The Diets of Redshank and Curlew

The authors present new information on the food of Redshank and Curlew at different seasons on the shores of The Wash, derived from field observations supported by an examination of pellets and droppings. They discuss the biases inherent in the method, and conclude that it is entirely adequate for general surveys, and obviates the need to collect birds from feeding flocks.     

A method for measuring some components of foraging of certain birds in the field

A rapid method for measuring the searching speed, and the delay imposed on searching by making a peck (pecking time) and handling a prey (handling time) of certain ground-feeding birds in the field is described and discussed. The method is illustrated with data from two species of wading birds.     

THE WINTER FEEDING ECOLOGY OF THE REDSHANK TRINGA TOTANUS

The aim of the work was to find out how the Redshank which over-winter on the Ythan estuary, Aberdeenshire, adapt their feeding activities to the short daylength in winter. To achieve this, the feeding behaviour and daily routine in winter were compared with those in autumn and spring. Corophium volutator was the main prey on the estuary during the day. However, the temperature of the mud greatly affected the diet in the two areas where the feeding behaviour was studied in detail. At temperatures above 6°C, most of the biomass ingested consisted of Corophium. However, in one study area in one winter, Macoma balthica was taken more frequently at lower than at high temperatures. In the other area, Nereis diversicolor was taken more frequently at low than at high temperatures. These changes in diet appeared related to changes in the behaviour of the prey affecting their availability and not due to the birds changing their preference at low temperatures. The ingestion rate was not affected by the change in diet in the first area but decreased at low temperatures in the second. The sizes of Macoma and Nereis taken by the birds did not vary seasonally. The sizes of Corophium taken decreased in winter as a result of a reduction in the size of those present in the substrate. It was concluded that there was no evidence of Redshank increasing their ingestion rate in winter to compensate for the short daylength. The proportion of the time spent feeding on the estuary in daylight was greater in winter than in spring. Redshank continued feeding at high water in the surrounding fields and on the estuary at night during the winter but not in either autumn or spring. During the winter they obtained less than 50% of their daily food requirements from the estuary in daylight, mainly because of the short daylength. Consequently, they had to collect the balance at night and at high water. Seasonal changes in the numbers of Redshank were recorded. After an autumn decrease in numbers, no decrease in numbers could be detected during the winter. These findings are discussed in relation to the possible difficulties for Redshank in collecting their daily food requirements during the winter. The impact of the birds on their main estuarine prey Corophium is also discussed.     

Mammalian social systems: Structure and function

Mammalian societies are complex socio-ecological systems controlled by the interactions of numerous internal constraints and external factors. We present a simple model describing these systems functionally in terms of the adaptive behavioural strategies for resource exploitation, predation avoidance and mating and rearing of young to maturity shown by the individuals that comprise them. The relations between species-specific limitations on the range of potential individual social behaviour and the environmental variables to which the system is responsive are analysed and hypotheses from correlational and analytical field studies examined. We advocate the continued development of sophisticated systems-analytical approaches to societal analysis taking into account the contrasting informational provenance of factors of different types. This is preferred to either an over-emphasis on environmental determination or excessively formalised neo-darwinian modelling based on assumptions from genetics and selection theory alone.     

Seasonal changes in the response of oystercatchers Haematopus ostralegus to human disturbance

The response of foraging animals to human disturbance can be considered as a trade-off between the increased perceived predation risk of tolerating disturbance and the increased starvation risk of not feeding and avoiding disturbance. We show how the response of overwintering oystercatchers Haematopus ostralegus to disturbance is related to their starvation risk of avoiding disturbance. As winter progresses, oystercatcher energy requirements increase and their feeding conditions deteriorate. To survive they spend longer feeding and so have less spare time in which to compensate for disturbance. Later in winter, birds approach a disturbance source more closely and return more quickly after a disturbance. Their behavioural response to disturbance is less when they are having more difficulty surviving and hence their starvation risk of avoiding disturbance is greater. These results have implications for studies which assume that a larger behavioural response means that a species is more vulnerable to disturbance. The opposite may be true. To more fully understand the impact of disturbance, studies should measure both behavioural responses and the ease with which animals are meeting their requirements. Conservation effort should be directed towards species which need to spend a high proportion of their time feeding, but still have a large response to disturbance.     

Individual and age differences in the feeding ecology of Oystercatchers Haematopus ostralegus wintering on the Exe Estuary, Devon

The habitats and foods used by a sample of marked individuals and by the whole population of Oystercatchers Haematopus ostralegus wintering on the Exe estuary, South Devon, are described. In the spring and early summer, only a few hundred immatures were present. Two thousand adults and several hundred juveniles arrived in late summer from the breeding grounds and remained until the following spring. The majority of the birds were then adults. At low water, most adults fed on the mussel Mytilus edulis beds and ate mussels. Most individuals specialised on this prey and ate little else. Some adults fed on mud-flats and sandflats within the estuary and along the coast, and specialised on a mixture of Nereis diversicolor and Scrobicularia plana or on Cerastoderma edule or Littorina spp. Though some juveniles ate mussels from the time they arrived, most did not. In winter, they took Scrobicularia on mudflats, earthworms Lumbricidae and leatherjackets Tipulidae larvae in fields and Spisula and mussels along the coast. At other times of year they mainly ate Nereis. However, as they grew older, more birds began to specialise on mussels, especially in their second, third and fourth summers when the adults were away. At high water, most birds roosted at the mouth of the estuary or along the coast. However, several hundred fed in the fields in winter with more doing so late in the winter on warm days on Neap tides when less time was available for feeding on the estuary in daylight. Birds feeding on Nereis and Scrobicularia at low water were most often seen in the fields, irrespective of age. Birds feeding on Mytilus and Littorina occurred there rarely, but no bird eating Cerastoderma was seen there. The findings are discussed in relation to age differences in feeding skills and ability to compete successfully with other birds.