BRANTA — Sora M. Estrella
Behavioural and feeding mechanisms flexibility in migratory shorebirds
Institution: University of Cadiz, Spain
Details: PhD 2006 (Completed)
Address: Coastal Wetlands Conservation Group. Dept. of Biology, School of Sciences of the Sea and Environment, University of Cádiz, P.O. Box 11510, Puerto Real, Spain (Jun 2007) Email
Subject Keywords: foraging, surface tension transport
Abstract
Most migrant waders depend at some stage of their migrations on aquatic systems, which have physical and chemical characteristics distinct from those present in terrestrial habitats. Since the feeding mechanisms employed by organisms are moulded in relation to the environment, their morphological limitations as well as the prey type, it follows that waders use feeding mechanisms enabling them to exploit aquatic systems. In this thesis I documented some of these feeding mechanisms along with other aspects of behavioural flexibility seen in waders when feeding in the aquatic environment.
Although previous laboratory studies suggest that many species of wader may be capable of surface tension feeding, no field studies have examined this possibility. Results show that Surface Tension Transport (STT) is a common feeding mechanism in small or medium-sized shorebird species feeding on small prey items in shallow water. Moreover, birds using STT transported prey up to 3.6 times faster than the theoretical value predicted previously and were capable of achieving high intake rates foraging on small prey items when they are available at high densities. It was also found that STT is used throughout the salinity, temperature and prey body mass ranges encountered by waders in their natural habitats.
Modulation of feeding behaviour in relation to type and/or size and/or position of prey is common among trophic generalists. Waders were found to modulate their bill gape in response to differences in prey size and position in the water column. Adjusting bill gape to prey size saves time and energy when feeding on small prey whereas modulation of bill gape in relation to prey position appears to be a response designed to overcome physical environmental constraints. Waders' bill gape was modulated frequently through the use of distal rhynchokinesis (flexing the upper mandible). Although the use of distal rhynchokinesis has been commonly associated with the deep probing feeding method, its use and occurrence was reported for the first time in wild long-billed waders feeding on small prey items suspended in water.
Foraging behaviour of typical plovers is highly stereotyped, and to date, the use of a sandpiper-like foraging method by typical plovers has been considered anecdotal. However it was found that sandpiper-like foraging method is common in Grey Plover (Pluvialis squatarola) and Ringed Plover (Charadrius hiaticula) feeding in prey-abundant pans, being particularly important for Ringed Plover, since up to 24% of the population foraged in this way at high tide. In light of these findings, the behavioural plasticity of foraging methods of typical plovers may be greater than has been assumed previously.
In conclusion waders display flexible feeding mechanisms and behaviour that allow them to cope with the different properties and conditions of the aquatic medium and to feed profitably on small prey present in them.