Klaus-Michael Exo

Exo, K-M. 1999. The significance of nocturnal feeding in waders. In: Adams, N.J. & Slotow, R.H. (eds) Proc. 22 Int. Ornithol. Congr., Durban: 2328. Johannesburg: BirdLife South Africa.

Species moving seasonally between tidal and non-tidal habitats face many different environmental conditions, e.g. different photoperiods and tides, acting as possible Zeitgebers. In tidal areas waders adjust their activity patterns according to the tidal cycle. Therefore, they are active both by day and night. However, quantitative data on night-time activity of free-living waders are scarce. We measured day and night-time activity in Oystercatchers Haematopus ostralegus quantitatively by four different methods: (1) Motion-sensitive transmitters, to record the entire daily patterns of different activities. (2) Triangulations to analyse home-range utilisation. (3) Automatic nest-balances to record food uptake. (4) Direct observations. In addition to the field studies, activity patterns were recorded of birds kept under different photoperiods and feeding schedules. The telemetric data indicate that the overall locomotor activity patterns, as well as the feeding activity, of Oystercatchers breeding in tidal areas was as high at night as during the day. Radiotracking indicated similar home ranges during day and night. Furthermore, food intake was similar during day and night. Hence, it follows that feeding efficiency during night-time was as high as during day-time. In contrast, activity patterns of inland-breeding Oystercatchers seem to be determined by the light-dark cycle with no activity occurring at night. The daily timing of feeding activities of captive Oystercatchers may support the 'supplementary food hypothesis'. Feeding at night occurred only when food availability during day-time was restricted.