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In this study we examined the effect of different livestock grazing treatments on breeding bird densities in a salt marsh habitat. To avoid an experiment on the large scale needed to directly measure grazing effects on bird densities, we followed a two-step approach. First, we measured vegetation micro-patterns (mosaic of lower vegetation and taller patches at 4x4 m) around Common Redshank Tringa totanus and Eurasian Oystercatcher Haematopus ostralegus nests and at random sites paired with these nests sites to judge suitability of micro-patterns for nest building. Secondly, we measured micro-patterns at 120 permanent plots in five different experimental grazing treatments to determine how grazing affects micro-patterns. We compared low stocking density of both cattle and of horses, high stocking density of cattle and of horses, and intermittent grazing with a high stocking density of cattle (i.e. yearly intervals of grazing and no grazing). Redshank and Oystercatcher nests occurred in sites with taller vegetation and more pronounced micro-patterns than found at random sites. Paddocks grazed with low densities of livestock or with a high density intermittent grazing treatment had micro-patterns preferred by the birds. We conclude that Redshanks and Oystercatchers may benefit in terms of potential nest sites from grazing at low livestock densities or at intermittent stocking densities through effects of grazing on micro-patterns in the vegetation.

Long time series allow us to look back in time and examine how birds responded to changes in their environment. During the second half of the 20(th) century, not only did Northern Lapwing Vanellus vanellus experience an increase in spring temperatures, their meadow habitat also changed dramatically due to agricultural intensification. In The Netherlands, eggs of Lapwing have been collected for consumption for ages, especially in the province of Frysl (a) over capn, and as the finding of the first egg of the season has been an important social event till today, first egg dates are archived. Here we present data on the dates at which the first egg of the season was found in Frysl (a) over capn, in 1897-2003. Somewhat to our surprise we found that the advance in the first egg date was primarily explained by increasing spring temperatures. Lapwings also laid earlier after wet winters, with little variance remaining to be explained by habitat changes. Still, at the same spring temperatures and winter rainfall, the first egg was laid on average three days earlier in 2000 compared with 1900. A complementary dataset on the date that the first egg was found in the loth successive Friesian municipality confirmed the strong effect of temperature and the additional effect of winter rainfall. The number of days between the first egg date for the province and the 10(th) municipality yielded a measure of breeding synchrony. The start of egg laying was more synchronous during cold springs. Our analysis thus shows that Lapwing laying date was primarily affected by climatic factors rather than by the considerable changes in breeding habitat.

Sexual size dimorphism is common in many bird species. A consequence of sexual size dimorphism is that the larger sex has larger food requirements, and also that it might be dominant over the smaller sex. We studied the foraging behaviour of Bar-tailed Godwits Limosa lapponica, a highly sexually size dimorphic species. Our interest is in intersexual competition and its consequences for habitat use. Male and female Bar-tailed Godwits were distributed non-randomly over the intertidal foraging areas. Males were concentrated on exposed mud flats, while females occurred more along the waterline. Also within these habitat types, the sexes associated with same-sex individuals. Males were more susceptible to intraspecific kleptoparasitism than females, which we suggest is the cause of the unequal spatial distribution of male and female Bar-tailed Godwits. Females seem to monopolise the better quality foraging areas, leaving males the rest.

Little is known about the fuel stores that arctic-breeding waders put on before departure from the breeding grounds. During a ship-based expedition to arctic Canada, we caught waders at seven, mainly coastal sites, within 68°-76°N and 139°-67°W, from 28 July to 31 August 1999. More than two hundred waders of twelve species were trapped, mainly White-rumped Calidris fuscicollis, Semipalmated C. pusilla, Baird’s C. bairdii and Buff-breasted Sandpipers Tryngites subruficollis. The vast majority of the birds were juveniles. Body masses and visual fat stores were low, close to the lowest values found anywhere during the non-breeding season for the different species. The relatively fattest birds were Buff-breasted Sandpipers, but they were still far from their maximum body mass on spring migration. We conclude that juvenile arctic waders depart from their natal areas with only small fuel stores, which is in concordance with a time-minimising migration strategy.

During southward migration the Wadden Sea is the meeting place of Red Knots Calidris canutus of two subspecies that breed in either western Siberia (C. c. canutus) or north Greenland and north-east Canada (C. c. islandica), but the details of their co-occurrence have not been described. In 1995-98 numbers of Red Knots in our study area in the western Dutch Wadden Sea usually built up in late July towards maxima of 10 000-20 000 individuals in August and early September. In each of these four years we attached tiny (1.3-1.8 g) radiotransmitters to a total of 95 molecularly sexed adults to determine the length of stay of different categories of birds. The 65 females (68%) predominated the samples, and among the females the majority (48 birds) was captured without traces of wing moult. In females, but not in males, birds caught in wing moult stayed significantly longer than non-moulting birds. Non-moulting females weighed up to 200 g and disappeared within three weeks after being marked. The timing of their disappearance corresponded with observed departures of flocks towards the southwest, and published departure times of canutus. The relationship between length of stay and mass at capture of these early departing non-moulting females suggests a daily mass gain of about 2.84 g d(-1). These birds had a mean bill length that was 1 mm (yet significantly) longer than those of the other female categories; a relatively long bill is a well known attribute of canutus. The much smaller sample of males with similar mass, moult and staging time characteristics did not show longer bill lengths and we are thus unable to unambiguously confirm the presence of canutus males in late July and early August; this bias remains to be functionally explained. Sex ratios were even in birds assignable to islandica.

In this paper an analysis is made of subtle behavioural differences between adult male and female Ovstercatchers feeding on Macoma balthica under field conditions and in captivity. Macoma is a tellinid bivalve that in the Dutch Wadden Sea is mainly preyed upon during spring and summer when it is buried at a shallow depth. males lift Macoma more, whereas females handle them mostly in situ. Both sexes handle a Macoma in situ faster than one lifted. Time loss of males in handling more lifted Macoma is compensated by the larger size of lifted Macoma, which yields more flesh. The time the birds need to find an edible Macoma is similar for both sexes, resulting in equal mean food intake rates for males and females in the field. Lifted Macoma are generally hammered and, since males with their short strong bills are more likely to hammer bivalves than females, this difference in bill morphology might explain why males more often lift Macoma than do females, especially as hammering produces a blunt bill tip which would reduce efficiency at opening Macoma in situ. However, none of the selected bill morphology variables showed a relationship within the sexes that explained the differences between the sexes.

We developed a model to calculate food intake by Oystercatchers from their weight gain between two incubation spells and correcting for the amount of excreta voided before the bird returned to its nest. The model predictions agreed well with estimated food consumption based on direct observations in the field, regardless of the type of prey consumed. The model was used to calculate food consumption of free-living Oystercatchers under circumstances when this could not be measured by direct observation; (1) when feeding downshore outside their territory and (2) while feeding during night-time low water periods. The rate of food intake outside the territory did net differ from the rate within the territory and averaged 0.9 g min(-1) fresh weight. Food intake rate in darkness did not differ from that during daylight (p = 0.96). The total amount of food consumed per low water period et night was higher in the male and lower in the female as compared to food consumed in daylight. The difference was probably induced by our activities which made the female very reluctant to incubate during the daylight hours. The average amount of food consumed differed hardly between day and night-time low water periods. Total food consumption over a 24 hour day was 162 +/- 88 g for the male and 196 +/- 13 g for the female. Accounting for the weight loss of the male over the observation period, the estimated energy expenditure is 535 and 565 kJ day(-1) for male and female, respectively. This is equivalent to 2.2 x BMR and strongly suggests that the incubation stage is a period when energy is conserved, rather than expended. Although the amount of food consumed per low water period varied greatly from one tidal cycle to the other, the birds appeared to balance intake with expenditure on a 24 hour basis. In the discussion we address the possible repercussions for the birds when they fail to keep this balance in the short run.

The available feeding time of coastal Oystercatchers varies from day to day due to the effect of wind direction and wind force on the water level. If the birds are not able to feed at all during a day, they lose 30 g, or 6% of their body weight. The body weight increases with the duration of the available feeding time, irrespective whether it is day or night. Oystercatchers continue to feed at night, at least in autumn and winter. Although wind force and wind direction affect the daily duration of the available feeding time, this variation fades away if calculated over a number of days, and therefore does not affect the birds in the long-term. Does the body weight increase, or decrease, with the higher costs of living associated with low temperatures and strong winds? A decrease in body weight with increased cost of living would suggest. that the birds are not able to find the extra food required to compensate for the higher maintenance level ('undercompensation'). An increase in body weight, on the other hand, would suggest that the birds in these difficult circumstances eat even more than needed in order to increase their body reserves in cases still worse conditions arrive ('overcompensation'). Unfortunately, the field data are confusing. The weak increase in body weight at low temperatures suggests an overcompensation, but the observed clear decrease in body weight with strong winds suggests an undercompensation. However, the increase of body weight with lower temperature is not large and is possibly due to intervening variables, so it is not clear whether this was an actual overcompensation. The negative effect of wind force on body weight is presumably caused by undercompensation in combination with a decrease in the feeding success.

Captive Oystercatchers consume daily 25-40 g dry flesh or 550-850 kJ, of which they metabolize 450-700 kJ. Free-living Oystercatchers eat more than captive birds but, contrary to expectation, this is not due to greater activity costs but to a higher body weight. When body weights are equal, free-living and captive Oystercatchers consume the same amount of food. The intake rate of Oystercatchers generally varies between 1 and 3 mg dry flesh feeding, but if non-feeding times are included, the crude intake rate usually varies between 1 and 1.5 mg s(-1). Extremely high intake rates, above 4 mg s(-1), are only observed in birds feeding during a short bout after a long resting period. According to Kersten & Visser (1996a) such high intake rates cannot be sustained fur long, because a maximum of 80 g wet flesh, equivalent to 12 g dry flesh, can be stored in the digestive tract and the processing rate does not exceed 4.4 mg wet flesh s(-1) or 0.66 mg ash-free dry weight (AFDW) s(-1). Due to this digestive bottleneck, the birds are forced to spend much time on the feeding area each day. Since the exposure time of their intertidal feeding ar eas is usually 5-6 hours, Oystercatchers cannot meet their daily energy requirements in a single low water period, which would often suffice if intake rate vias the limiting factor. For a given length of the feeding period, the bottleneck model predicts the maximum crude intake intake, called CIRmax, that can be achieved, i.e. the highest intake rate including the non-feeding time. When the birds are able to feed for less than three hours, the achieved crude intake rate usually remains far below this maximum, suggesting that the rare at which prey are found and eaten deter mines the intake rate. The consumption is also usually less than would be allowed by digestive constraint when the birds feed for twelve hours or longer, because the birds at thermoneutral conditions do not need more than 36 g a day. When the birds spend three to twelve hours on the feeding area, the average consumption is usually close to, or below the predicted maximum. However, in a few cases, the maximum was clearly exceeded. These studies do not invalidate the bottleneck model, because there is ample reason to believe that food consumption was overestimated A detailed investigation of the many sources of error indicates that food consumption is more likely to be overestimated than underestimated in field studies.

We predict the intake rate and prey choice of Oystercatchers feeding along the Frisian coast, Dutch Wadden Sea, combining the optimal prey choice model (Charnov 1976) with detailed measurements of the widely fluctuating food supply. Assuming that the birds maximize their intake rate, the birds should never eat Mussels Mytilus edulis during 10 years of observa- tions, Mya arenaria during two short periods, Macoma blathica and Scrobicularia plana during most summers and Cockles Cerastoderma edule in most winters. Observations on feeding Oystercatchers confirmed the predictions. Due to the seasonal variation in burying depth of Scrobicularia and Macoma, these prey were in winter, if not inaccessible, hardly worthwhile exploiting because of the increase of handing time and searching time with burying depth. Hence, the seasonal variation in intake rate was very large in these deep-living prey compared to surface prey, such as Cockles and Mussels. Consequently, Oystercatchers usually switch from surface to deep-living prey in spring and back to surface prey in autumn in order to maximize their intake rate. Oystercatchers will never achieve a high intake rate when they feed on small prey, even when these prey would occur in extremely high densities. The reason for this is that the yield of small prey during handling is even less than the intake rate during feeding of 1 mg ash-free dry weight (AFDW) s(-1) which Oystercatchers need to meet their energy demands juring the limited feeding periods in the tidal habitat. Since Oystercatchers eat only large bivalves, they might be vulnerable because cohorts of prey may disappear completely before they can be harvested. Despite the very large annual variation in the biomass of the different prey species in the Wadden Sea, the total food supply harvestable by Oystercatchers is large enough for them to stay ir, the area, unless ice covers the tidal flats. However, Oystercatchers cannot survive in the Wadden Sea when their diet is restricted to one or two prey species. They need to switch between at least 3 or 4 prey species. For the same reason, the birds have to roam over feeding areas measuring ar least some ten's of km(2). The winter remains a difficult period, however. The mortality is higher in winter than in summer and increases with the severity of the winter. Besides, the winter mortality increases when the food consumption is reduced, due to either a low intake rate and/or a short feeding time. Therefore, the wintering numbers of Oystercatchers in the Wadden Sea are limited during circumstances which occur in only some of the winters, viz. when ice covers the feeding areas and the harvestable food supplies are low. The total biomass of the five bivalve species in the study area amounted to 81 g ash-free dry flesh (AFDW) m(-2), on average. The annual production was 56 g m(-2), but only 32 g m(-2) can be considered as exploitable by Oyster catchers. Oystercatchers did not han est the 9 g m(-2) year(-1) produced by large Mya living out of I-each of the bill, nor the 5 g m(-2) produced by bivalves too small to be eaten by Oystercatchers. Moreover 9 g m(-2) disappeared during disasters (e.g. frost) and could not be eaten by birds. Oystercatchers consumed 12 g m(-2) year(-1), on average, thus more than the 10 g m(-2) taken by all other shorebird species together. Half of the prey biomass disappearing due to mortality between August and March could be attributed to Oystercatcher predation. The predation pressure by Oystercatchers was much lower iu Scrobicularia and Macoma. In contrast, 80% of the second year Mya was eaten by Oystercatchers in some months. The numbers of Oystercatchers feeding in the study area were weakly related to the annual variations in the total food supply, but strongly related to those of the harvestable food supply. This high correlation must be due to two causal relationships: the bird density increases with the intake rate, and intake rate increases with the harvestable food supply.

Three individually-tested adult Oystercatchers took different proportions of shallow-buried bivalves Mya arenaria and Scrobicularia plana from a mixture on offer in an experimental situation. Two birds, taking mainly or exclusively Scrobicularia, selected the species as predicted by a random search model based on encounter rate. The third bird actively selected for the less abundant Mya. All birds showed size selection within the prey species. Diet composition was correlated with the efficiency of the birds in handling the prey items. Optimal foraging theory adequately explained the observed prey species- and size selection by the birds. A short-term learning effect was found in two of the Oystercatchers. These birds managed to reduce their handling time during the course of the experiments, mainly by cutting the flesh loose from the shell faster. Cutting was always the most time-consuming component of prey handling but relatively more when a bird took the less preferred prey species. By rapidly improving their cutting technique for a specific prey, the birds increased their profitability. This adaptive behaviour contradicts the view of the Oystercatcher as a conservative specialist, which has problems when alternative prey should be taken in times of food shortage, although under natural conditions prey switching is hampered by morphological constraints.