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

Timing games in the reproductive phenology of female Pacific salmon (Oncorhynchus spp.)

We use a game-theoretic framework to investigate the reproductive phenology of female kokanee (Oncorhynchus nerka). As in the other semelparous species of Pacific salmon, females construct nests in gravel, spawn with males, bury their fertilized eggs, and defend their nest sites until they die several days later. Later-breeding females may reuse previous nest sites, and their digging behavior is thought to subject previously buried eggs to mortality. Using game-theoretic models, we show that females can reduce this risk by allocating resources to longevity (the period between arrival and death) as opposed to eggs. Waiting before territory settlement is also expected if it allows females to conserve energy and delay senescence. The models demonstrate how these costs and benefits interact to select for a seasonal decline in longevity, a well-known phenomenon in the salmonid literature, and a seasonal decline in wait duration. Both of these predictions were supported in a field study of kokanee. Female state of reproductive maturity was the most important proximate factor causing variation in longevity and wait duration. With more than 30% of territories being reused, dig-up is likely an important selective force in this population

Death and danger at migratory stopovers: Problems with "predation risk"

Dierschke (2003) recently published a paper entitled, ``Predation hazard during migratory stopover: are light or heavy birds under risk?¿¿ He measured the body condition of 11 species of passerine migrants depredated by feral cats and raptors at an offshore stopover site, and used these data to address two hypotheses: (1) predation risk is higher for heavy birds because it decreases the take-off ability, and (2) predation risk is higher for lean birds because their need to feed increases their exposure to predators. Dierschke found that lighter weight individuals were strongly disproportionately represented among the victims, and concluded that ``it seems that the importance of reduced escape performance caused by the carrying of fuel loads is overestimated and possibly not biologically significant in terms of predation risk¿¿. We found Dierschke¿s data and analysis interesting and exciting, and believe that fundamentally, we interpret his results much as he does: lighter birds are killed disproportionately because they spend more time feeding and/or do so in ``riskier¿¿ habitats. But we disagree with the inference that massspecific ``predation risk¿¿ is therefore of little ecological importance or biological significance. In this commentary we consider the reasoning behind his conclusions, focusing particularly on the meaning of ``predation risk¿¿

Effects of predation danger on migration strategies of sandpipers

We examine the potential selective importance of predation danger on the evolution of migration strategies of arctic-breeding calidrid sandpipers. Adult calidrids truncate parental care for reasons not obviously related to levels of food abundance on the breeding areas or at migratory stopover sites, suggesting that a different trade-off occurs between providing additional care and adult survivorship. The southward migrations of adult western sandpipers precede those of migratory peregrine falcons by almost a month. By moving early and quickly, adults remain ahead of migrant falcons all the way to their non-breeding areas, where they rapidly moult flight feathers. They complete the moult just as falcons arrive in late September¿October. By migrating early, they avoid exposure to falcons when they are unusually vulnerable, due to the requirements for fuelling migratory flight and of wing feather moult. Juvenile western sandpipers migrate south just as falcon numbers start to increase, but do not moult flight feathers in their first winter. Pacific dunlin use an alternative strategy of remaining and moulting in Alaska after falcons depart, and migrating to their overwintering sites after migrants have passed. East of the Rocky Mountains, the southbound migration of falcons begins 4¿6 weeks later. Southbound semipalmated sandpipers make extended migratory stopovers, but their lengths of stay shorten prior to falcon migration to the sites in September. Predation danger also may affect the evolution of migration routes. Southbound western sandpipers fly directly from Alaska to southern British Columbia, in contrast to the multi-stage journey northward along the Alaska panhandle. We estimate that a direct flight would be more economical on northward migration, but may be avoided because it would expose sandpipers to higher mass-dependent predation danger from migratory falcons, which travel north with sandpipers. By contrast, few raptors are present in Alaska during preparation for the southward flight. A temporal and spatial window of safety may also permit semipalmated sandpipers to become extremely vulnerable while preparing for trans-Atlantic southward flights. Danger management may account for the these previously enigmatic features of calidrid migration strategies, and aspects of those of other birds

Ashmole's Halo as the outcome of a predator-prey game

Many seabird species breed in large colonies, from which they make repeated excursions to collect prey for delivery to offspring. This necessarily confines their foraging activities to a region around or near the colony, which is constrained by their powers of mobility. The cumulative impact in this region is hypothesized to be large enough to reduce the density of prey, a phenomenon referred to as “Ashmole’s halo.” Considering its important role in thought about seabird life histories, Ashmole’s halo has received remarkably little formal investigation. Here we model the formation of Ashmole’s halo. In a previous theoretical study, Gaston et al. (2007) derived the size and shape of the halo, using a set of simple assumptions. In that model, prey density was reduced by exploitation alone: prey individuals had no explicit behavioral options with which to evade or elude predators, such as moving away from a zone of high depredation. The objective of the model developed here is to assess the role of prey behavior in the occurrence of Ashmole’s halo. In our model, both seabirds and their prey make strategic choices about foraging location: seabirds do so to maximize the delivery of prey to offspring, while fish do so to maximize fitness (growth and survivorship). The situation is a predator-prey game, because for both seabirds and fish the best choice of foraging location depends on the choices made by all other players. Our simulations show that the halo develops much more quickly and is deeper and stronger when prey individuals are able to adjust their location adaptively (i.e. to maximize fitness), than when a random (i.e. diffusion) process governs prey movement as prey density is altered by exploitation. These results broaden the conditions under which Ashmole’s halo could be pronounced enough to affect seabird biology

Do Arctic breeding geese track or overtake a green wave during spring migration?

Geese breeding in the Arctic have to do so in a short time-window while having sufficient body reserves. Hence, arrival time and body condition upon arrival largely influence breeding success. The green wave hypothesis posits that geese track a successively delayed spring flush of plant development on the way to their breeding sites. The green wave has been interpreted as representing either the onset of spring or the peak in nutrient biomass. However, geese tend to adopt a partial capital breeding strategy and might overtake the green wave to accomplish a timely arrival on the breeding site. To test the green wave hypothesis, we link the satellite-derived onset of spring and peak in nutrient biomass with the stopover schedule of individual Barnacle Geese. We find that geese track neither the onset of spring nor the peak in nutrient biomass. Rather, they arrive at the southernmost stopover site around the peak in nutrient biomass, and gradually overtake the green wave to match their arrival at the breeding site with the local onset of spring, thereby ensuring gosling benefit from the peak in nutrient biomass. Our approach for estimating plant development stages is critical in testing the migration strategies of migratory herbivores