TWO Cormorant colonies in The Netherlands (Naardermeer and Oostvaardersplassen), exploiting the same water bodies but situated at different distances from them, were compared with respect to daily variations in exact fishing sites and corresponding variations in time budget and fish consumption. Mean flying distances between colony and fishing site were estimated at 22 and 15 km respectively. Birds from the Naardermeer colony carried out less trips but of a longer duration than birds from Oostvaardersplassen, most markedly so in the chick rearing period (2 trips per day of 185 min vs. 3 trips of 165 min). Daily fluctuations in time spent away from the colony were clearly dependent on daily shifts in main fishing sites. On days when fishing was concentrated at larger distances, individual foraging trips lasted longer, due to the increase in flying time. Neither net fishing time nor daily fish consumption, as estimated by pellet analyses, compensated for the increment in time and energy expenditure on those days. It was estimated that the average daily energy expenditure would amount to about 2.8 . BMR (basal metabolic rate) in birds from Naardermeer and to about 2.7 . BMR in birds from Oostvaardersplassen. Fish consumption estimates based on pellet analyses led to an estimated DME (daily metabolisable energy) of 2 . BMR for both colonies. Thus, an overall negative energy balance became apparent, resulting in estimated mass losses throughout the breeding season of on average 980 and 860 g for Naardermeer and Oostvaardersplassen birds, respectively. Mass losses are likely to be higher with increasing travelling distances, indicating that travelling distance may influence reproductive output. This could be one of the factors causing consistently lower reproductive outputs at the Naardermeer throughout the years.
|Number of pages||12|
|Publication status||Published - 1995|