Energetic and behavioral consequences of migration: an empirical evaluation in the context of the full annual cycle

Seasonal migrations are used by diverse animal taxa, yet the costs and benefits of migrating have rarely been empirically examined. The aim of this study was to determine how migration influences two ecological currencies, energy expenditure and time allocated towards different behaviors, in a full annual cycle context. We compare these currencies among lesser black-backed gulls that range from short- (< 250 km) to long-distance (> 4500 km) migrants. Daily time-activity budgets were reconstructed from tri-axial acceleration and GPS, which, in conjunction with a bioenergetics model to estimate thermoregulatory costs, enabled us to estimate daily energy expenditure throughout the year. We found that migration strategy had no effect on annual energy expenditure, however, energy expenditure through time deviated more from the annual average as migration distance increased. Patterns in time-activity budgets were similar across strategies, suggesting migration strategy does not limit behavioral adjustments required for other annual cycle stages (breeding, molt, wintering). Variation among individuals using the same strategy was high, suggesting that daily behavioral decisions (e.g. foraging strategy) contribute more towards energy expenditure than an individual's migration strategy. These findings provide unprecedented new understanding regarding the relative importance of fine versus broad-scale behavioral strategies towards annual energy expenditures.

Going to sleep with a full belly: Thermal substitution by specific dynamic action in shorebirds

Many bird species occupy habitats where environmental temperatures fall well below their thermoneutral zone (TNZ), so they must deal with high energy costs of thermoregulation to keep in heat balance. In such circumstances, specific dynamic action (SDA) - also referred to as heat increment of feeding - could be used to substitute for these high thermoregulatory costs. If birds ingest food before going to roost in cold environments, the SDA will be beneficial as an energy-conserving mechanism by thermal substitution. We investigated the magnitude and duration of SDA in a small-sized shorebird, the dunlin Calidris alpina, while feeding on living prey. We simulated in the aviary the food availability of a semidiurnal tidal cycle, and calculated the thermal substitution by SDA below their TNZ at the beginning of the "high tide" (resting period), after feeding ad libitum during the "low tide" (feeding period). Within TNZ (25 °C), dunlins consumed 12% (2.15 kJ) of the gross energy intake in excess by the SDA, with a duration of ~95 min. At 10 °C, i.e. below the lower critical limit of TNZ, SDA magnitude and duration were reduced by 29% and 31%, respectively. The amount of food ingested significantly affected the duration and magnitude of SDA, as well as the dunlin's body temperature. Thermal substitution by SDA saved 11% of the dunlin's theoretical daily energy requirement during winter. This thermal substitution could be commonly used by birds going to roost in cold climates. Interacting with other different behavioral and/or physiological strategies would help to maintain lower energetic costs and enhance survival in cold environments.

Low fitness at low latitudes: Wintering in the tropics increases migratory delays and mortality rates in an Arctic breeding shorebird

Evolutionary theories of seasonal migration generally assume that the costs of longer migrations are balanced by benefits at the non‐breeding destinations.

We tested, and rejected, the null hypothesis of equal survival and timing of spring migration for High Arctic breeding sanderling Calidris alba using six and eight winter destinations between 55°N and 25°S, respectively.

Annual apparent survival was considerably lower for adult birds wintering in tropical West Africa (Mauritania: 0.74 and Ghana: 0.75) than in three European sites (0.84, 0.84 and 0.87) and in subtropical Namibia (0.85). Moreover, compared with adults, second calendar‐year sanderlings in the tropics, but not in Europe, often refrained from migrating north during the first possible breeding season. During northward migration, tropical‐wintering sanderlings occurred at their final staging site in Iceland 5–15 days later than birds wintering further north or south. Namibia‐wintering sanderlings tracked with solar geolocators only staged in West Africa during southward migration.

The low annual survival, the later age of first northward migration and the later passage through Iceland during northward migration of tropical‐wintering sanderlings, in addition to the skipping of this area during northward but not southward migration by Namibia‐wintering sanderlings, all suggest they face issues during the late non‐breeding season in West Africa.

Migrating sanderlings defy long distances but may end up in winter areas with poor fitness prospects. We suggest that ecological conditions in tropical West Africa make the fuelling prior to northward departure problematic.

Natural and sexual selection drive multivariate phenotypic divergence along climatic gradients in an invasive fish

Invasive species that rapidly spread throughout novel distribution ranges are prime models to investigate climate-driven phenotypic diversification on a contemporary scale. Previous studies on adaptive diversification along latitudinal gradients in fish have mainly considered body size and reported either increased or decreased body size towards higher latitudes (i.e. Bergmann's rule). Our study is the first to investigate phenotypic divergence in multiple traits, including sexually selected traits (size and shape of the male copulatory organ, the gonopodium) of invasive Gambusia affinis in China. We studied body size, life history traits and morphological variation across populations spanning 17 degrees of latitude and 16 degrees of longitude. Even though we found phenotypic variation along climatic gradients to be strongest in naturally selected traits, some sexually selected traits also showed systematic gradual divergence. For example, males from southern populations possessed wider gonopodia with increased armament. Generally, males and females diverged in response to different components of climatic gradients (latitudinal or longitudinal variation) and in different trait suites. We discuss that not only temperature regimes, but also indirect effects of increased resource and mate competition (as a function of different extrinsic overwinter mortality rates) alter the selective landscape along climatic gradients.

Prey type and foraging ecology of Sanderlings Calidris alba in different climate zones: are tropical areas more favourable than temperate sites?

Sanderlings (Calidris alba) are long-distance migratory shorebirds with a non-breeding range that spans temperate and tropical coastal habitats. Breeding in the High Arctic combined with non-breeding seasons in the tropics necessitate long migrations, which are energetically demanding. On an annual basis, the higher energy expenditures during migration might pay off if food availability in the tropics is higher than at temperate latitudes. We compared foraging behaviour of birds at a north temperate and a tropical non-breeding site in the Netherlands and Ghana, respectively. In both cases the birds used similar habitats (open beaches), and experienced similar periods of daylight, which enabled us to compare food abundance and availability, and behavioural time budgets and food intake. During the non-breeding season, Sanderlings in the Netherlands spent 79% of their day foraging; in Ghana birds spent only 38% of the daytime period foraging and the largest proportion of their time resting (58%). The main prey item in the Netherlands was the soft-bodied polychaete Scolelepis squamata, while Sanderlings in Ghana fed almost exclusively on the bivalve Donax pulchellus, which they swallowed whole and crushed internally. Average availability of polychaete worms in the Netherlands was 7.4 g ash free dry mass (AFDM) m⁻², which was one tenth of the 77.1 g AFDM m⁻² estimated for the beach in Ghana. In the tropical environment of Ghana the Sanderlings combined relatively low energy requirements with high prey intake rates (1.64 mg opposed to 0.13 mg AFDM s⁻¹ for Ghana and the Netherlands respectively). Although this may suggest that the Ghana beaches are the most favourable environment, processing the hard-shelled bivalve (D. pulchellus) which is the staple food could be costly. The large amount of daytime spent resting in Ghana may be indicative of the time needed to process the shell fragments, rather than indicate rest.

Sex-specific winter distribution in a sexually dimorphic shorebird is explained by resource partitioning

Sexual size dimorphism (SSD) implies correlated differences in energetic requirements and feeding opportunities, such that sexes will face different trade-offs in habitat selection. In seasonal migrants, this could result in a differential spatial distribution across the wintering range. To identify the ecological causes of sexual spatial segregation, we studied a sexually dimorphic shorebird, the bar-tailed godwit Limosa lapponica, in which females have a larger body and a longer bill than males. With respect to the trade-offs that these migratory shorebirds experience in their choice of wintering area, northern and colder wintering sites have the benefit of being closer to the Arctic breeding grounds. According to Bergmann's rule, the larger females should incur lower energetic costs per unit of body mass over males, helping them to winter in the cold. However, as the sexes have rather different bill lengths, differences in sex-specific wintering sites could also be due to the vertical distribution of their buried prey, that is, resource partitioning. Here, in a comparison between six main intertidal wintering areas across the entire winter range of the lapponica subspecies in northwest Europe, we show that the percentage of females between sites was not correlated with the cost of wintering, but was positively correlated with the biomass in the bottom layer and negatively with the biomass in the top layer. We conclude that resource partitioning, rather than relative expenditure advantages, best explains the differential spatial distribution of male and female bar-tailed godwits across northwest Europe.

Costs, benefits, and fitness consequences of different migratory strategies

The relative fitness of individuals across a population can shape distributions and drive population growth rates. Migratory species often winter over large geographic ranges, and individuals in different locations experience very different environmental conditions, including different migration costs, which can potentially create fitness inequalities. Here we used energetics models to quantify the trade-offs experienced by a migratory shorebird species at locations throughout the nonbreeding range, and the associated consequences for migratory performance, survival, and breeding habitat quality. Individuals experiencing more favorable winter conditions had higher survival rates, arrived on the breeding grounds earlier, and occupied better quality breeding areas, even when migration costs are substantially higher, than individuals from locations where the energy balance on the wintering grounds was less favorable. The energy costs and benefits of occupying different winter locations can therefore create fitness inequalities which can shape the distribution and population-wide demography of migratory species.

Variable and complex food web structures revealed by exploring missing trophic links between birds and biofilm

Food webs are comprised of a network of trophic interactions and are essential to elucidating ecosystem processes and functions. However, the presence of unknown, but critical networks hampers understanding of complex and dynamic food webs in nature. Here, we empirically demonstrate a missing link, both critical and variable, by revealing that direct predator-prey relationships between shorebirds and biofilm are widespread and mediated by multiple ecological and evolutionary determinants. Food source mixing models and energy budget estimates indicate that the strength of the missing linkage is dependent on predator traits (body mass and foraging action rate) and the environment that determines food density. Morphological analyses, showing that smaller bodied species possess more developed feeding apparatus to consume biofilm, suggest that the linkage is also phylogenetically dependent and affords a compelling re-interpretation of niche differentiation. We contend that exploring missing links is a necessity for revealing true network structure and dynamics.

Biofilm grazing in a higher vertebrate: the western sandpiper, Calidris mauri

We show that a higher vertebrate can graze surficial intertidal biofilm, previously only considered a food source for rasping invertebrates and a few specialized fish. Using evidence from video recordings, stomach contents, and stable isotopes, we describe for the first time the grazing behavior of Western Sandpipers (Calidris mauri) and estimate that biofilm accounts for 45-59% of their total diet or 50% of their daily energy budget. Our finding of shorebirds as herbivores extends the trophic range of shorebirds to primary consumers and potential competitors with grazing invertebrates. Also, given individual grazing rates estimated at seven times body mass per day and flock sizes into the tens of thousands, biofilm-feeding shorebirds could have major impacts on sediment dynamics. We stress the importance of the physical and biological processes maintaining biofilm to shorebird and intertidal conservation.

Phenotypic flexibility of body composition in relation to migratory state, age, and sex in the western sandpiper (Calidris mauri)

We investigated the flexibility of body composition in relation to seasonally variable demands for endurance flight capacity and hyperphagia in a migratory shorebird. Migrating western sandpipers were sampled in spring and fall while refueling at a north temperate stopover and were compared with nonmigrating birds captured at a tropical wintering area in Panama. Sandpipers weighed 25% more at stopover, and nearly 40% of migratory mass increase consisted of lean body components. Most organs and flight muscles were 10%-100% larger during migration, and the greatest relative size increases occurred in the digestive system (including liver). Birds preparing to initiate spring migration from Panama deposited only fat, suggesting that changes in lean body components take place after migration has begun, possibly through training effects. Sex did not influence body composition. Juveniles making their first southward migration were similar to adults in structural size and body mass but had substantially enlarged alimentary tracts. Sandpipers appeared to deposit lean mass during stopover in fall but not in spring. The dramatic enlargement of the digestive system in this small species that makes short flights and fuels frequently contrasts with the reduction of digestive components in larger species that fuel only once or twice by making one or two very long flights to their destination.

Effects of weather on daily body mass regulation in wintering dunlin

We investigated the influence of changes in weather associated with winter storms on mass balance, activity and food consumption in captive dunlin (Calidris alpina) held in outdoor aviaries, and compared the aviary results with weather-related body mass differences in free-living dunlin collected at Bolinas Lagoon, California. Captive birds fed ad libitum increased their body mass at higher wind speeds and lower temperatures, suggesting regulation of energy stores, whereas free-living birds exhibited patterns suggesting thermoregulatory limits on body mass regulation. Daily energy expenditure in aviary dunlin was 2.85 kJ g d–1, or 2.8× basal metabolic rate (BMR), with thermostatic costs averaging 59 % of daily expenditure. Slight but significant increases in body mass and energy expenditure in captive birds on rainy days, adjusted for possible external water mass, suggested rainfall as a proximate cue in regulating daily body mass. Body mass changes under artificial rainfall indicated similar results, and field masses suggested that free-living birds have greater body mass on days with measurable rainfall. Increased activity costs under artificial rainfall were associated with an increase in maintenance activities, relative to controls. Whether activity costs increased on days with natural rates of rainfall was unclear. Our results are consistent with current hypotheses regarding the role of body mass regulation in providing insurance against increased starvation risk during deteriorating thermal or foraging conditions, or in reducing the costs of extra mass as conditions improve.

The adjustment of avian metabolic rates and water fluxes to desert environments

We tested the hypothesis that birds in arid environments, where primary productivity is low and surface water is scarce, have reduced energy expenditure and water loss compared with their mesic counterparts. Using both conventional least squares regression and regression based on phylogenetically independent contrasts, we showed that birds from desert habitats have reduced basal and field metabolic rates compared with species from mesic areas. Previous work showed that desert birds have reduced rates of total evaporative water loss when exposed to moderate environmental temperatures in the laboratory. We tested whether reduced rates of total evaporative water loss translate into low field water fluxes. Conventional ANCOVA indicated that desert birds have reduced water fluxes, but an analysis based on phylogenetically independent contrasts did not support this finding, despite the wide array of taxonomic affiliations of species in the data set. We conclude that the high ambient temperatures, the low primary productivity, and the water scarcity in desert environments have selected for or resulted in reduced rates of energy expenditure and evaporative water loss in birds that live in these climes.

Energetics of free-ranging mammals, reptiles, and birds

We summarize the recent information on field metabolic rates (FMR) of wild terrestrial vertebrates as determined by the doubly labeled water technique. Allometric (scaling) relationships are calculated for mammals (79 species), reptiles (55 species), and birds (95 species) and for various taxonomic, dietary, and habitat groups within these categories. Exponential equations based on body mass are offered for predicting rates of daily energy expenditure and daily food requirements of free-ranging mammals, reptiles, and birds. Significant scaling differences between various taxa, dietary, and habitat groups (detected by analysis of covariance with P < or = 0.05) include the following: (a) The allometric slope for reptiles (0.889) is greater than that for mammals (0.734), which is greater than that for birds (0.681); (b) the slope for eutherian mammals (0.772) is greater than that for marsupial mammals (0.590); (c) among families of birds, slopes do not differ but elevations (intercepts) do, with passerine and procellariid birds having relatively high FMRs and gallinaceous birds having low FMRs; (d) Scleroglossan lizards have a higher slope (0.949) than do Iguanian lizards (0.793); (e) desert mammals have a higher slope (0.785) than do nondesert mammals; (f) marine birds have relatively high FMRs and desert birds have low FMRs; and (g) carnivorous mammals have a relatively high slope and carnivorous, insectivorous, and nectarivorous birds have relatively higher FMRs than do omnivores and granivores. The difference detected between passerine and nonpasserine birds reported in earlier reviews is not evident in the larger data set analyzed here. When the results are adjusted for phylogenetic effects using independent contrasts analysis, the difference between allometric slopes for marsupials and eutherians is no longer significant and the slope difference between Scleroglossan and Iguanian lizards disappears as well, but other taxonomic differences remain significant. Possible causes of the unexplained variations in FMR that could improve our currently inaccurate FMR prediction capabilities should be evaluated, including many important groups of terrestrial vertebrates that remain under- or unstudied and such factors as reproductive, thermoregulatory, social, and predator-avoidance behavior.

Estimating metabolic costs for homeotherms from weather data and morphology: an example using calidridine sandpipers

It is often desirable to estimate the metabolic costs incurred by homeothermic organisms of differing morphology living in different real or hypothetical environmental conditions. To address this problem, we describe a method, based on previously published empirical allometric and heat-transfer equations, that allows a rough estimate to be made of the daily maintenance metabolic costs (i.e., basal and thermoregulatory costs) incurred by a bird in a simple cold two-dimensional environment. The model uses widely available weather variables (temperature, wind speed, and global solar radiation), morphological variables (body mass, height of body's centre of gravity, diameter of torso), and a habitat variable (height of vegetation). We apply the model to weather data from the Canadian Arctic to predict daily metabolic costs for two calidridine sandpiper species (Calidris canutus and C. minutilla) during the summer. The model is extremely sensitive to error in the slope and intercept of the allometric equation predicting conductance from body mass, but is generally robust to other model parameters. Using ambient temperature (Ta) in place of operative temperature (Te) has only a minor (3.5%) effect on predicted metabolic costs, so, given that Teis difficult to estimate, we recommend this substitution (at least for arctic latitudes, where solar radiation is of reduced importance). The model predicts metabolic rates similar to those obtained from an equation based on a heated taxidermic mount for C. canutus, thereby providing some measure of validation. The model can easily be modified to predict metabolic costs for other groups of birds or mammals.