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The feeding behaviour and ecology of Common, Sandwich and to a lesser extent, Arctic Terns was studied on the estuary of the River Ythan, Aberdeenshire. Populations of the three species nested at the mouth of the estuary and the ratio of Sandwich j Common : Arctic Terns during the incubation period was approximately 26 : 6 : 1 in 1972 and 1.25 s 3*5;1 in 1973. The numbers of each species feeding on the estuary varied tidally, daily and seasonally. Maximum numbers fed at high tide on spring tides when large numbers of fish entered the estuary. The numbers feeding at high tide on neap tides and at low tide were very low. At both high and low tide proportionately more Common Terns fed on the estuary than either Sandwich or Arctic Terns. Seasonal changes in the relative use of the estuary by the Common Tern population were related to the birds' breeding cycle and to the food available to them on the estuary. Seasonal changes in the Arctic Tern were related mostly to changes in the population size through large late summer influxes of migrant birds. During the high tide period the distribution of feeding terns moved upstream on the flow tide and downstream on the ebb tide, probably following the distribution of the prey species. At high tide all species fed close together in a loose flock but at low tide Common Terns fed solitarily and defended territories along the edge of the water channel. Individuals probably occupied the same territories from day to day. The prey caught by the terns varied tidally and seasonally. At low tide, non-shoaling benthic crustacea and fish were caught. At high tide shoaling surface-living fish were taken; predominantly Ammody-tidae early in the season and Clupeidae later in the season. This seasonal change was related to migrations of the prey species into and out of inshore waters. The weight of the average catch obtained by Common and Sandwich Terns increased seasonally, partly as a result of the change of prey type and partly as a result of the growth of the prey species. Throughout most of the season, all three species took the same prey types and prey lengths at high tide. When carrying food to their mates or chicks Common and Sandwich Terns selected the larger and heavier prey items. Items caught close to the colony were taken back more frequently than items caught farther away. Common Terns modified their searching behaviour according to the type and abundance of their prey. Searching was done from a stationary hovering position when catching benthic prey at low tide and mostly in forward flight when catching surface-living prey at high tide. When searching an area of low abundance surface-living prey they made fewer, smaller changes in flight direction, tended more to alternate left with right changes and flew faster than when searching an area of high prey abundance. Common and Sandwich Terns used three distinct prey capture techniques, differing essentially in the depths to which the birds penetrated the water. The use of the shallower techniques increased as wind-speed and the wave amplitude of the water increased, probably in relation to the visibility of the prey. Sandwich Terns used proportionately less of the shallower techniques than Common Terns so that they became completely submerged more frequently. The success of capture attempts depended upon the technique used; the deeper the penetration into the water the less successful the attempts were. Capture success in Common and Sandwich Terns did not change significantly with the state of tide over either the low or the high tide periods. Prey capture rates were high for a short time at the start of the high tide period but then fell and remained more or less constant throughout the rest of the period. Capture rates of benthic prey during the low tide period were related to the state of tide, being highest at low tide. Prey capture rates of Common and Sandwich Terns were about 5 times higher at high tide on spring tides than on neap tides. The difference was related to the number of fish entering the estuary. Prey capture rates of Common Terns at low tide and of Common and Sandwich Terns at high tide decreased with increasing windspeed. At low tide this was a result of a decrease in the success of capture attempts but at high tide it was a result of a decrease in the rate of capture attempts with no change in their success. The decrease was greater in the Sandwich Tern than in the Common Tern. As the density of feeding terns increased at high tide, interactions between terns became more frequent leading to an increase in the failure of capture attempts in the air. In the Sandwich Tern this resulted in a decrease in the overall success of capture attempts but in the Common Tern it did not. There were no significant differences between Common and Sandwich Terns in their overall prey capture rates and success throughout the season. |
