Critical thresholds of disturbance by people and raptors in foraging wading birds

Intertidal areas support during the non-breeding season many wading birds Charadrii that may often take flight in response to the presence of people or of birds of prey on their intertidal feeding and roosting grounds. Disturbance can cause birds to spend energy flying away and to lose feeding time while relocating to different feeding areas, where the increased bird densities may intensify competition from interference and, if of sufficient duration, from prey depletion. Until now, there has been no method for establishing how frequently birds can be put to flight before their fitness is reduced. We show how individual-based behavioural models can establish critical thresholds for the frequency with which wading birds can be disturbed before they die of starvation. It uses oystercatchers Haematopus ostralegus in the baie de Somme, France where birds were put to flight by disturbance up to 1.73 times/daylight hour. Modelling shows that the birds can be disturbed up to 1.0-1.5 times/h before their fitness is reduced in winters with good feeding conditions (abundant cockles Cerastoderma edule and mild weather) but only up to 0.2-0.5 times/h when feeding conditions are poor (scarce cockles and severe winter weather). Individual-based behavioural models enable critical disturbance thresholds to be established for the first time.     

Modelling the effect of environmental change on shorebirds: A case study on Poole Harbour, UK

An individuals-based model, MORPH, was used to assess the quality of Poole Harbour, UK, for five overwintering shorebirds: dunlin Calidris alpina, redshank Tringa totanus, black-tailed godwit Limosa limosa, oystercatcher Haematopus ostralegus and curlew Numenius arquata. Site quality, and the effect of environmental change, was measured as predicted overwinter survival. Dunlin had the highest prey biomass densities and were the least likely to be affected by reductions in their food supply, lower temperatures or loss of terrestrial habitats. Black-tailed godwits and curlew had the lowest prey biomass densities and were the most likely to be affected by reductions in their food supply, lower temperatures and loss of terrestrial habitats. All five shorebird species were seriously affected by simulated sea-level rise. Conservation issues identified for the Poole Harbour SPA were the relatively low densities of larger size classes of polychaete worms, the importance of maintaining and managing surrounding terrestrial habitats and the effect of sea-level rise on the length of time for which intertidal food supplies are available.     

Roost availability may constrain shorebird distribution: Exploring the energetic costs of roosting and disturbance around a tropical bay

High tides force shorebirds from their intertidal feeding areas to refuges known as roosts. This paper explores the energetic costs of roost disturbance of great knot (Calidris tenuirostris) and red knot (C. canutus) at Roebuck Bay, North-western Australia, assessing disturbance levels at different roost sites through direct observation and automatic radio-telemetry, and applying physiological equations and predictive roost choice models to estimate energetic costs of disturbance through a complete tidal cycle. The study area had a variety of roosts, but use of each was constrained by conditions of tide and time. The roost most suitable for shorebirds on daytime high tides of intermediate height experienced high levels of disturbance from both natural sources (birds of prey) and humans. Flight costs caused by disturbance at this site exceeded the costs of flying to and roosting at the nearest alternative roost, 25 km away. However, shorebirds did not roost at the alternate site, possibly because of the risk of heat stress in a prolonged flight in tropical conditions. Increases in disturbance levels at just one of the roost sites of Roebuck Bay would increase energetic costs substantially, and could easily reach the point at which feeding areas accessed from this roost cannot be used without incurring a net energy deficit. Roost availability can therefore limit access to feeding areas and hence limit population size. Adequate provision and management of roost sites is accordingly an important consideration in conservation of sites used by coastal shorebirds.     

Distance to high-tide roosts constrains the use of foraging areas by dunlins: Implications for the management of estuarine wetlands

Shorebirds are declining all around the world, mostly due to deterioration of the estuarine habitats used in winter and migration. Estuaries cover small areas, so it is essential to guarantee that shorebirds can access all the tidal flats where they usually feed at low-tide.Studying use of space by dunlins (Calidris alpina) in the Tagus estuary (Portugal), we noted that lack of suitably located high-tide roosts can limit the access of shorebirds to feeding habitats. Density of dunlins on foraging areas declined significantly with distance to the nearest roost, and fewer than 20% individuals foraged more than 5 km from two roosts where they were dye-marked.So to permit full access to feeding areas it is important to maintain a network of suitably located high-tide roosts. We developed a GIS modelling methodology to evaluate the adequacy of existing roost networks, and to estimate the consequences of losing or creating new roosts. The methodology requires maps with the location of roosts and foraging habitats, and knowledge of the distances that birds are willing to fly to reach foraging areas. It quantifies the proportion of foraging areas close to the existing roosts and the average distance that birds have to fly to reach potential feeding sites.Applying this methodology to the Tagus estuary we concluded that lack of roosts probably explains why the intertidal flats in the north-west of the estuary are underused by shorebirds. A modelling exercise suggested that this gap could be eliminated by creating a roost in an old drained wetland area. We also modelled the impact of the loss of two roosts that are currently threatened. Without them almost half of the available feeding areas will be too far from roosts to be efficiently used by dunlins, and possibly by other shorebirds.     

Landscape context mediates influence of local food abundance on wetland use by wintering shorebirds in an agricultural valley

While it is widely understood that local abundance of benthic invertebrates can greatly influence the distribution and abundance of wetland birds, no studies have examined if wetland landscape context can mediate this relationship. We studied the influence of wetland food abundance and landscape context on use of agricultural wetlands by wintering dunlin (Calidris alpina) and killdeer (Charadrius vociferus) in the Willamette Valley of Oregon, USA, over two winters (1999-2000, 2000-2001) of differing rainfall and subsequent habitat distribution. We monitored bird use (frequency of occurrence and abundance) at a sample of wetlands differing in local food abundance (density and biomass) and landscape context [adjacent shorebird habitat (defined as ha of wet habitat with less than 50% vegetative cover and within a 2-km radius) and nearest neighbor distance]. We evaluated predictive models for bird use using linear regression and the Cp criterion to select the most parsimonious model. During the dry winter (2000-2001), dunlin exhibited greater use of sites with higher invertebrate density and biomass but also with more adjacent shorebird habitat and closest to a wetland neighbor. However, neither landscape context nor food abundance were important predictors of dunlin use during the wet winter (1999-2000). Use of sites by killdeer was unrelated to either local food abundance or landscape context measures during both winters. Our findings contribute to a growing recognition of the importance of landscape structure to wetland birds and highlight a number of implications for the spatial planning and enhancement of wetlands using a landscape approach.     

Factors affecting the behavioural responses of whooper swans (Cygnus c. cygnus) to various human activities

The effects of human activity on bird behaviour and distribution have been studied extensively in recent years, but variation in their response to disturbance is still poorly understood. Here, we analyse variation in the behaviour of wintering whooper swans Cygnus c. cygnus, to determine whether their susceptibility to human activity changes with time, location and the type of disturbance involved. Overall, the swans' feeding activity varied within and between years, and in relation to feeding site, but there was less variation in the amount of time spent alert. Disturbance frequency resulting from human activity was lower with increasing flock size and with increased distance to the nearest road or track. Distances that humans could approach before alerting the birds similarly varied with field characteristics (e.g. size and proximity to roads or tracks), and also with the type of disturbance involved. The distance at which >5% of the flock became alert because of human activity decreased with the number of previous disturbance incidents in the day, indicating that swans become less sensitive to disturbance events if daily disturbance frequency is high, but there was no evidence that habituation to disturbance persisted over longer periods. The time taken for the birds to resume undisturbed behaviour varied with the duration of the disturbance event, which in turn depended on the type of disturbance involved, with pedestrians alerting the birds for longer periods than vehicles and aircraft. Recovery rates following disturbance were also associated with field size, flock size and the proportion of the flock alerted. Feeding activity was influenced by a range of variables, including year, season, field location, crop type and the number of days that the flock had used the field (32.9% of variance in the data explained by these variables), with disturbance factors explaining an additional 4.9% of variance in the proportion feeding per hour. Conversely, alert activity was influenced mainly by disturbance events. The range of factors influencing the swans' feeding behaviour, and variability in their response to human activity, has implications for management programmes and for attempts to predict the effects of human activity on the birds at a local and larger scale.     

Nakdong estuary barrage and reclamation project: Preliminary results of the botanical,macrozoobenthic and ornithological studies

The Nakdong Estuary Barrage and Reclamation Project has been investigated by studies on the vegetation, macrozoobenthos and birds in the Nakdong river delta, Republic of Korea. This study deals with the preliminary results, obtained during October to November 1983, while the environmental impacts of the barrage were being discussed.The total biomass of Scirpus triqueter, the estuary's most important foodplant for waterfowl (mainly for swans), was assessed at approximately 38·3 tonnes dry wt. Of this amount only the roots and rhizomes (9·6 tonnes dry wt) are expected to be consumed by birds. Aquatic weeds in the nearby fresh Jukrim branch provided another food source (5·8 tonnes dry wt).The wintering waterfowl also used other feeding areas such as farmland and freshwater impoundments—almost all herbivorous birds participated in nightly feeding migrations.At 12 stations, spread over 20·5 km² of intertidal flats of the estuary, a total of 40 macrozoobenthic species were found. Crustaceans dominated with 19 species, while molluscs and polychaetes were represented by 11 and 10 species, respectively. For all stations together an average biomass of 14·6 g ash-free dry wt (ADW) m⁻² (range 3·1–51·9 g) was calculated, dominated by molluscs with 9·5 g, while the share of crustaceans and polychaetes was only 2·9 g and 2·2 g, respectively. In terms of weight, the bivalve Corbicula japonica was dominant with an average of 4·4 g ADW m⁻².During three censuses 10 000 waders and 25 000 to 30 000 waterfowl were counted. Among them were two species not recorded before in Korea: the ruff and the canvasback. Of all nonpasserines the wigeon (with a peak of 6800) and the dunlin (with a peak of 9100) were most numerous, while the ducks proved to be the most dominant (64%) group of birds.In the beginning of October the wader density in the estuary amounted to 5·2 birds ha⁻¹ intertidal area exposed at low tide.     

Inland feeding by brent geese branta bernicla in Sussex,England

The numbers and pattern of distribution of dark-bellied brent geese Branta b. bernicla feeding inland around Chichester and Pagham Harbours, West Sussex in winter 1979/80 are described.A maximum of 11,000 birds fed inland. Taking the winter as a whole, 70% fed within 200 m of the coast. 68% of inland-feeding birds occurred on grass. This was partly influenced by deliberate scaring of the geese from cereals. 14% of the available area of grass around Chichester was used by geese but only 6% was heavily grazed. Of the area of cereals, 13% was used with 4% heavily grazed.Unless disturbed, the brent geese usually remained inland for most of the day, regardless of the state of the tide, returning to roost overnight on intertidal areas. The more uniformly distributed food supply in fields enabled the geese to feed in larger, more compact flocks than they could do on the estuary.The provision of grassland refuges is suggested as a means of alleviating conflict with agriculture. For Chichester Harbour up to five refuge areas may be required where the geese can feed undisturbed. Each refuge should not only provide the food requirements of the geese but be large enough to provide a buffer against human disturbance on surrounding land. The total refuge area should probably be not less than about 400 ha, on which many of the normal farming practices can continue. Refuges should be situated adjacent to the coast and the grass sward needs to be closely grazed by livestock or mown in the summer to provide a suitable short sward for the wintering geese.     

Movements of wintering shorebirds within the Firth of Forth: Species differences in usage of an intertidal complex

The movements of seven species of waders within and through the estuarine complex of the Firth of Forth, Scotland, were studied by dye-marking and ringing in three winters, 1978–1981. The species studied formed two groups (which may be part of a continuum). Grey plovers Pluvialis squatarola, turnstones Arenaria interpres, oystercatchers Haematopus ostralegus and redshanks Tringa totanus tended to stay within the same part of the estuary throughout the winter. Bar-tailed godwits Limosa lapponica, dunlins Calidris alpina and knots C. canutus ranged more widely. Knots dispersed rapidly throughout the whole of the Firth, and probably elsewhere. Retraps of ringed birds indicated year-to-year differences broadly similar to the within-winter ones. The results are compared with data from elsewhere, and are discussed in relation to large-scale reclamation proposals for the Forth.     

The distribution,population size,movements and conservation of the African black oystercatcher Haematopus moquini

The African black oystercatcher Haematopus moquini is endemic to southern Africa and occurs as a breeding species between Seal Island, South West Africa/Namibia and Mazeppa Bay, Transkei. Vagrants outside this area are rare further north than the Hoanib Estuary, South West Africa/Namibia and further east than Transkei. The present adult population is estimated to be 4781 birds of which 53% are found in the southwestern Cape Province, South Africa. Offshore islands support 29% of the world population and are identified as important localities for African black oystercatchers with a high breeding production. These birds face threats from human disturbance, expecially from off-road vehicles on the mainland, from introduced mammalian predators at islands and from competition with man for food and bait organisms. Ten islands and seven mainland sites are isolated as supporting more than 1% of the world population, and, together with their intertidal zones, are recommended for Nature Reserve status.     

Bird collisions with power lines: Failing to see the way ahead?

Visual fields were determined in three bird species representative of families known to be subject to high levels of mortality associated with power lines; kori bustards Ardeotis kori, Otididae, blue cranes Anthropoides paradisea, Gruidae and white storks Ciconia ciconia, Ciconiidae. In all species the frontal visual fields showed narrow and vertically long binocular fields typical of birds that take food items directly in the bill under visual guidance. However, these species differed markedly in the vertical extent of their binocular fields and in the extent of the blind areas which project above and below the binocular fields in the forward facing hemisphere. The importance of these blind areas is that when in flight, head movements in the vertical plane (pitching the head to look downwards) will render the bird blind in the direction of travel. Such movements may frequently occur when birds are scanning below them (for foraging or roost sites, or for conspecifics). In bustards and cranes pitch movements of only 25° and 35° respectively are sufficient to render the birds blind in the direction of travel; in storks head movements of 55° are necessary. That flying birds can render themselves blind in the direction of travel has not been previously recognised and has important implications for the effective mitigation of collisions with human artefacts including wind turbines and power lines. These findings have applicability to species outside of these families especially raptors (Accipitridae) which are known to have small binocular fields and large blind areas similar to those of bustards and cranes, and are also known to be vulnerable to power line collisions.     

A strategy for baseline monitoring of estuary Special Protection Areas

Intertidal estuaries in north-west Europe provide essential feeding grounds for internationally important numbers of overwintering shorebirds. Monitoring of the food supply available in these intertidal habitats is central to shorebird conservation strategy. In this paper, we describe the methods used to do a baseline survey of the Exe estuary Special Protection Area, south-west England. A grid system of sampling, linked to the Ordnance Survey National Grid, was used to ensure a systematic and repeatable coverage of the whole estuary. Cluster analysis was used to classify sites according to their invertebrate biomass densities. Classification of sites into groups was verified using environmental correlates. Results from these analyses were used to map habitat patches for the whole estuary. We show how, once a baseline survey such as this has been undertaken, monitoring of shorebird food supplies over time should be achievable with reduced sampling effort.     

Conservation of Pteropus livingstonii based on roost site habitat characteristics on Anjouan and Moheli, Comoros islands

Livingstone's fruit bats, Pteropus livingstonii, (Pteropodidae: Chiroptera) are endemic to only two islands, Moheli and Anjouan of the Comoros Islands in the West Indian Ocean. With an estimated population of less than 2000 individuals, P. livingstonii is listed as Critically Endangered on the International Union for the Conservation of Nature's Red List. Yet the habitat characteristics of these giant fruit bats, or flying foxes, are poorly known. I conducted field research during 5 months in 1998 focused on finding new roost sites, measuring characteristics of both roost sites and roost trees at new as well as previously identified roost sites, and recording the same characteristics for a set of absence sites. I identified six new roost sites allowing me to measure the characteristics of 15 known roost sites and seven absence sites. The study spanned both dry and rainy seasons and I found a significant increase in the number of individuals at roost sites during the rainy season. Bat roost sites were associated with presence of water, southeast aspect, steep slopes, and presence of several native tree species. Roost habitat was characterized based on biotic, abiotic and human factors. This research on habitat characteristics is contributing to the development of a conservation action plan for the Livingstone's fruit bats. However, further research on dry season roost sites and distribution as well as feeding sites is necessary in order to better understand their seasonal habitat requirements and therefore ensure the conservation of P. livingstonii.     

Effects of disturbance and habitat loss on papyrus-dwelling passerines

Throughout the world, but particularly in tropical regions habitat loss and disturbance are considered detrimental to biodiversity. We examine the effects of disturbance by harvesting, burning and habitat fragmentation on six bird species associated with papyrus (Cyperus papyrus) swamps. The presence or absence of these species was verified in 93 wetlands in southern Uganda between June and August 2003. Disturbance was estimated directly by observation and indirectly from examining vegetation structure. Habitat fragmentation was quantified by delineating swamps on Landsat ETM images and applying ‘Fragstats’ to calculate relevant patch metrics. The occurrence of all six species was affected positively by increased swamp size, but birds were more likely to occur in small swamps at high altitude. The shape and proximity of swamps to neighbouring wetlands had little effect on occurrence. As altitude, habitat fragmentation and disturbance were correlated, determining avian responses to disturbance was problematic. However, the occurrence of all species was affected by disturbance, often showing unimodal responses, if disturbance was not considered in conjunction with habitat fragmentation. When the effects of habitat fragmentation and disturbance were analysed together, only the occurrences of papyrus yellow warbler and white-winged warbler were affected (positively) by disturbance. Results suggest that papyrus-dwelling passerines, except papyrus gonolek, are tolerant of low intensities of disturbance, a novel finding in the tropics. We recommend that policy-makers do not prohibit harvesting, thus generating good will and encouraging rural householders to comply with additional conservation policy.     

Identifying road mortality threat at multiple spatial scales for semi-aquatic turtles

Prioritizing sites for localized mitigation measures, and forecasting the effect of interventions on an endangered population, requires an understanding of the spatial scales at which threat processes operate. Road mortality is among the greatest threats to semi-terrestrial freshwater turtles due to the group’s life-history traits. Declining throughout much of their range, spotted (Clemmys guttata) and Blanding’s turtles (Emydoidea blandingii) are exposed to high road densities and traffic volumes in the northeastern United States. We examine the distribution of roadkill risk for spotted and Blanding’s turtles at three spatial scales. Tortuosity during upland movements was used to predict road-crossing locations at the single-movement scale. A gravity model of wetland-to-wetland interactions was then developed to identify road mortality hot spots at a broader road segment scale. Finally, road-crossing risk was assessed at the scale of focal areas that support distinct populations, using a population viability analysis to evaluate the consequences of road mortality on resident populations. The observed spatial variability of road mortality risk was high for single road crossing movements, limiting the effectiveness of static mitigation measures conducted at this scale. At the broader road segment scale, road mortality hotspots were evident. The demographic risk associated with roads varied widely among discrete populations, with probabilities of extinction over 100-year projections reaching 5.1% for spotted turtles, and 58.8% for Blanding’s turtles. We conclude that conservation interventions are most likely to be effective in mitigating the effects of road mortality when implemented at the road segment and population scales.     

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

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.     

Roost selection by barbastelle bats (Barbastella barbastellus, Chiroptera: Vespertilionidae) in beech woodlands of central Italy: consequences for conservation

We investigated roost selection by Barbastella barbastellus in a mountainous area of central Italy. Twenty-five bats, mostly lactating females, were radio-tracked to 33 roost trees. Trees in unmanaged woodland were favoured as roost trees; woodland subject to limited logging was used in proportion to availability, and areas where open woodland and pasture occurred were avoided. Selection depended on tree condition (dead beech trees were preferred) and height (roost trees were taller than random ones). Cavity selection was based on cavity type, height and entrance direction: roost cavities were mainly beneath loose bark, at a greater height above ground and facing south more frequently than random cavities. Untouched areas of mature woodland should be preserved to provide roosting conditions for B. barbastellus. In logged areas, harvesting protocols should save dead and mature trees; frequent roost switching and small colony size imply that large numbers of roost trees are needed.     

Environmental factors and spatial scale influence shorebirds’ responses to human disturbance

The extent of a shorebird’s response to a human disturbance depends on the associated energetic or predation risk costs. These costs are influenced by a suite of environmental variables, operating at several temporal and spatial scales. Here, I measured prey availability, distance to forest cover, cloud cover, and wind speed, in addition to human and shorebird densities to examine how human presence affected habitat choice, relative to these environmental variables, at Pachena Beach, British Columbia. In a standardised experiment, I also approached feeding flocks to determine if environmental factors influenced the time take for shorebirds to resume feeding. Binary logistic models suggested that people did not displace shorebirds. Instead, shorebirds were preferentially selecting areas further from forest cover that may have had lower predation risk. The time taken for shorebirds to resume feeding after a human disturbance was greater in the morning and in areas of low prey availability. This suggests that shorebirds respond more to a disturbance when the foraging cost is lower indicating that behavioural responses may not necessarily reflect the potential fitness costs of human disturbance.     

Traditional salt-pans hold major concentrations of overwintering shorebirds in Southeast Asia

Shorebirds are declining worldwide due to loss and degradation of critical breeding and wintering habitats. Some human-modified habitats, particularly salt-pans which are used by shorebirds in many regions of the world, may help substitute for natural habitats lost for a wide range of species during migration. We studied the influence of landscape characteristics on species richness, abundance, and diversity of shorebirds at 20 sites covering most of the Inner Gulf of Thailand, a landscape with a long history of salt farming. Sites with salt-pans present held significantly higher species richness, abundance and diversity of shorebirds. Areas with larger proportions given over to aquaculture tended to have lower species richness, abundance and diversity. Generalized additive models indicated that landscapes with a larger proportion of tidal flats in conjunction with salt-pans were the best predictors of sites with higher species richness, abundance and diversity. Landscape configurations with higher richness, abundance and diversity of shorebirds also tended to be less fragmented and contained slightly larger patches. Shorebirds appeared to use ponds with exposed mud in salt-pans as both roosting sites and supplementary feeding grounds during high tide. Traditional salt-pans therefore proved to contribute significantly to maintenance of overwintering shorebird populations in this landscape and should be investigated elsewhere in Asian coastal zones. Collaboration between researchers, salt farmers and planning authorities as to how best to maintain salt-pans as potential shorebird roost sites such as in the Inner Gulf of Thailand is urgently needed in order to maintain habitat for shorebird populations in critical wintering and staging areas of this flyway.     

Reducing incidental mortality of seabirds with an underwater longline setting funnel

A demersal longline fishery for Patagonian toothfish (Dissostichus eleginoides) that commenced off the Prince Edward Islands during 1996 has killed significant proportions of locally breeding albatrosses and petrels. As one of a suite of mitigation measures, we tested the efficacy of a Mustad underwater setting funnel to reduce incidental mortality of seabirds. The funnel, which deploys the longline 1-2 m beneath the sea surface, was used on 52% of 1714 sets (total effort 5.12 million hooks) over a 2-year period. Used in conjunction with a bird-scaring line, overall seabird bycatch rate was low (0.022 birds per 1000 hooks), and was dominated by white-chinned petrels (Procellaria aequinoctialis) (88% of the 114 birds killed). Bycatch rate was three times lower when the funnel was used both by day and at night. Daytime catch rates with the funnel were less than those attained during night sets without the funnel. In conjunction with other mitigation measures, underwater setting offers a significant reduction in seabird mortality in this fishery and could increase fishing efficiency by allowing daytime setting. However, small numbers of albatrosses were caught during daytime sets with the funnel, and its use for daytime sets should be closely monitored.