on pollen colour dimorphisms in Nigella degenii: morph-specific differences in pollinator visitation, fertilisation success and siring ability

Two subspecies of Nigella degenii (Ranunculaceae) possess a dimorphism in pollen colour and vary extensively in frequency of the two morphs in natural populations. Here we investigate the role of selection on pollen colour during the pollination phase in the two subspecies and its potential contribution to the maintenance of this colour variation. In a combination of common garden experiments and field observations, we obtained data on pollinator visitation rates and explored the effect of pollen colour on fertilisation success and siring ability under conditions of low vs. high pollen competition. In experimental gardens, naïve pollinators responded differently to plants with different pollen colour, but the favoured morph varied between dates and locations, and colour morphs were not visited in a frequency-dependent manner. Donor plants with dark pollen had a reproductive advantage (higher seed set) in single-donor pollinations, but the realised siring ability (measured by progeny morph ratio) was highly variable between different two-donor crosses with no general bias towards the light or dark morph. Therefore, although the dark pollen type appears to have a general selective advantage in terms of fertilisation success, our data are also consistent with a scenario involving the maintenance of both colour morphs, particularly under conditions of high pollen competition, a variable genetic background and/or spatial or temporal variation in the pollinator fauna.     

Testing the interactive effects of carotenoids and polyunsaturated fatty acids on ejaculate traits in the guppy Poecilia reticulata (Pisces: Poeciliidae)

Using the polyandrous livebearing guppy Poecilia reticulata, this study revealed no main effects of carotenoids in the diet on ejaculate traits, but significant main effects of polyunsaturated fatty acids (PUFAs) on sperm viability and weak but significant interacting effects of both nutrients on sperm length. Collectively, these findings not only add evidence that PUFAs are critical determinants of sperm quality, but also provide tentative evidence that for some traits these effects may be moderated by carotenoid intake.     

Contrasting sexual selection on males and females in a role-reversed swarming dance fly, Rhamphomyia longicauda Loew (Diptera: Empididae)

Although there are several hypotheses for sex-specific ornamentation, few studies have measured selection in both sexes. We compare sexual selection in male and female dance flies, Rhamphomyia longicauda (Diptera: Empididae). Swarming females display size-enhancing abdominal sacs, enlarged wings and decorated tibiae, and compete for nuptial gifts provided by males. Males preferentially approach large females, but the nature of selection and whether it is sex-specific are unknown. We found contrasting sexual selection for mating success on structures shared by males and females. In females, long wings and short tibiae were favoured, whereas males with short wings and long tibiae had a mating advantage. There was no assortative mating. Females occupying potentially advantageous swarm positions were large and, in contrast to selection for mating success, tended to have larger tibiae than those of rivals. We discuss our findings in the context of both the mating biology of dance flies, and the evolution of sexual dimorphism in general.     

Rainfall and flowering synchrony in a tropical shrub: Variable selection on the flowering time ofErythroxylum havanense

Summary We tested the adaptive significance of flowering synchrony by means of a quantitative analysis of selection and by flowering induction experiments with the deciduous shrubErythroxylum havanense. Temporal schedules of flower and fruit production were determined for a local population (in three sites) in a Mexican seasonal forest for 2 years (1987–1988). The consequences of natural variation in flowering time (flowering initiation day) on maternal reproductive success (fecundity) were evaluated. We observed high levels of inter- and intraindividual flowering synchrony in 1987, but not in 1988 and this contrast was related to differences in rainfall patterns between the two years. A significant proportion (15.4%) of the phenotypic variation in flowering initiation day was accounted for by environmental variance. The expression of phenotypic variance of flowering time and, consequently, the opportunity for selection to act, are controlled by annual variation in rainfall. Despite the between-year difference in flowering synchrony, we detected a relatively intense directional selection on flowering initiation day in both years, but selection coefficients were of opposite sign (standardized directional gradients were –0.326 and 0.333 for 1987 and 1988, respectively). For both years there was a significant relationship between individual relative fitness and the number of neighbouring flowering plants in a given day, suggesting positive frequency-dependent selection.     

Natural selection on body size is mediated by multiple interacting factors: a comparison of beetle populations varying naturally and experimentally in body size

Body size varies considerably among species and among populations within species, exhibiting many repeatable patterns. However, which sources of selection generate geographic patterns, and which components of fitness mediate evolution of body size, are not well understood. For many animals, resource quality and intraspecific competition may mediate selection on body size producing large-scale geographic patterns. In two sequential experiments, we examine how variation in larval competition and resource quality (seed size) affects the fitness consequences of variation in body size in a scramble-competing seed-feeding beetle, Stator limbatus. Specifically, we compared fitness components among three natural populations of S. limbatus that vary in body size, and then among three lineages of beetles derived from a single base population artificially selected to vary in size, all reared on three sizes of seeds at variable larval density. The effects of larval competition and seed size on larval survival and development time were similar for larger versus smaller beetles. However, larger-bodied beetles suffered a greater reduction in adult body mass with decreasing seed size and increasing larval density; the relative advantage of being large decreased with decreasing seed size and increasing larval density. There were highly significant interactions between the effects of seed size and larval density on body size, and a significant three-way interaction (population-by-density-by-seed size), indicating that environmental effects on the fitness consequences of being large are nonadditive. Our study demonstrates how multiple ecological variables (resource availability and resource competition) interact to affect organismal fitness components, and that such interactions can mediate natural selection on body size. Studying individual factors influencing selection on body size may lead to misleading results given the potential for nonlinear interactions among selective agents.