Migratory Waders Research Articles

Hunter Valley wetland birds raise conservation issues.

The last few years, particularly 1982-83, have been exciting times for ornithologists in the Newcastle area. Unusual seasonal conditions in the area, coupled with Australia wide drought have resulted in remarkable surges in the population of water birds frequenting the Lower Hunter estuary wetlands and have again caused focus to be turned to controversial issues. Egrets have returned to nest after having ceased breeding in the area in the early 1970’s, this time to establish a rookery close to a built-up area within the city boundary. The first recorded Australian sighting of the Hudsonian godwit, a North American wader, on Kooragang Island, the breeding of black swans in suburban lagoons, a visit to these same lagoons by a jabiru, and the arrival of the rare freckled duck and large flocks of glossy ibis have been just some of the highlights. These occurrences have attracted attention Australia wide. The arrival of the godwit in early 1983 brought more than 100 bird enthusiasts to Newcastle from as far afield as Queensland, Northern Territory and Victoria, as well as other New South Wales centres, hoping to catch a glimpse of the rare visitor amongst the hoses of migratory waders on the mud and sand flats of Kooragang Island. This island, which has been established to carry a population of between 10,000 and 20,000 birds, is well known to Australian naturalists. It has even attracted attention overseas because of the threat of encroaching industrialisation to the wintering habitat of some 190 species, with an article in the New York Times of September 14, 1974, entitled “Australian Factory Site Would Peril Bird Migration” (Stewart, 1974).

Extended Primary Moult as an Adaptation of Adult Wood SandpipersTringa glareolato Their Freshwater Habitats in Southern Africa

Migrant waders using freshwater habitats are hypothesized to have slower primary moult than waders using coastal habitats. We chose the Wood Sandpiper Tringa glareola as a representative species using the freshwater habitats and compare its moult pattern with a range of fresh-water and coastal wader species to test the habitat hypothesis. Only fragmentary descriptions of Wood Sandpipers' primary moult in their sub-Saharan non-breeding quarters had existed. We analysed the primary moult formulae of 1496 adult Wood Sandpipers obtained in southern Africa. The Underhill & Zucchini moult model was used to estimate the timing and duration of moult for all 10 primaries combined and for each primary individually. We also estimated the rate of production of feather material during moult. Adult Wood Sandpipers arrive in southern Africa between late July and November, and depart from mid-March to April. Suspension of moult was observed in 56 birds (7.5%) after two to nine primaries had been replaced. The remaining birds performed a continuous complete primary moult, with average start and completion dates of 21 August and 30 December, respectively; estimated duration was 131 days. The overall rate of production of primary feather material was uniform, achieved by growing up to five small inner primaries simultaneously at the beginning of the moult but only one or two simultaneously while the large outer primaries were growing. Primary moult of adult Wood Sandpipers took longer but ended earlier than in similar-sized waders using coastal habitats. Compared with waders using coastal habitats, Wood Sandpipers prolonged moult by shedding their primaries at longer intervals and by extending the growth period of each primary. The longer primary moult and its earlier ending compared with coastal waders are probably adaptations to Wood Sandpipers' use of freshwater habitats, which in southern Africa provide unpredictable food supplies and might require nomadic movements between ephemeral inland wetlands.

Transfer of heavy metals in the food chain earthworm Black-tailed godwit (Limosa limosa): Comparison of a polluted and a reference site in The Netherlands

The Black-tailed godwit (Limosa limosa) is a migratory wader that favours wet meadows for breeding. The species has a Red List status in The Netherlands, as it strongly declined in numbers since the 1960s. Intensification of agriculture and land use change resulting in habitat loss are considered major causes of this decline. In some areas the breeding habitat is contaminated with heavy metals. Adult godwits mainly feed on earthworms in the breeding season, which are known to accumulate heavy metals from the soil. In this paper we investigate the transfer of heavy metals from the soil to the Black-tailed godwit, which may have an additive negative effect on the viability of local populations. We measured heavy metal concentrations in soil, earthworms, and godwit eggs and feathers at a polluted and a reference site. The results suggest that Lead, Mercury and Cadmium are transferred from the soil to godwits even though the species spends only a few months in the breeding area during the year.

Steep past and future population decline in an arctic wader: dynamics and viability of Baltic Temminck's stints Calidris temminckii

Decline of migrant wader populations worldwide implies that their demography may be affected by phenomena occurring in a large geographical scale. Knowledge about vital rates affecting population growth and viability helps in finding the processes behind the declines. We estimated the rate of population growth (λ) of a dramatically declined population of Temminck's stints Calidris temminckii using a ten year capture–recapture data. We assessed the viability of the population by projections based on λ and its variation. We also studied sensitivity of λ to changes in vital rates (breeding success, recruitment and adult survival). Both adult and juvenile survival and breeding success were lower than c. 30 years earlier. Adult survival seemed to have dropped during the study period. Population growth appeared strongly negative and viability analyses predicted an almost certain extinction within a few decades. Adult survival formed the main component of the growth rate suggesting that population crash is mainly caused by a drop in adult survival. Still, sensitivity analyses showed that a moderate rise in adult survival or any other single vital rate would not make the population viable. Most probably, survival has dropped due to factors operating along migration routes and wintering areas, at sites shared by birds from other parts of the breeding range. Therefore, the observed population crash and decline in adult survival may indicate parallel changes in poorly monitored arctic populations. Migration habits (continental wide flyways and scattered wintering areas) imply that factors responsible for the drop in survival have a wide geographical range.

Non-breeding habitat preference affects ecological speciation in migratory waders

Models of ecological speciation predict that certain types of habitat should be more conducive to species diversification than others. In this study, I test this hypothesis in waders of the sub-order Charadrii using the number of morphological sub-species per species as an index of diversity. I classified all members of this clade as spending the non-breeding season either coastally or inland and argue that these represent fundamentally different environments. Coastal mudflats are characterised by high predictability and patchy worldwide distribution, whilst inland wetlands are widespread but unpredictable. The results show that migratory species that winter coastally are sub-divided into more sub-species than those that winter inland. This was not the case for non-migratory species. I argue that coastal environments select for more rigid migratory pathways, whilst inland wetlands favour more flexible movement patterns. Population sub-division could then result from the passive segregation of breeding sites or from the active selection for assortative mating of ecomorphs.

Transport of brine shrimps via the digestive system of migratory waders: dispersal probabilities depend on diet and season

Waterbirds are known to disperse invertebrate propagules that survive gut passage, but there is very little information about how the probability of dispersal changes at different times of the annual cycle when birds move in different directions, or how it is affected by changes in diet. We studied internal transport of brine shrimp Artemia cysts by migratory waders in the Odiel saltworks in south-west Spain. Viable cysts of parthenogenetic Artemia were abundant in the faeces and regurgitated pellets of redshank Tringa totanus, pellets of spotted redshank T. erythropus, and faeces of black-tailed godwit Limosa limosa during spring and/or autumn migrations in 2001–2002, but were not recorded during winter. Godwits did not produce pellets, and spotted redshank faeces were not sampled. Significant correlations between the number of cysts in a pellet or faecal sample and the proportion of that sample constituted by Artemia adults suggested that most cysts were ingested while in the ovisacs of gravid females. The proportion of cysts destroyed during digestion increased when accompanied by harder food items or grit, and when fewer cysts were ingested. The median number of intact cysts was higher in redshank faeces than in their pellets, but cysts extracted from pellets were more likely to hatch. A higher proportion of redshank pellets contained Artemia cysts in spring than in autumn, but more redshank migrated through the area in autumn. Significantly fewer cysts were recorded in redshank pellets in winter than in spring or autumn. Our results confirm that there is potential for long-distance dispersal of Artemia cysts via waders during both northwards (spring) and southwards (autumn) migrations.

Ecology, disturbance and restoration of coastal saltmarsh in Australia: a review

It is clear that saltmarshes are a unique and important component of the coastal biosphere of Australia. Their contribution ranges from stabilisation of fine sediments and providing an excellent protective buffer between land and sea, to their diverse blend of terrestrial and marine fauna. Further, saltmarsh plants are highly specialised and adapted to fill a harsh niche allowing them to act in roles that other vegetation types cannot. Saltmarsh habitats are recognised for their importance to migratory waders under the Ramsar convention, but it is becoming increasingly evident that they are also important to a variety of commercially valuable fish and native mammal species. Activities that are detrimental to saltmarshes continue and need to be addressed in order to conserve remaining saltmarsh areas. In general, urbanisation of the catchment has lead to filling of saltmarshes, tidal restriction, use by recreational vehicles, grazing, trampling and increased sedimentation and nutrient runnoff allowing colonisation and invasion of mangroves. These disturbances have a number of ecological consequences ranging from weed infestation to complete changes in the species composition and ecology. Reversing the disturbance is not always simple and can require extensive groundwork to be successful. Rehabilitation of existing saltmarsh areas has been a successful means to enhance this habitat. In general, it requires relatively little effort to remove weeds and fence off areas to regenerate naturally. Saltmarsh areas have been shown to respond well to this type of manipulation. Restoration and creation require substantial effort and planning to ensure a successful outcome. However, given the right environmental combinations of elevation, tide and salinity, saltmarsh will establish and grow. To speed the process transplantation of saltmarsh plants can be considered either from donor sites or plants propagated in green houses.

Internal transport of seeds by migratory waders in the Odiel marshes, south‐west Spain: consequences for long‐distance dispersal

Waders (Charadriiformes) undergo particularly long migratory flights, making them ideal vectors for long‐distance dispersal. We present a study of dispersal of plant seeds by migratory waders in the Odiel saltworks in south‐west Spain. This is the first field study to demonstrate excretion of viable seeds by waders. Viable seeds of Mesembryanthemum nodiflorum (Aizoaceae), Sonchus oleraceus (Asteraceae) and Arthrocnemum macrostachyum (Chenopodiaceae) were frequent in pellets and faeces of redshank Tringa totanus, spotted redshank Tringa erythropus, and black‐tailed godwit Limosa limosa during spring and autumn migrations, but less frequent during winter. Another 11 seed types were recorded at low density. More intact M. nodiflorum seeds were present in redshank faeces than in their pellets, but seeds extracted from pellets were more likely to germinate. More S. oleraceus seeds were transported per redshank pellet in spring, but more redshank migrated through the area in autumn. The distributions of the plants transported are consistent with an important role for long‐distance dispersal by waders. M. nodiflorum and S. oleraceus are introduced weeds in the Americas and Australasia, and dispersal by birds may contribute to their rapid spread. Although S. oleraceus is generally thought to be wind‐dispersed, birds may be responsible for longer distance dispersal events.

Changes in the waterbirds and other biota of Lake Yumberarra , an episodic arid zone wetland

Lake Yumberarra is a small freshwater lake (151 ha) in the Currawinya National Park in southwestern Queensland. During 1995-2003 it held water much of the time in two fillingdrying cycles, one initiated by riverine flood water and the second by local runoff. Water in both was clear, alkaline and generally fresh, but became saline as it dried. Zooplankton comprised 27 crustaceans and a few rotifers with the copepod Boeckella triarticulata dominant. Numbers were greatest soon after filling and in each spring and least towards drying. At least 44 taxa of macroinvertebrates inhabited the littoral zone, with hemipterans and chironomids dominant. Waterplants, chiefly Myriophyllum verrucosum, grew abundantly in the first half of each filling cycle. A limited fish fauna inhabited the lake, with carp present only in the first cycle. The lake was used extensively by 59 species of waterbirds, with four foraging groups dominant at different times probably reflecting fluctuating abundance of their food resources. Breeding in the lake was not widespread, nor was the lake used much by migratory waders, unlike some other wetlands in the Paroo. Its unusually high species richness and bird densities means the lake is important for feeding and maintaining waterbird diversity in an arid landscape.

Spring arrivals of migrant waders in Iceland in the 20th century

Records of first sightings of Icelandic‐breeding waders and high‐Arctic passage migrants have been made in most parts of Iceland since 1902. Two sets of records of first sightings are used here to analyse temporal changes in arrival dates during the 20th century and the effects of weather conditions. First sightings became earlier in the first half of the 20th century, as local spring temperatures increased. They ceased to do so after 1960, when temperatures were decreasing slightly, though the associations between arrival dates and annual temperatures were weak. In years with more than five records for a species, the earliest sightings were 5–9 days before the median dates of all first sightings. Though earliest sightings were delayed in cool Icelandic springs, those median dates showed little correlation with local spring temperatures. Six species arrived later when the winter or spring values of the North Atlantic Oscillation (NAO) were high, but seven species showed no significant associations with the winter NAO. Sightings tended to be later in springs in which westerly or cyclonic systems were prevalent over Ireland and Scotland. First sightings in Iceland were substantially later than in the Outer Hebrides, though in closer synchrony with the peaks of passage there than with arrival and passage dates in Shetland and north Norway. Sustained observations at a few key sites should be useful in tracking the responses of waders to variations in climate and other environmental changes.

Autumn Migration of Waders (Charadrii) in the Middle Vistula Valley in 2004-2005

Autumn Migration of Waders (Charadrii) in the Middle Vistula Valley in 2004-2005 Studies were conducted in 2004-2005 in the period: 3 July - 26 August. The study area was located in the middle Vistula valley, near village Pawłowice, about 20 km north to the town of Dęblin (51°36'N, 21°38'E). The area covers a 2.5 km long section of not regulated river with ca 2 km2 of sandy beach. This site is an attractive resting and foraging place for migrating waders. Altogether 25 species of migrant waders were observed there. The dominants were: Lapwing (Vanellus vanellus) - 52.5-59.3%, Common Sandpiper (Actitis hypoleucos) - 9.8-18.9%, Wood Sandpiper (Tringa glareola) - 10.5-11.1%. For the Lapwing, Greenshank (Tringa nebularia), Ringed Plover (Charadrius hiaticula), Little Ringed Plover (Charadrius dubius), Common Snipe (Gallinago gallinago) and Ruff (Philomachus pugnax) the pattern of migration dynamics in the middle Vistula valley was similar to some earlier observations done in this area on the turn of the 1970s. In the Common Sandpiper in the 1970s-1980s no distinct migration peak was recorded, while in 2004-2005 one migration wave extended in time was observed. Migration of the Wood Sandpiper in the 1970s-1980s was characterized by a single migration peak, but in 2004-2005 migration pattern of this species had distinctly two peaks.

Incremental ecological exposure risks from contaminated sediments in an urban estuarine river

Estuaries in urban regions present unique environmental management challenges. Ecosystems in urban estuaries are typically impacted by habitat loss and degradation, watershed modification, and nonpoint and point sources of many chemicals. Restoring such systems requires an understanding of the relative contribution of various stressors to overall ecological conditions and an understanding of shifting patterns of stress over time. In this article, we present the results of a multiparameter environmental assessment of a quintessential urbanized waterway: the lower Passaic River in the vicinity of Newark, New Jersey, USA. To provide the foundation for effective management decision making, we quantified baseline conditions (habitat losses and degradation), chemical concentrations in sediment and biota relative to published toxic effect levels, direct toxicity of sediments to benthic organisms, and food-web mediated risks to fish-eating birds. Habitat losses have been severe (greater than 85% of wetlands, nearly 100% of the total length of tidal and nontidal tributaries, and 100% of natural shoreline habitat have been lost), resulting in substantial habitat constraints on biota. Despite this, biological communities are present in the lower Passaic. In general, concentrations of toxic chemicals in surface sediments have fallen with time, and natural recovery processes are proceeding. Chemical concentrations remain high enough to impair survival of amphipods, but not amphipod growth or polychaete growth or survival as measured in laboratory bioassays using field-collected sediment. Fish and blue crab body burdens of some metals, PCBs, and the pesticide, DDT, are at concentrations sufficiently high to exceed toxicity thresholds. The resident fish-eating bird--the belted kingfisher--is at exposure risk from some metals, PCBs, and polychlorinated dibenzo-p-dioxins and dibenzo furans (PCDD/Fs). Migratory waders--the herons and egrets--are not at risk from chemical exposure. These complex findings suggest that restoring the lower Passaic River to ecological health is a correspondingly complex task. Habitat constraints must be eased for biotic components of the ecosystem, and chemical effects must be reduced for those organisms that are present. Only a coordinated, multidisciplinary restoration program will succeed in this challenging environment.

Life history varies with migratory distance in western sandpipers Calidris mauri

The propensity of migratory waders to remain on the non‐breeding grounds during the arctic breeding season (“oversummer”) in their first biological year of life (“juveniles”) may be latitude, and thus migratory distance dependent. We compared the extent of preparation for northward migration of western sandpipers Calidris mauri spending the non‐breeding season in México and Panamá during 1995–1998. During winter residency and premigratory periods, we measured body mass and scored the extent of dull basic versus bright alternate breeding plumage of captured juveniles and adults (second biological year or older), and obtained additional plumage scores from observations of uniquely colour banded birds. Nearly all western sandpipers in México prepared for northward migration by increasing body mass and moulting into breeding plumage. In Panamá, most adults prepared for migration, but few, if any, juveniles did so. Patterns of body mass and breeding plumage development do not generally support the hypothesis that oversummering by juveniles results directly from less efficient foraging or from resource competition with adults. We suggest instead that costs directly associated with migratory distance per se influence the life history strategies of sandpipers spending the non‐breeding seasons at different latitudes. This latitudinal difference should interact with the well documented sex‐ratio cline in non‐breeding distribution (male western sandpipers predominating in northern parts of the range and females in southern parts). This suggests that females have more conservative life histories, prioritizing first year survivorship, relative to males that instead weight first‐year breeding opportunities.

Incremental Ecological Exposure Risks from Contaminated Sediments in an Urban Estuarine River

Estuaries in urban regions present unique environmental management challenges. Ecosystems in urban estuaries are typically impacted by habitat loss and degradation, watershed modification, and nonpoint and point sources of many chemicals. Restoring such systems requires an understanding of the relative contribution of various stressors to overall ecological conditions and an understanding of shifting patterns of stress over time. In this article, we present the results of a multiparameter environmental assessment of a quintessential urbanized waterway: the lower Passaic River in the vicinity of Newark, New Jersey, USA. To provide the foundation for effective management decision making, we quantified baseline conditions (habitat losses and degradation), chemical concentrations in sediment and biota relative to published toxic effect levels, direct toxicity of sediments to benthic organisms, and food-web mediated risks to fish-eating birds. Habitat losses have been severe (greater than 85% of wetlands, nearly 100% of the total length of tidal and nontidal tributaries, and 100% of natural shoreline habitat have been lost), resulting in substantial habitat constraints on biota. Despite this, biological communities are present in the lower Passaic. In general, concentrations of toxic chemicals in surface sediments have fallen with time, and natural recovery processes are proceeding. Chemical concentrations remain high enough to impair survival of amphipods, but not amphipod growth or polychaete growth or survival as measured in laboratory bioassays using field-collected sediment. Fish and blue crab body burdens of some metals, PCBs, and the pesticide, DDT, are at concentrations sufficiently high to exceed toxicity thresholds. The resident fish-eating bird—the belted kingfisher—is at exposure risk from some metals, PCBs, and polychlorinated dibenzo-p-dioxins and dibenzo furans (PCDD/Fs). Migratory waders—the herons and egrets—are not at risk from chemical exposure. These complex findings suggest that restoring the lower Passaic River to ecological health is a correspondingly complex task. Habitat constraints must be eased for biotic components of the ecosystem, and chemical effects must be reduced for those organisms that are present. Only a coordinated, multidisciplinary restoration program will succeed in this challenging environment.

Are long‐distance migrant passerines faithful to their stopover sites?

Long‐distance migrant passerines are well known to often display high levels of philopatry to breeding and wintering grounds. One could expect that similar selective pressures and similar navigation skills would result in their being faithful to stopover sites, a pattern that has been described for several populations of migratory waders and waterfowl. In this paper, we develop the argument that passerines should suffer from higher costs and receive lower benefits from stopover site faithfulness than waterfowl and waders. Based on Alerstam's (1979)“optimal drift strategy” theory and other considerations, we predict that passerines should have lower stopover site fidelity than geese and waders, and that site faithfulness should decrease with increasing distance from either end of the migratory journey. We present results from a long‐term study on the stopover ecology of migrant passerines in southern Portugal that support these predictions and show that, for species and populations that neither nest nor winter in this country, few individuals are faithful to the stopover site. On the other hand, populations that included individuals at (or near to) the start or the end of the migratory journey, had much higher return rates. We could not find any evidence that species linked to scarce habitats, such as wetlands, were more site faithful. Our results are in agreement with several other reports, but in apparent contrast to previous conclusions resulting from some studies involving Old World warblers. These differences are discussed and it is argued that there is no solid evidence to suggest that site fidelity should be important for passerines at stopovers far away from breeding or wintering grounds, meaning that there is a large within‐individual variability in the precise migratory routes and stopover sequence used each year.

Gluttony in migratory waders – unprecedented energy assimilation rates in vertebrates

Maximum energy assimilation rate has been implicated as a constraint on maximal sustained energy expenditure, on biomass production, and in various behavioural and life history models. Data on the upper limit to energy assimilation rate are scarce, and the factors that set the limit remain poorly known. We studied migratory waders in captivity, given unlimited food supply around the clock. Many of these waders assimilated energy at rates of seven to ten times basal metabolism, exceeding maximum rates reported for vertebrates during periods of high energy demand, for example during reproduction and in extreme cold. One factor allowing the high energy assimilation rates may be that much of the assimilated energy is stored and not concomitantly expended by muscles or other organs. The remarkable digestive capacity in waders is probably an adaptation to long and rapid migrations, putting a premium on high energy deposition rates. The upper limit to daily energy assimilation in vertebrates is clearly higher than hitherto believed, and food availability, total daily feeding time and, possibly, the fate of assimilated energy may be important factors to take into account when estimating limits to energy budgets in animals.

Basal metabolic rate in migratory waders: intra‐individual, intraspecific, interspecific and seasonal variation

Summary Basal metabolic rates (BMR) were measured in 36 adult and 119 juvenile waders of 19 species on autumn migration in southern Sweden. In a comparison with literature data, it was found that juvenile BMR was generally lower than at the onset of migration in the Arctic and slightly higher than on African wintering grounds. The seasonal differences may reflect local physiological adaptations or possibly a gradual decline from high premigratory levels due to growth. Our data contradict the idea that BMR is high during migration as an adaptation to generally high levels of energy expenditure. The allometric exponent, scaling BMR to body mass, was significantly higher within individuals (1·19) and within species (1·82) than among species (0·62). The high intra‐individual exponent indicates that non‐fat tissues, with a high metabolic activity, are involved in the mass changes during migratory stopover. The high intraspecific exponent indicates that tissues with a high metabolic activity contributed disproportionately to variation in body mass among individuals or that larger individuals had elevated mass specific metabolic rates of some tissues.

Abundance and distribution of waterbirds on the Knysna Estuary

Twelve waterbird counts were undertaken on the Knysna Estuary during January (summer) and July (winter) from 1993 to 1998. A total of 67 species were recorded; a median of 5343 waterbirds were present during summer, 76% of them Palearctic migrants, dropping to a median of 2336 waterbirds in winter. Waterbird numbers were about a third lower than normal during January 1997 following a flood in November 1996. There was a shift from a summer avifauna comprised predominantly of invertebrate-feeding migrant waders to a winter avifauna comprised mainly of piscivorous species. During the summer, 52% of the birds were Curlew Sandpipers and Grey Plovers, Greenshanks, Kelp Gulls and Whimbrels each made up more than 6% of the avifauna. During the winter Kelp Gulls, Cape and Reed Cormorants, Little Egrets and Sacred Ibises together comprised 62% of the avifauna. The majority of invertebrate-feeding birds were found on the intertidal mudbanks around the Ashmead Channel and on the Brenton side of the estuary below the rail bridge, whereas the shallow water areas between the rail and N2 bridges were important to piscivorous birds, especially during the winter. The Knysna Estuary, being the largest estuarine system in the warm temperate region of South Africa, supports the largest number of birds of any estuarine system between Cape Agulhas and Durban Bay. However, bird densities are not as high as on other important bird estuaries and it is hypothesised that this may be due to recreational disturbance and to the relatively low density and availability of macrobenthic invertebrates.

A new pressure sensory mechanism for prey detection in birds: the use of principles of seabed dynamics?

We demonstrate a novel mechanism for prey detection in birds. Red knots (Calidris canutus), sandpipers that occur worldwide in coastal intertidal areas, are able to detect their favourite hard-shelled prey even when buried in sand beyond the reach of their bills. In operant conditioning experiments designed to find out whether the birds could tell buckets containing only wet sand from buckets containing hard objects in wet sand, we show that they detect the presence not only of deeply buried live bivalves but also of stones. The latter finding virtually excludes, under experimental conditions, prey-detection mechanisms based on vision, acoustics, smell, taste, vibrational signals emitted by prey, temperature gradients and electromagnetic fields. A failure to discriminate between food and non-food trays with dry sand indicates that pore water is involved. Based on the presence of large arrays of Herbst corpuscles (sensory organs that can measure the acceleration due to changes in pressure), the specifics of foraging technique and the characteristics of sediments on which red knots feed, we deduce that the sensory mechanism involves the perception of pressure gradients that are formed when bills probe in soft sediments in which inanimate objects block pore water flow. To our knowledge, this mechanism has not been described before. It is argued that repeated probing in soft, wet sediments allows red knots to induce a residual pressure build-up of sufficient strength to detect the pressure disturbance caused by a nearby object. The cyclic process of shaking loosely packed sand grains followed by gravitational settling into a closer packing, leads, owing to insufficient drainage of the sediment, to a locally increased pressure disturbance that is 'pumped up' at each shake.

Preliminary investigation of nocturnal habitat use by migratory waders (Order Charadriformes) in northern New South Wales

Nocturnal and diurnal habitat use by migrant waders was studied at three sites in the Richmond estuary, northern New South Wales. Numerous species of migrant wader showed significant differences in habitat use between night and day. Sanderling (Calidris alba), red-necked stint (Calidris ruficollis) and, possibly, double-banded plover (Charadrius bicinctus) foraged on mudflats during the day but moved to ocean beaches to forage during night low tides. Pacific golden plover (Pluvialis fulva), greater sandplover (Charadrius leschenaultii), eastern curlew (Numenius madagascariensis) and bar-tailed godwit (Limosa lapponica) displayed less significant differences in habitat use, although densities increased at night on moist sandy mudflats. The numbers of terek sandpiper (Xenus cinereus) and curlew sandpiper (Calidris ferruginea) decreased on sandy mudflats at night although where these birds went is unknown. Nocturnal foraging by X. cinereus may be restricted to moonlit nights. Greenshank (Tringa nebularia) also displayed differences in habitat use by foraging more on moist sandy mud at night and less in sea grass. No significant differences were found for whimbrel (Numenius phaeopus). Habitat use by both N. madagascariensis and N. phaeopus appears related to lunar phase. Six hypotheses are postulated to explain the observed differences: (1) exploitation of changes in prey behaviour and availability; (2) predator avoidance; (3) human avoidance; (4) increased light on mudflats from urban areas; (5) avoidance of roosting flocks of gulls and terns; and (6) relationship with prevailing environmental conditions. The data presented provide a basis from which further work will be undertaken.