Constraints of feeding on Salicornia ramosissima by wigeon Anas penelope: an experimental approach

The coastal marshes of the Charente-Maritime (western France) are a major wintering area for wigeon Anas penelope. In these marshes, wigeon feed mainly on grasses, while foraging on Salicornia (a fleshy, succulent halophyte) is uncommon. In order to understand the reason for this under-exploitation, an experimental study was carried out with captive wigeon in autumn 1998 on a Salicornia ramosissima marsh in the Charente-Maritime. Birds were unable to maintain their body weight when feeding on Salicornia. Measurements of food chemical composition and metabolisability, as well as instantaneous intake rate of the birds (g/min) could not explain these weight losses. However, the time budget of wigeon revealed that they spent a maximum of 10–11 h per 24 h consuming Salicornia, whereas 18–19 h would have been needed to meet their daily energy requirements. The daily foraging time on Salicornia may have been limited by physiological constraints due to: (1) a high ash content (mainly salt, about 34% of dry matter), and/or (2) a digestive bottleneck, because of the waxy cuticle covering the leaves, which is likely to have constrained processing rate of Salicornia ears in the gut of birds. In the discussion, we address the question of the potential geographical differences in the use of Salicornia by birds.