Time and energy constraints of fishing behaviour in breeding Cormorants <i>Phalacrocorax carbo sinensis</i> at lake IJsselmeer, The Netherlands

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.</p

Mass fishing by Cormorants <i>Phalacrocorax carbo sinensis</i> at lake IJsselmeer, The Netherlands:A recent and successful adaptation to a turbid environment

The habit of mass fishing by Cormorants at lake IJsselmeer, The Netherlands, is a recent phenomenon. During the first half of the 1970s the birds changed behaviour probably as a result of the deteriorating under water visibility in the lake (3-4 m water depth). The behavioural switch coincided with years of high numbers of Smelt Osmerus eperlanus and Ruffe Gymnocephalus cernuus present in the southeastern part of lake Markermeer, the birds' main fishing area at that time. Social fishing by Cormorants is directed towards the catch of relatively small, pelagically dwelling fish. It is argued that for a large water system where social fishing is the rule, a minimum colony size of c. 1000 pairs is required. Typically each colony had one socially fishing group (4000-5000 birds) that slowly changed position through the course of the day. Depending on the direction of the wind the flock's position could greatly change between days. Hunting speed was measured and coincided with maximum swimming speed of medium sized fish prey (15-25 cm). Hunting speed increased during the season probably as a result of the greater swimming speeds of the fish at higher temperatures. Intake rate was closely linked to the birds' position within the flock indicating local depletion of the fished water layer. Mass fishing was especially rewarding at intermediate light intensities under water (50-80 cm Secchi depth, or 300-500 mu E.m(-2)s(-1) at 40 cm depth). The habit of pushing up the fish against the light back-ground of the clear top water layer was only possible when wind caused no greater turbidity than 40 cm Secchi depth (100 mu E.m(-2)s(-1)) which is considered a breakpoint for this kind of behaviour. Adapting the habit of mass fishing effectively enabled the birds to exploit the turbid, rapidly changing environment which resulted in the extension of the foraging range thus maximising colony size relative to the resources available.</p

Size selection in diving Tufted Ducks Aythya fuligula explained by differential handling of small and large mussels Dreissena polymorpha

We studied prey size selection of Tufted Ducks feeding on fresh-water mussels under semi-natural conditions. In experiments with non-diving birds, we found that Tufted Ducks use two techniques to handle mussels. Mussels less than 16 mm in length are strained from a waterflow generated in the bill (suction-feeding), while larger mussels are picked up and handled singly. Suction-feeding on small mussels proved to be more profitable. In the non-diving situation, the ducks preferentially took the smallest mussels on offer. It is suggested that smaller mussels incur lower costs of crushing mussel shells in the gizzard. Ducks diving for mussels preferred suction-feeding on all mussels up to about 16 mm in order to minimize the costs of diving. The selectivity for small mussels increased with diving depth (1-5 m), probably because ducks diving deeper spend more time at the bottom collecting small mussels, before a larger mussel is taken to the surface and ingested afterwards. We conclude that the two handling techniques we described for Tufted Duck open up an extensive feeding niche for benthic feeding diving ducks

Size selection in diving Tufted Ducks Aythya fuligula explained by differential handling of small and large mussels Dreissena polymorpha

We studied prey size selection of Tufted Ducks feeding on fresh-water mussels under semi-natural conditions. In experiments with non-diving birds, we found that Tufted Ducks use two techniques to handle mussels. Mussels less than 16 mm in length are strained from a waterflow generated in the bill (suction-feeding), while larger mussels are picked up and handled singly. Suction-feeding on small mussels proved to be more profitable. In the non-diving situation, the ducks preferentially took the smallest mussels on offer. It is suggested that smaller mussels incur lower costs of crushing mussel shells in the gizzard. Ducks diving for mussels preferred suction-feeding on all mussels up to about 16 mm in order to minimize the costs of diving. The selectivity for small mussels increased with diving depth (1-5 m), probably because ducks diving deeper spend more time at the bottom collecting small mussels, before a larger mussel is taken to the surface and ingested afterwards. We conclude that the two handling techniques we described for Tufted Duck open up an extensive feeding niche for benthic feeding diving ducks

Solitary foraging in sand pits by breeding Cormorants <i>Phalacrocorax carbo sinensis</i>:Does specialised knowledge about fishing sites and fish behaviour pay off?

Though most Cormorants from the colonies exploiting the lake IJsselmeer area in The Netherlands nowadays have taken up the habit of fishing in large groups, some individuals are still observed fishing solitarily. This phenomenon occurs predominantly during the coldest months at well-defined patches, often located in sand pit regions. A case study in a sand pit in the early spring of 1982 revealed that Cormorants from Oostvaardersplassen took advantage of the fish located at their winter refuges inside the pit by specifically diving along the pit's steepest slopes. Thus, they were able to achieve a more than average daily ration of about 475 g of fish within a mere 15 minutes of foraging in the second half of March. Later on, the site became less profitable and the birds gradually left. The importance of high intake rates in early spring is stressed, in view of time consuming activities like nest site occupation, pair formation and accumulation of body reserves for egg-laying, incubation and chick feeding. It is suggested that only the 'higher quality' birds may have the experience and skill required to use solitary fishing techniques as a profitable alternative to social fishing in early spring.</p