Nigel A. Clark

Rapid population decline in red knots: fitness consequences of decreased refuelling rates and late arrival in Delaware Bay

Most populations of migrant shorebirds around the world are in serious decline, suggesting that vital condition–dependent rates such as fecundity and annual survival are being affected globally. A striking example is the red knot (Calidris canutus rufa) population wintering in Tierra del Fuego, which undertakes marathon 30 000 km hemispheric migrations annually. In spring, migrant birds forage voraciously on horseshoe crab eggs in Delaware Bay in the eastern USA before departing to breed in Arctic polar deserts. From 1997 to 2002 an increasing proportion of knots failed to reach threshold departure masses of 180–200 g, possibly because of later arrival in the Bay and food shortage from concurrent over–harvesting of crabs. Reduced nutrient storage, especially in late–arriving birds, possibly combined with reduced sizes of intestine and liver during refuelling, had severe fitness consequences for adult survival and recruitment of young in 2000–2002. From 1997 to 2002 known survivors in Delaware Bay were heavier at initial capture than birds never seen again, annual survival of adults decreased by 37% between May 2000 and May 2001, and the number of second–year birds in wintering flocks declined by 47%. Population size in Tierra del Fuego declined alarmingly from 51 000 to 27 000 in 2000–2002, seriously threatening the viability of this subspecies. Demographic modelling predicts imminent endangerment and an increased risk of extinction of the subspecies without urgent risk–averse management.

Changes in commercially fished shellfish stocks and shorebird populations in the Wash, England

Abstract The Wash, in eastern England, supports internationally important populations of 11 species of shorebird. A major commercial shellfishery provides potential conflict between fishermen and nature conservation interests. During the 1990s, high fishing mortality and low recruitment substantially reduced the stocks of cockle (Cerastoderma edule) and mussel (Mytilus edulis). Population models, constructed from estimates of survival and recruitment, indicated that declines in the availability of cockles and mussels were associated with changes in oystercatcher (Haemotopus ostralegus) survival between 1970 and 1998, including three periods of mass mortality, and also the recruitment of juvenile birds to both oystercatcher and knot (Calidris canutus) populations. Emigration of knot may also have taken place. Cockle recruitment was dependent on climatic conditions, whereas mussel populations tended to be stable. The decline in mussel stocks, due to over-fishing, increased the vulnerability of the oystercatcher population to mass-mortality episodes in poor cockle years. The key to preventing major oystercatcher kills in future is to ensure sufficient mussels are available in poor cockle years. Recent cultivation of mussels in inter-tidal areas has been beneficial and is an important management tool for maintaining bird populations.

Effects of Horseshoe Crab Harvest in Delaware Bay on Red Knots: Are Harvest Restrictions Working?

Each May, red knots (Calidris canutus rufa) congregate in Delaware Bay during their northward migration to feed on horseshoe crab eggs (Limulus polyphemus) and refuel for breeding in the Arctic. During the 1990s, the Delaware Bay harvest of horseshoe crabs for bait increased 10-fold, leading to a more than 90% decline in the availability of their eggs for knots. The proportion of knots achieving weights of more than 180 grams by 26–28 May, their main departure period, dropped from 0.6–0.8 to 0.14–0.4 over 1997–2007. During the same period, the red knot population stopping in Delaware Bay declined by more than 75%, in part because the annual survival rate of adult knots wintering in Tierra del Fuego declined. Despite restrictions, the 2007 horseshoe crab harvest was still greater than the 1990 harvest, and no recovery of knots was detectable. We propose an adaptive management strategy with recovery goals and annual monitoring that, if adopted, will both allow red knot and horseshoe crab populations to recover and permit a sustainable harvest of horseshoe crabs.

A guide to the provision of refuges for waders : an analysis of 30 years of ringing data from the Wash, England

1. It has been proposed to establish roost refuges to limit the disturbance to internationally important over-wintering populations of waders on the Wash. 2. Waders have been caught regularly on the Wash since 1959. By 1993, 24 576 oystercatchers, 4125 grey plovers, 38 041 knots, 96 801 dunlins and 11 729 redshanks had been ringed at 85 roost sites around the Wash. 3. Analysis of the movements between roosts provides a means of determining the relative mobility of waders and the effect of spacing on the availability of the proposed refuges to waders during normal roost movements. This analysis does not take into account either movements between feeding sites nor between roosting and feeding sites. 4. The Wash was divided into five sections according to observed wader movements between roosts and feeding grounds. At least 79% of inter-roost movements of oystercatchers, grey plovers, knots, dunlins and redshanks within a year were restricted to a single section. Juvenile waders changed section more frequently than adults. Waders were almost as site faithful between-year as within-year. 5. A model, based on the variation in retrap rate with distance of the three least mobile wader species within a section of the Wash, was used to describe wader dispersal between roosts. The model was then used to estimate the effect of interrefuge distance on the proportion of waders reaching at least one refuge during normal roost movements. 6. Between roosts waders dispersed over short distances within a section of the Wash. Placing refuges 2, 4 and 7 km apart for grey plovers, 2.5, 5 and 10 km apart for dunlins within-year, and 3.5, 5.5 and 9.5 km apart for redshanks put the refuges within reach of 90, 75 and 50% of the respective populations of these species during their normal roost movements. To validate the sectional approach, the analysis was repeated using roosting movements of dunlins over another section and over the whole Wash; the results were similar to those obtained in the original analysis. 7. The implications of wader dispersal for refuge creation are discussed.

TEST OF A BEHAVIOR-BASED INDIVIDUAL-BASED MODEL: RESPONSE OF SHOREBIRD MORTALITY TO HABITAT LOSS

In behavior-based individual-based models (IBMs), demographic functions are emergent properties of the model and are not built into the model structure itself, as is the case with the more widely used demography-based IBMs. Our behavior-based IBM represents the physiology and behavioral decision making of individual animals and, from that, predicts how many survive the winter nonbreeding season, an important component of fitness. This paper provides the first test of such a model by predicting the change in winter mortality of a charadriid shorebird following removal of intertidal feeding habitat, the main effect of which was to increase bird density. After adjusting one calibration parameter to the level required to replicate the observed mortality rate before habitat loss, the model predicted that mortality would increase by 3.65%, which compares well with the observed increase of 3.17%. The implication that mortality was density-dependent was confirmed by predicting mortality over a range of bird densities. Further simulations showed that the density dependence was due to an increase in both interference and depletion competition as bird density increased. Other simulations suggested that an additional area of mudflat, equivalent to only 10% of the area that had been lost, would be needed by way of mitigation to return mortality to its original level. Being situated at a high shore level with the flow of water in and out impeded by inlet pipes, the mitigating mudflat would be accessible to birds when all mudflats in the estuary were covered at high tide, thus providing the birds with extra feeding time and not just a small replacement mudflat. Apart from providing the first, and confidence-raising, test of a behavior-based IBM, the results suggest (1) that the chosen calibration procedure was effective; (2) that where no new fieldwork is required, and despite being parameter rich, a behavior-based IBM can be parameterized quickly (few weeks), and thus cheaply, because so many of the parameter values can be obtained from the literature and are embedded in the model; and (3) that behavior-based IBMs can be used to explore system behavior (e.g., the role of depletion competition and interference competition in density-dependent mortality).

Demographic consequences of migratory stopover: linking red knot survival to horseshoe crab spawning abundance

Understanding how events during one period of the annual cycle carry over to affect survival and other fitness components in other periods is essential to understanding migratory bird demography and conservation needs. Previous research has suggested that western Atlantic red knot (Calidris canutus rufa) populations are greatly affected by horseshoe crab (Limulus polyphemus) egg availability at Delaware Bay stopover sites during their spring northward migration. We present a mass-based multistate, capture-recapture/resighting model linking (1) red knot stopover mass gain to horseshoe crab spawning abundance and (2) subsequent apparent annual survival to mass state at the time of departure from the Delaware Bay stopover area. The model and analysis use capture-recapture/resighting data with over 16,000 individual captures and 13,000 resightings collected in Delaware Bay over a 12 year period from 1997–2008, and the results are used to evaluate the central management hypothesis that red knot populations can be influenced by horseshoe crab harvest regulations as part of a larger adaptive management effort. Model selection statistics showed support for a positive relationship between horseshoe crab spawning abundance during the stopover and the probability of red knots gaining mass (parameter coefficient from the top model b = 1.71, = 0.46). Our analyses also supported the link between red knot mass and apparent annual survival, although average estimates for the two mass classes differed only slightly. The addition of arctic snow depth as a covariate influencing apparent survival improved the fit of the data to the models (parameter coefficient from the top model b = 0.50, = 0.08). Our results indicate that managing horseshoe crab resources in the Delaware Bay has the potential to improve red knot population status.

The response of common terns Sterna hirundo to power lines: An assessment of risk in relation to breeding commitment, age and wind speed

Abstract Common terns breed within the industrial complex at Shotton Steel Works, North Wales. Each journey between the breeding colony and the main feeding areas on the Dee estuary requires the terns to negotiate two spans of power lines. Here we investigated the risk of collision with power lines in relation to the time demands on adults, the age of birds and wind speed. Systematic observations were made of terns journeying to and from the estuary during four phases of the breeding season (courtship, incubation, nestling and juvenile). Casualty surveys of the ground beneath the power lines were also completed, and in order to quantify the potential removal rate of bird casualties by predators, bird corpses were placed underneath the power lines and recounted after two, five and 14 days. There was a greater than three-fold increase in the frequency of combined journeys made by terns from courtship to the nestling phase and this increase coincided with an increase in the proportion of adult terns which passed under or between the wires of the power lines. Terns also flew closer on average to the top earth wire during the nestling and fledging phases than during the courtship or incubation phases. Juvenile terns flew consistently closer to wires than adults with most juvenile crossings being Common terns are agile birds and suffered only low rates of direct mortality through collisions with power lines. However, the demands placed on parent birds during the breeding season caused a significant increase in their vulnerability to collision. This is a factor which has not previously been studied and has implications for a wider range of species.

Staging Behavior in Red KNOT (Calidris Canutus) in Delaware BAY: Implications for Monitoring Mass and Population Size

Abstract.— Many migratory birds use staging sites to gain essential resources to fuel their ongoing migration. Understanding staging strategies reveals much about migration systems and is essential if one is concerned with monitoring population trends and mass gains, two of the principal methods for assessing the “health” of a migratory population. In spring 2004, we investigated the staging behavior in Delaware Bay of Red Knot (Calidris canutus) using mark-recapture techniques and resightings of birds marked in the preceding spring. Individuals staged for 11–12 days, which declined to 8–10 days late in the season. Arrivals were asynchronous, but departures tended to be synchronized. A simple sensitivity analysis showed that the mark-recapture analysis estimated length of stay to within +10% and confirmed biases in monitoring trends and mass gains using peak counts and mass-on-date regressions. Alternative methods using staging duration to estimate passage population size and mass gains were shown to be unbiased. Using these methods, we estimated a passage population size in 2004 of 18,000 Red Knot that arrived at an average mass of 111 g and, on average, gained mass at 7.2 g day-1. Thus, in 2004, the passage population was substantially smaller than the recent peak count of 50,360 in 1998, which confirms a significant decline in the number of Red Knot staging in Delaware Bay. Use of refined techniques such as these is essential if management decisions such as those in Delaware Bay are to be based on firm scientific advice.

Shorebird Communities on British Estuaries: Factors Affecting Community Composition

Environmental gradients across 109 British estuaries were examined in relation to the communities of shorebirds in winter. Ordination and classification techniques were employed on the 5-year means of peak-winter high-tide counts for the 13 most common species, converted to densities, for the two periods, 1969-75 (first period) and 1981 -85 (second period), in order to quantify changes in community composition between the two periods. Multiple regression within GLIM was then used to relate the DCA ordination scores to physical, climatic, geographic and water chemistry variables. In an ordination analysis of the combined data for the two periods, axis 1 had a left-right gradient of species favouring coastal lagoons (avocet) to those favouring sandy estuaries (bar-tailed godwit, sanderling, knot)

Birds of the Severn Estuary and Bristol Channel: Their current status and key environmental issues

The Severn Estuary and Bristol Channel encompass a number of designated sites supporting populations of waterbirds and seabirds that are of national or international importance, including the Severn Estuary and Burry Inlet Special Protection Areas (SPAs)/Ramsar Sites and Carmarthen Bay, the UKs first marine SPA. Here, we provide an overview of the present numbers and trends of the waterbirds and seabirds using these sites, updating previous reviews undertaken prior to these designations. We further provide a summary of the main issues that have affected the status of the areas bird populations. Declines in the numbers of waders on the Severn Estuary and the southwest over the last two decades have been linked to climate change. The Sea Empress oil-spill impacted both breeding seabirds and the wintering Common Scoters in Carmarthen Bay, though numbers of the latter recovered 3years after the spill. At the Burry Inlet, Oystercatcher numbers have fallen over the last 25years and considerable research has been undertaken into the conflict with cockle and mussel fisheries. A long-term study at Cardiff Bay, at the mouth of the Severn, revealed a significant impact on the survival of Redshanks following its impoundment and has helped to further understanding of responses of waterbirds to estuarine habitat loss. The potential impacts of the construction of a tidal power scheme on the Severn Estuary are also discussed.