How Waders Feed

by Paul Finn

Shorebirds are categorised by their use of the interface between land and sea. In this environment the cycle of the tides, rather than night and day, is the major influence. Shorebirds technically constitute the order Charadriiformes, which includes skuas, gulls, terns and waders. The term 'waders' refers to the wading birds which include plovers, sandpipers, curlews, snipes and others (Pringle 1987).

The majority of waders in Moreton Bay are utilising the intertidal habitat solely for feeding. There are two main feeding techniques employed by waders. The 'sandpiper' strategy is characterised by a slow methodical walk with periodical bill probing into prey burrows within the sediment. This is a tactile method of hunting, that is, birds using the sandpiper strategy rely on detecting unseen prey by touching them. Oystercatchers and various sandpipers probe with an open bill to increase the effective touching area (Zwarts and Blomert 1992). Recent research has discovered that Red Knots have sensory organs within their bills that can measure changes in pressure caused by prey or inanimate objects within wet substrates (Piersma et al. 1998). This enables Red Knots to detect prey without having to physically touch them.

The 'plover' strategy, on the other hand, is characterised by a run-stop-search sequence. Each foraging move ends in a search pause or a prey capture where the bird pecks prey directly from the surface of the flat. Due to their morphology plovers are fixed-method foragers. That is, they are limited to a visual method of locating prey. Plovers have large eyes for hunting visually and short beaks which restrict them from penetrating deep into the sediment. Waders which typically employ the sandpiper strategy (such as: Eastern Curlew; Whimbrel; Godwits; Knots; and Sandpipers) are more versatile foragers. These birds also have the option of foraging visually when it is profitable to do so. Eastern Curlew, for example, have extraordinarily long bills and probe deep into the burrows of crabs, yabbies, and worms. They may initially utilise visual cues to detect burrows that are in use, such as worm castings or freshly maintained entrances, and concentrate their probing efforts on these active burrows.

While feeding, waders should aim to maximise their intake rate of prey. However, maximum food intake rates may incur associated costs or risks. For example, oystercatchers that stab into bivalves too hastily or vigorously run the risk of damaging their bills (Urfi et al. 1996). Also, as search speed increases so too does the amount of energy expended, conversely, the ability to detect prey declines (Speakman and Bryant 1993). Therefore, the optimal search speeds that waders employ are often considerably slower than what they can possibly achieve. Most wader species, with the exception of avocets, swallow one prey item at a time. Avocets swallow many prey at a time in a 'packet' that contains prey, mud and plant material (Moreira 1995). The rate of food processing in waders is restricted by the capacity of the gut to process food. Waders cannot continually ingest food over an entire low tide period as they hit a 'digestive bottleneck'. Therefore, digestive pauses are necessary throughout the feeding period to empty the contents of the digestive tract (Zwarts and Blomert 1992).

In a mixed community of waders, it is the combination of their morphological diversity in bill (culmen) and leg (tarsus) length, specialisation in different feeding strategies and utilisation of different prey items and sections of the flat that enable them to coexist and make efficient use of the available space without too much competition (Lane 1987, Whitfield 1990).

References

Lane, B. (1987) 'Shorebirds in Australia' Nelson, Melbourne.

Moreira, F. (1995) The winter feeding ecology of Avocets Recurvirostra avosetta on intertidal areas. Diet and feeding mechanisms. Ibis 137: pages 99-108.

Piersma, T., van Aelst, R., Kurk, K., Berkhoudt, H. and Maas, L.R.M. (1998) A new pressure sensory mechanism for prey detection in birds: the use of principles of seabed dynamics? 'Proceedings of the Royal Society of London'. 265, 1377-1383.

Pringle, J.D. (1987) 'The Shorebirds of Australia. Angus and Robertson, NSW.

Speakman, J.R. and Bryant, D.M. (1993) The searching speeds of foraging shorebirds: redshank (Tringa totanus) and oystercatcher (Haematopus ostralegus). 'The American Naturalist'. 142: 296-319.

Urfi, A.J., Goss-Custard, J.D. and Durell, S.E.A. le V. dit (1996) The ability of oystercatchers Haematopus ostralegus to compensate for lost feeding time: field studies on individually marked birds. 'Journal of Applied Ecology". 33: 873-883.

Whitfield, P. (1990) Individual feeding specialization of wintering turnstone Arenaria interpres. 'Journal of Animal Ecology' 59: 193-211.

Zwarts, L. and Blomert, A.-M. (1992) Why knot Calidris canutus take medium-sized Macoma balthica when six prey species are available. 'Marine Ecology Progress Series'. 83:113-128.