Anita Koolhaas

Interactions between stomach structure and diet choice in shorebirds

Captive Red Knots (Calidris canutus) fed soft food pellets developed atrophied stomachs, and were reluctant to eat their usual hard-shelled mollusc prey. An interspecific comparison among shorebirds showed that wild Red Knots and other intact-mollusc-eating species have gizzards with relatively great mass but very small proventriculi. Within six different shorebird species, the heavier individuals usually had the heavier stomachs as well, but in Red Knots and Bar-tailed Godwits (Limosa lapponica) we identified heavy premigrant individuals with reduced stomach masses, suggesting a reallocation of protein reserves before long-distance flights. In both species reduced stomach mass was associated with a relatively soft diet. We were unable to show that during adjustment of stomachs to hard-shelled prey, such prey are broken down to smaller fragments. We attribute this to the counteractive influence of the pylorus during adjustment. We summarize the suggested stomach/diet interactions as a network of causal relationships and feedback loops involving the type of diet and gizzard mass. We identify two basic modifiers of gizzard mass: one working via endurance training and disuse atrophy; and another involving endocrine and/or neural mechanisms. It is likely that, in the course of their annual cycle, shorebirds are prevented from achieving maximal digestive performance owing to seasonal changes in feeding habitats and diet enforced by their long-distance migrations.

Scale and intensity of intertidal habitat use by knots Calidris canutus in the Western Wadden Sea in relation to food, friends and foes

Abstract In August–October 1988–1992 we studied the distribution and abundance of knots Calidris canutus around Griend in the western Wadden Sea, and the extent to which these can be explained by benthic prey availability and presence of avian predators. Numbers in the nonbreeding season showed monthly averages of 10 000 to 25 000 birds. Over 100 000 knots were recorded on three occassions. Knots feed in large flocks, individual birds usually experiencing 4 000 to 15 000 flock-mates. The Siberian-breeding/west-African wintering canutus subspecies passed through in late July and early August. Otherwise the Greenlandic/Canadian breeding islandica subspecies was present. Over the period 1964–1992 there were no clear trends in the number of knots, but canutus -knots were particularly abundant in July–August 1991, whereas in 1992 both subspecies were absent. Macoma balthica was the preferred prey of both subspecies. Hydrobia ulvae, Mytilus edulis and Cerastoderma edule were eaten when Macoma was absent close to the surface of the sediment. As Macoma buried deeper from July onwards, canutus faced better average feeding conditions than islandica later in the year. The spatial distributon of knots feeding on the intertidal flats around Griend was best explained by the harvestable biomass of the prevalent prey species in a particular year and season, i.e. Macoma (main prey when their harvestable biomass densities were greater than ca 0.8 g AFDM per m 2 ) and Cerastoderma , and by the avoidance of situations where they run the risk of attack by bird-eating birds. Flocks of knots covered most of the intertidal flats in the Western Dutch Wadden Sea in a couple of tidal cycles. This is about 800 km 2 , much larger than the equivalent area used by knots on their wintering grounds in Mauritania (10–15 km 2 ), a difference that is correlated with prey spectrum, prey availability and predictability.

Long flights do not influence immune responses of a long-distance migrant bird: a wind-tunnel experiment.

Heavy physical work can result in physiological stress and suppressed immune function. Accordingly, long-distance migrant birds that fly for thousands of km within days can be expected to show immunosuppression, and hence be more vulnerable to infections en route. The red knot Calidris canutus Linnaeus is a long-distance migrant shorebird. We flew red knots the equivalent of 1500 km over 6 days in a wind tunnel. The humoral and cell-mediated immune responses of the flyers were compared to those of non-flying controls. Humoral immunity was measured as antibody production against injected diphtheria and tetanus antigens, and cell-mediated response as phytohemagglutinin-induced wing-web swelling. Blood corticosterone levels, which may modulate immune function, were measured in parallel. The long flights had no detectable effects on humoral or cell-mediated immune responses, or on corticosterone levels. Thus, flight performance per se may not be particularly stressful or immunosuppressive in red knots. Some birds assigned as flyers refused to fly for extended periods. Before flights started, these non-flyers had significantly lower antibody responses against tetanus than the birds that carried out the full flight program. This suggests that only birds in good physical condition may be willing to take on heavy exercise. We conclude that these long-distance migrants appear well adapted to the work load induced by long flights, enabling them to cope with long flight distances without increased stress levels and suppression of immunity. Whether this also applies in the wild, where the migrating birds may face adverse weather and food conditions, remains to be investigated.

Dredging for edible cockles (Cerastoderma edule) on intertidal flats : short-term consequences of fisher patch-choice decisions for target and non-target benthic fauna

Intertidal flats in the Dutch Wadden Sea are protected by national and international treaties. Still, mechanical dredging for edible cockles Cerastoderma edule was allowed in 74% of 1200 km(2) of interticlal flats. Cumulatively, between 1992 and 2001, 19% of the intertidal area was affected by mechanical cockle-dredging at least once. On the basis of a grid of 2650 stations sampled annually, we evaluate the extent to which cockle-dredging from 1998 to 2003 was selective with respect to non-target macrozoobenthic intertidal fauna. In all 4 years that comparisons could be made, to-be-dredged areas contained greater diversity of macrobenthic animals than areas that remained undredged. Targeted cockles were 2.5 times more abundant in areas that were to be dredged shortly, but other species also occurred in higher densities in these areas. Small amphipods and some bivalves occurred less in to-be-dredged areas than elsewhere. In terms of short-term responses to dredging, four non-target species showed a significant decrease in abundance 1 year after dredging. Only Tellina tenuis showed an increase a year after dredging.

Reversed optimality and predictive ecology: burrowing depth forecasts population change in a bivalve.

Optimality reasoning from behavioural ecology can be used as a tool to infer how animals perceive their environment. Using optimality principles in a ‘reversed manner’ may enable ecologists to predict changes in population size before such changes actually happen. Here we show that a behavioural anti-predation trait (burrowing depth) of the marine bivalve Macoma balthica can be used as an indicator of the change in population size over the year to come. The per capita population growth rate between years t and t+1 correlated strongly with the proportion of individuals living in the dangerous top 4 cm layer of the sediment in year t: the more individuals in the top layer, the steeper the population decline. This is consistent with the prediction based on optimal foraging theory that animals with poor prospects should accept greater risks of predation. This study is among the first to document fitness forecasting in animals.

Behavioural evidence for heat-load problems in Great Knots in tropical Australia fuelling for long-distance flight

Abstract Migratory shorebirds that live in the tropics prior to embarking on long (>5000 km) flights may face heat-load problems. The behaviour of a large sandpiper, the Great Knot (Calidris tenuirostris), was studied in Roebuck Bay, north-west Australia, from February to April 2000. We determined the incidence of heat-reduction behaviour in foraging and roosting birds in relation to breeding-plumage score (an index of migratory preparation) and microclimate variables. Heat-reduction behaviour (primarily raising the back feathers) was significantly related to breeding-plumage score and solar radiation. Raising back feathers may reduce the external heat load for a bird, or increase convective or cutaneous evaporative cooling. The results suggest that managing heat loads in tropical-wintering waders may become more difficult close to departure on migration.

Treatment of capture myopathy in shorebirds: a successful trial in northwestern Australia

Abstract Shorebirds held during banding activities can develop muscle cramps, especially when temperatures are high and birds are heavy. Such capture myopathy can be fatal or render birds vulnerable to predators. We rehabilitated Great Knots (Calidris tenuirostris), Red Knots (C. canutus), Bar-tailed Godwits (Limosa lapponica), and Red-necked Stints (C. ruficollis) in northwestern Australia. We kept birds in slings (if cramped) or in a small cage (if able to walk) and gave them daily standing exercises. Recovery of severely cramped birds took up to 14 d, which may reflect a critical period of tissue regeneration. Of 15 knots (8 Red and 7 Great) taken into captivity, 12 were rehabilitated and released. The resighting rate after the breeding season of the rehabilitated birds was the same as for other birds color-banded during our research, indicating that the rehabilitation was successful. We conclude that rehabilitating cramped shorebirds is possible though time-consuming. A sex bias in susceptibility to capture myopathy is suggested by seven of the eight Red Knots treated being male; the sex ratio in the local population was 1:1.

Wind effects on prey availability: How northward migrating waders use brackish and hypersaline lagoons in the Sivash, Ukraine

Abstract Large numbers of waders migrating northward in spring use the Sivash, a large system of shallow, brackish and hypersaline lagoons in the Black Sea and Azov Sea region (Ukraine). The bottoms of these lagoons are often uncovered by the wind. Hence, for waders the time and space available for feeding depend on wind conditions. In hypersaline lagoons the benthic and pelagic fauna was very poor, consisting mainly of chironomid larvae (0.19 g AFDM·m −2 ) and brine shrimps Artemia salina , respectively. Brine shrimp abundance was correlated with salinity, wind force, wind direction and water depth. Dunlin Calidris alpina and curlew sandpiper Calidris ferruginea were the only species feeding on brine shrimp. As brine shrimp densities are higher in deeper water, smaller waders such as broad-billed sandpipers Limicola falcinellus are too short-legged to reach exploitable densities of brine shrimp. In brackish lagoons the benthic and pelagic fauna was rich, consisting of polychaetes, bivalves, gastropods, chironomid larvae, isopods and amphipods (8.9 to 30.5 g AFDM·m −2 ), but there were no brine shrimps. Prey biomass increased with the distance from the coast, being highest on the site that was most frequently inundated. Dunlin, broad-billed sandpiper and grey plover Pluvialis squatarola were the most abundant birds in the brackish lagoon. Due to the effects of wind-tides only a small area was usually available as a feeding site. Gammarus insensibilis was the alternative prey resource in the water layer, and their density varied with wind direction in the same way as brine shrimp. Curlew sandpipers and dunlins in the hypersaline lagoons and broad-billed sandpipers in the brackish lagoons often changed feeding sites, probably following the variation in prey availability. Only because of the large size and variety of lagoons are waders in the Sivash always able to find good feeding sites.

Are birds stressed during long-term flights? A wind-tunnel study on circulating corticosterone in the red knot

During endurance flight most birds do not feed and have to rely on their body reserves. Fat and protein is catabolised to meet the high energetic demands. Even though the hormonal regulation of migration is complex and not yet fully understood, the adrenocortical hormone corticosterone crystallizes to play a major role in controlling physiological traits in migratory birds during flight. However, results from field studies are partially equivocal, not least because data from birds during endurance flight are hard to get and present mostly a momentary shot. A wind-tunnel experiment offered the possibility to measure repeatedly under controlled conditions the effect of long flights on the stress hormone corticosterone. In a long-distance migrating shorebird, the red knot Calidris canutus, we measured plasma concentrations of corticosterone within 3 min and after a restraint time of 30 min directly after 2h and 10h non-stop flights, respectively, and during rest. Baseline corticosterone levels were unchanged directly after the flights, indicating that endurance flight did not affect corticosterone levels. The adrenocortical response to restraint showed the typical rise in birds during rest, while birds after a 2 or 10h flight substantially decreased plasma corticosterone concentrations. We suggest that the negative adrenocortical response to restraint after flight is part of the mechanism to reduce the proteolytic effect of corticosterone to save muscle protein and to avoid muscle damaging effects.

No evidence for melatonin-linked immunoenhancement over the annual cycle of an avian species

The winter immunoenhancement hypothesis associates long nights and increased exposure to melatonin with enhanced immune function in winter when resource availability is low and the chances of becoming ill are high. Thus, increased exposure to melatonin in the winter could be adaptive for species facing difficult winter conditions. This idea has found some support in studies of resident mammals. In birds, the link between day length and melatonin over the annual cycle is weaker, and contributions of melatonin to seasonal timing are unclear. Furthermore, many species, especially migrants, do not experience the most difficult conditions of their annual cycle in winter. In this study, we tested whether the winter immunoenhancement hypothesis holds in an avian species, the red knot Calidris canutus. We found that melatonin duration and amplitude varied significantly over the annual cycle with the highest values occurring in winter. However, peaks did not correspond to the winter solstice or with annual variation in immune function. Our findings do not support the winter immunoenhancement hypothesis in knots and question whether the idea that immune function should be bolstered in winter can be generalized to systems where winter is not the most difficult time of the year.