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Includes Text S1 (Knot husbandry), Text S2 (Gizzard and pectoral muscle mass measurements), Text S3 (Model implementation procedures), Table S1 (Age- and year-specific covariance matrices) and Table S2 (RMA slopes)

Overview of authorsâ proof corrections. (DOCX 27 kb)

1. Birds of marine environments have specialized glands to excrete salt, the saltglands. Located on the skull between the eyes, the size of these organs is expected to reflect their demand, which will vary with water turnover rates as a function of environmental (heat load, salinity of prey and drinking water) and organismal (energy demand, physiological state) factors. On the basis of inter- and intraspecific comparisons of saltgland mass (msg) in 29 species of shorebird (suborder Charadrii) from saline, fresh and mixed water habitats, we assessed the relative roles of organism and environment in determining measured msg species. 2. The allometric exponent, scaling dry msg to shorebird total body mass (mb), was significantly higher for coastal marine species (0 88, N = 19) than for nonmarine species (0 43, N = 14). Within the marine species, those ingesting bivalves intact had significantly higher msg than species eating soft-bodied invertebrates, indicating that seawater contained within the shells added to the salt load. 3. In red knots (Calidris canutus), dry msg varied with monthly averaged ambient temperature in a U-shaped way, with the lowest mass at 12 5 degrees C. This probably reflects increased energy demand for thermoregulation at low temperatures and elevated respiratory water loss at high temperatures. In fuelling bar-tailed godwits (Limosa lapponica), dry msg was positively correlated with intestine mass, an indicator of relative food intake rates. These findings suggest once more that saltgland masses vary within species (and presumably individuals) in relation to salt load, that is a function of energy turnover (thermoregulation and fuelling) and evaporative water needs. 4. Our results support the notion that msg is strongly influenced by habitat salinity, and also by factors influencing salt load and demand for osmotically free water including ambient temperature, prey type and energy intake rates. Saltglands are evidently highly flexible organs. The small size of saltglands when demands are low suggests that any time costs of adjustment are lower than the costs of maintaining a larger size in this small but essential piece of metabolic machinery.

Many bird species are site-faithful, not only to breeding sites but also to the areas used during migration and wintering. For Red Knots we already knew that they use very large home-ranges in the Wadden Sea (800 km2, equivalent to half the Dutch part of this area), whereas they have very small home-ranges (10-15 km2) in their wintering area on the Banc d'Arguin in Mauritania. Here, the main question is whether Red Knots restrict themselves to half the Dutch Wadden Sea or maybe also move between the western (West) and the eastern parts (East) of this area (Fig. 1) and if so, to which extent? We analysed resighting data from a long-term colour-ringing project on this species (Tab. 1), and applied radiotransmitters on 20 Red Knots in West and 20 in East after erecting 11 automatic receiving stations throughout the Wadden Sea (Fig. 1) to investigate the exchange between West and East within a period of two months. Many Knots caught in West ('West-Knots') were seen in East and vice versa, but the fraction of West-Knots seen in West was 2.8 times higher than the fraction of EastKnots resighted in West. In the same way, the fraction East-Knots resighted in East was 2.1 times larger than the fraction of West-Knots seen in East. So there was much exchange between the two areas but there is also site fidelity to the catching area in subsequent years. The fraction Knots resighted in important Red Knot areas abroad did not differ much between the two catching areas (Tab. 3). The telemetry experiment showed that East-Knots moved to West in the course of the autumn while WestKnots stayed within West, though they used a large area there (Fig. 2). Half of the East-Knots visited West more than once during a period of one month and one bird moved at least seven times between these areas. It is likely that this bird made use of tailwinds to make these flights (Tab. 5). The data presented support the known existence of large home-ranges in this species in the temperate zone and show that part of the individuals have even larger ranges than hitherto estimated. The main reason for this seems the unpredictable presence and/or availability of food resources (mainly small shellfish). It is suggested that the higher average temperatures in winter in West than in East and the smaller migration distance from West to alternative wintering areas in Britain could be the reasons why many East Knots move to West in the course of the autumn.

1. Millions of shorebirds migrate each year through a small number of highly productive staging areas where they often conflict with fisheries interests. Delaware Bay, USA, is a major shorebird stopover site where, in spring, many thousands of shorebirds undergo rapid mass gain by feeding on the eggs of commercially harvested horseshoe crabs Limulus polyphemus. 2. Environmental factors may cause deviations from the best migration schedule. We used within-year mass gain data from red knot Calidris canutus caught in Delaware Bay between 1998 and 2005 to determine the degree of flexibility individuals have to vary migration speed. 3. Mass gain by birds below 133 g was shown to comprise 15.3% fat (39 kJ g(-1)), the remainder being lean mass (6 kJ g(-1)). Above this critical level, fat comprised 83.9% of mass deposition. The rates of energy deposition (kJ d(-1)) were therefore fundamentally different between the two states but were among the highest ever recorded among vertebrates (5-7 x basic metabolic rate). 4. A total of 36-62% of the variation in observed rates of energy deposition between 1998 and 2002 was explained by a year factor, date and mass at initial capture and interaction terms, such that light-weight birds at the end of May had rates of mass gain or energy deposition two to three times higher than birds of similar mass in mid-May, indicating that birds were attempting to achieve a certain mass by a certain date. In 2003 and 2005, this relationship broke down as a result of lower densities of eggs. 5. Synthesis and application. The maintenance of high densities of crab eggs required for high rates of mass gain in red knot requires severe cuts in, or the complete cessation of, the crab harvest, reduced human and raptor-related disturbance as well as management of beaches to provide sufficient crab-spawning habitat. These findings are widely applicable to other systems where harvesting activities come into conflict with migrating animals and show that certain sections of the population, in this case the long-distance migrants from South America, will be impacted more than short-distance migrants whose physiology may give them access to altto alternative food resources.

Reversible phenotypic changes, such as those observed in nutritional organs of long-distance migrants, increasingly receive the attention of ornithologists. In this paper we review the cost-benefit studies that have been performed on the flexible gizzard of Red Knots Calidris cunutus. By varying the hardness of the diet on offer gizzard mass could experimentally be manipulated, which allowed quantification of the energetic costs and benefits as a function of gizzard size. These functions were used to construct an optimality model of gizzard mass for Red Knots on migration and during winter. Two possible currencies were assumed, one in which Knots aim to balance their energy budget on a daily basis (satisficers), and one in which Knots aim to maximise their daily energy budget (net rate maximisers). The model accurately predicted variation in gizzard mass that we observed (1) between years, (2) within years, and (3) between sites. Knots maintained satisficing gizzards during winter and rate-maximising gizzards when fuelling for migration. The model-exercise revealed the importance of digestive constraints and quality of prey in the life of Knots.

1. There is world-wide concern about the effects of bottom-dredging on benthic communities in soft sediments. In autumn 1988, almost a third of the 50-km(2) intertidal system around the island of Griend in the western Dutch Wadden Sea was suction-dredged for edible cockles Cerastoderma edule and this study assessed subsequent effects. An adjacent area not directly touched by this fishery and an area from which the mussel Mytilus edulis beds were removed, served as reference areas. 2. Sediment characteristics, together with the total stock size and settlement densities of Cerastoderma, Baltic tellin Macoma balthica and soft-shelled clam Mya arenaria, were documented during 11 successive autumns before (August-September 1988) and after (August-September 1989-98) the suction-dredging event in fished and unfished areas. Four other areas in the Dutch Wadden Sea, where changes in densities of juvenile bivalves from 1992 to 1998 were measured, served as additional reference locations. 3. Between 1988 and 1994, median sediment grain size increased while silt was lost from sediments near Griend that were dredged for cockles. The initial sediment characteristics were re-attained by 1996. 4. After the removal of all Mytilus and most Cerastoderma, the abundance of Macoma declined for 8 years. From 1989 to 1998, stocks of Cerastoderma, Macoma and Mytilus did not recover to the 1988 levels, with the loss of Cerastoderma and Macoma being most pronounced in the area dredged for cockles. Declines of bivalve stocks were caused by particularly low rates of settlement in fished areas until 1996, i.e. 8 years after the dredging. 5. A comparison of settlement in the short (1992-94) and medium term (1996-98) after cockle-dredging in several fished and unfished areas spread over the entire Dutch Wadden Sea, showed a significant negative effect of dredging on subsequent settlement of Cerastoderma. Macoma also declined, but not significantly. 6. We conclude that suction-dredging of Cerastoderma had long-lasting negative effects on recruitment of bivalves, particularly the target species, in sandy parts of the Wadden Sea basin. Initially, sediment reworking by suction-dredging (especially during autumn storms) probably caused losses of fine silts. Negative feedback processes appeared to follow that prevented the accumulation of fine-grained sediments conducive to bivalve settlement.

Supplementary material. (PDF 430 kb)

Supplementary material. (PDF 430 kb)