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).

Impacts of man-made landscape features on numbers of estuarine waterbirds at low tide.

The potential impact of human disturbance on wintering waterbirds using intertidal mudflats was considered by relating their numbers to the presence of nearby footpaths, roads, railroads, and towns. Data were obtained for six English estuaries from the Wetland Bird Survey Low Tide Count scheme. Counts were undertaken monthly from November to February, and data were available for an average of 2.8 years per estuary for the period 1992-1993 to 1999-2000. Count sections and the positions of man-made landscape features were mapped using a GIS. Generalized linear models tested whether bird numbers varied according to the estuary, month, area, whether or not the section bordered water, and the proportion of each section within a specified distance of each landscape feature. In addition, the proximity of sections to the nearest footpath access point was considered. Numbers of six of nine species, northern shelduck (Tadorna tadorna), red knot (Calidris canutus), dunlin (Calidris alpina), black-tailed godwit (Limosa limosa), Eurasian curlew (Numenius arquata) and common redshank (Tringa totanus), were significantly lower where a footpath was close to a count section, while those of brant (Branta bernicla) were greater. Northern shelduck, black-bellied plover (Pluvialis squatarola), dunlin, and black-tailed godwit numbers were reduced close to railroads and those of common ringed plover (Charadrius hiaticula), black-bellied plover, and Eurasian curlew close to roads. Common ringed plover numbers were greater close to towns. The relative distances to which species were affected by footpaths corresponded to published information concerning their flight distances in response to human disturbance. The study provided evidence that sustained disturbance associated with footpaths, roads, and railroads reduced local habitat quality for waterbirds and the carrying capacity of estuaries.

Impacts of disturbance from construction work on the densities and feeding behavior of waterbirds using the intertidal mudflats of Cardiff Bay, UK.

The impact of disturbance from construction work around Cardiff Bay, south Wales, on the densities and feeding behavior of seven waterbird species was studied over an 11-year period. Construction of a barrage across the mouth of the bay has subsequently resulted in its impoundment; other major works included the construction of a bridge carrying a divided highway. Construction work disturbance significantly reduced the densities of five species--green-winged teal (Anas crecca), Eurasian oystercatcher (Haematopus ostralegus), dunlin (Calidris alpina), Eurasian curlew (Numenius arquata), and common redshank (Tringa totanus)--on adjacent intertidal mudflats, and thus the overall carrying capacity of the bay. Construction work also reduced the feeding activity of Eurasian oystercatcher, dunlin, and common redshank on these mudflats. The possible impact of the loss of birds from these mudflats upon the populations that the bay supported is discussed. Evidence from other local studies suggests that the displacement of common redshank from these mudflats did not contribute to a decline in this species.