Trade-offs, condition dependence and stopover site selection by migrating sandpipers

Western sandpipers Calidris mauri on southward migration fly over the Gulf of Alaska to the Strait of Georgia, British Columbia, where they stop for a few days to replenish reserves before continuing. In the Strait, individuals captured on the extensive tidal mudflats of the Fraser estuary (∼25000 ha) are significantly heavier (2.71 g, or >10% of lean body mass) than those captured on the small (<100 ha) mudflat of nearby Sidney Island. Previous work has shown that the difference cannot be attributed to seasonal timing, size, age or gender effects, and here we compare predictions made by six hypotheses about a diverse set of data to explain why, partway through a migratory journey of ∼10000 km, birds have such different body masses at two stopover sites within 40 km of each other. The ‘trade-off’ hypothesis – that the large Fraser estuary offers safety from predators, but a lower fattening rate, while the small Sidney Island site is more dangerous, but offers a higher fattening rate – made six successful predictions, all of which were upheld by the data. All other hypotheses failed at least one prediction. We infer that calidrid sandpipers arriving in the Strait of Georgia with little fat remaining (and therefore low body mass) choose to take advantage of the high feeding rate at small sites like Sidney Island because they are less vulnerable to avian predators than are individuals with higher fat reserves, who instead elect to feed at large open sites like the Fraser estuary mudflats

Widespread Horizontal Genomic Exchange does not Erode Species Barriers among Sympatric Ducks

Background: The study of speciation and maintenance of species barriers is at the core of evolutionary biology.During speciation the genome of one population becomes separated from other populations of the same species,which may lead to genomic incompatibility with time. This separation is complete when no fertile offspring isproduced from inter-population matings, which is the basis of the biological species concept. Birds, in particularducks, are recognised as a challenging and illustrative group of higher vertebrates for speciation studies. There aremany sympatric and ecologically similar duck species, among which fertile hybrids occur relatively frequently innature, yet these species remain distinct.Results: We show that the degree of shared single nucleotide polymorphisms (SNPs) between five species ofdabbling ducks (genus Anas) is an order of magnitude higher than that previously reported between any pair ofeukaryotic species with comparable evolutionary distances. We demonstrate that hybridisation has led to sustainedexchange of genetic material between duck species on an evolutionary time scale without disintegrating speciesboundaries. Even though behavioural, genetic and ecological factors uphold species boundaries in ducks, wedetect opposing forces allowing for viable interspecific hybrids, with long-term evolutionary implications. Based onthe superspecies concept we here introduce the novel term “supra-population” to explain the persistence of SNPsidentical by descent within the studied ducks despite their history as distinct species dating back millions of years.Conclusions: By reviewing evidence from speciation theory, palaeogeography and palaeontology we propose afundamentally new model of speciation to accommodate our genetic findings in dabbling ducks. This model, weargue, may also shed light on longstanding unresolved general speciation and hybridisation patterns in higherorganisms, e.g. in other bird groups with unusually high hybridisation rates. Observed parallels to horizontal genetransfer in bacteria facilitate the understanding of why ducks have been such an evolutionarily successful group ofanimals. There is large evolutionary potential in the ability to exchange genes among species and the resultingdramatic increase of effective population size to counter selective constraints

Evolution and Connectivity in the World-Wide Migration System of the Mallard: Inferences from Mitochondrial DNA

Background: Main waterfowl migration systems are well understood through ringing activities. However, inmallards (Anas platyrhynchos) ringing studies suggest deviations from general migratory trends and traditions inwaterfowl. Furthermore, surprisingly little is known about the population genetic structure of mallards, andstudying it may yield insight into the spread of diseases such as Avian Influenza, and in management andconservation of wetlands. The study of evolution of genetic diversity and subsequent partitioning thereof duringthe last glaciation adds to ongoing discussions on the general evolution of waterfowl populations and flywayevolution. Hypothesised mallard flyways are tested explicitly by analysing mitochondrial mallard DNA from thewhole northern hemisphere.Results: Phylogenetic analyses confirm two mitochondrial mallard clades. Genetic differentiation within Eurasia andNorth-America is low, on a continental scale, but large differences occur between these two land masses (FST =0.51). Half the genetic variance lies within sampling locations, and a negligible portion between currentlyrecognised waterfowl flyways, within Eurasia and North-America. Analysis of molecular variance (AMOVA) atcontinent scale, incorporating sampling localities as smallest units, also shows the absence of population structureon the flyway level. Finally, demographic modelling by coalescence simulation proposes a split between Eurasiaand North-America 43,000 to 74,000 years ago and strong population growth (~100fold) since then and littlemigration (not statistically different from zero).Conclusions: Based on this first complete assessment of the mallard’s world-wide population genetic structure weconfirm that no more than two mtDNA clades exist. Clade A is characteristic for Eurasia, and clade B for North-America although some representatives of clade A are also found in North-America. We explain this pattern byevaluating competing hypotheses and conclude that a complex mix of historical, recent and anthropogenic factorsshaped the current mallard populations. We refute population classification based on flyways proposed byornithologists and managers, because they seem to have little biological meaning. Our results have implications forwetland management and conservation, with special regard to the release of farmed mallards for hunting, as wellas for the possible transmission of Avian Influenza by mallards due to migration

Foraging opportunity: a method of monitoring shorebird migration and overwintering sites in a changing environment

Roberts Bank within the Fraser River estuary, BC contains important migratory stopover and overwintering habitat for shorebirds such as the western sandpiper (Calidris mauri) and the Pacific dunlin (Calidris alpina pacifica). Shorebirds are especially abundant during northward migration, with single-day counts numbering into the hundreds of thousands of birds. Previous research and ecological theory have demonstrated that site usage by shorebirds is influenced by numerous factors, including prey availability and predation risk. We developed a concept termed “foraging opportunity” that quantifies shorebird food availability (biofilm, meiofauna, and macrofauna) in relation to predation danger from hunting falcons. Foraging opportunity was determined across Roberts Bank during northward migration and evaluated against shorebird usage for the same period. Model results agreed with prior research and foraging theories, demonstrating good alignment between prey resources and shorebird usage in safer foraging areas and a shift in usage into areas with less prey, but safer foraging conditions, when high prey abundances were located in more dangerous conditions close to shore. As shorebird migration and overwintering sites are potentially affected by a changing environment, including climate change and anthropogenic effects, we suggest foraging opportunity techniques as a method of understanding and monitoring site quality and shorebird distribution patterns in a changing world

Apex predator behaviour in a changing Salish Sea: determining the role bald eagle foraging behaviour plays in nutrient cycling and terrestrial food webs under diminishing salmon populations

The semelparous salmonid species are the ecological foundation of Pacific Northwest coastal ecosystems, which span across the international border to encompass the Salish Sea. The annual return of salmonids to their natal streams and decomposition of their carcasses deposits marine-derived nutrients into freshwater ecosystems, which are integral to food web dynamics, nutrient cycling and habitat quality. Bald eagles (Haliaeetus leucocephalus) are apex predators that congregate along riverbanks to scavenge accumulated carcasses, which results in individuals using piracy to steal food items from others. Winners fly to nearby trees to consume carcasses, which are dropped to the forest floor and decomposed by terrestrial invertebrates. While this transfer of nutrients is critical to terrestrial habitat quality, the current decline in salmon populations in the Salish Sea could alter this cycle. Thus, the aim of our study is to determine if a decrease in carcass abundance will alter the piracy rates among eagles and the amount of marine-derived nutrients that are transferred into forests. To accomplish this, field observations on piracy rates will be compared to a simulated crash in salmon populations in an Individual-Based Model to determine how eagles alter their behaviour under decreased carcass abundance. The results will be compared to an isotope analysis of terrestrial invertebrates from perch tree locations. Predicted results include: a) the piracy rate in the IBM will be higher when fewer carcasses are available, b) less nutrients will be transferred to terrestrial ecosystems under low carcass abundance and high competition and c) the N and C isotopic signatures in terrestrial invertebrates under perch trees will be higher than controls. The results of this study will be used in collaboration with other fields to take an ecosystem-based approach to ensure coastal food web dynamics, nutrient cycling and habitat quality are priorities in our transboundary salmon management strategies

Winter Body Mass and Over-Ocean Flocking as Components of Danger Management by Pacific Dunlins

Background: We compared records of the body mass and roosting behavior of Pacific dunlins (Calidris alpinapacifica) wintering on the Fraser River estuary in southwest British Columbia between the 1970s and the 1990s.‘Over-ocean flocking’ is a relatively safe but energetically-expensive alternative to roosting during the high tideperiod. Fat stores offer protection against starvation, but are a liability in escape performance, and increase flightcosts. Peregrine falcons (Falco peregrinus) were scarce on the Fraser River estuary in the 1970s, but their numbershave since recovered, and they prey heavily on dunlins. The increase has altered the balance between predationand starvation risks for dunlins, and thus how dunlins regulate roosting behavior and body mass to manage thedanger. We therefore predicted an increase in the frequency of over-ocean flocking as well as a decrease in theamount of fat carried by dunlins over these decades.Results: Historical observations indicate that over-ocean flocking of dunlins was rare prior to the mid-1990s andbecame common thereafter. Residual body masses of dunlins were higher in the 1970s, with the greatestdifference between the decades coinciding with peak peregrine abundance in October, and shrinking over thecourse of winter as falcon seasonal abundance declines. Whole-body fat content of dunlins was lower in the 1990s,and accounted for most of the change in body mass.Conclusions: Pacific dunlins appear to manage danger in a complex manner that involves adjustments both in fatreserves and roosting behavior. We discuss reasons why over-ocean flocking has apparently become morecommon on the Fraser estuary than at other dunlin wintering sites

INVITATION PAPER (C.P. ALEXANDER FUND): FORAGING OF INDIVIDUAL WORKERS IN RELATION TO COLONY STATE IN THE SOCIAL HYMENOPTERA

Workers of social insects are members of colonies that survive and reproduce together. Therefore, the behavioral activities of individual workers should be integrated with colony state. We here summarize and discuss the relationship between colony state and foraging behavior of individual workers under the provisional assumption that the colony is a unit. We argue that colony state can be described by a number of variables that should relate to fitness components in order to be meaningful. Among the possible candidates, colony population size seems to have an overriding importance in many respects, as shown by its relation to fitness components such as survival probability and reproductive performance. Other important variables include colony demography, i.e. caste or size distributions, nutritional status, or queen number. Each of these variables has been shown to affect fitness components; however, the evidence is rather scanty. We also discuss the evidence that variation in colony state variables relates to variation in individual worker behavior. Nutritional status (i.e. low or high levels of food stores) and colony size have been shown repeatedly to affect individual behavior. However, most of the evidence comes from the honey bee. Some studies suggest that behavioral responses are hierarchically structured. More work needs to be done to investigate the actual mechanisms of integration of individual behavior with colony state. Some knowledge has accumulated about the processes that govern recruitment to food sources. We conclude this review by discussing some concepts and problems for further research. These include the concept of a preferred colony state to which the colony should return after disturbance through the behavioral activities of the workers. Further theoretical elaboration and empirical investigations may help to elucidate whether this concept is useful and necessary. A largely neglected issue concerns the number versus effort problem, i.e. whether individuals should work harder or more workers should be allocated to a task that is in demand. We propose a simple scenario that suggests testable predictions. Finally, we discuss how colony state, individual work load, and the dependence of worker mortality rate on activity level may interact to generate different short-term foraging strategies that workers should adop