The breeding biology and ecology of Forster's Tern (Sterna forsteri) at Delta, Manitoba
McNicholl, Martin K.
This report records the basic breeding biology, as well as information on food, effects of weather, and some inter-specific relationships of Forster's Tern (Sterna forsteri) at a Delta Marsh colony in 1968 and 1969. Forter's Terns at Delta usually nested on muskrat houses or floating vegetation in Scirpus "islands" on open marsh bays, but occasionally also on mud flats. One to several nests were on a given muskrat house or raft of dead Scirpus, and nests on floating vegetation showed a greater tendency to be well built than those on active or dilapidated muskrat houses. Measurements are given for substrate diameter, nest diameter, cup depth, distance from and height above water, water depth at nests, and distances between nests. Clutch size data indicate that clutch sizes of two and three were most frequent, and that clutch size data from colonies in unstable environments are probably reliable only if daily checks are made. Measurements are given for egg length, greatest width, and elongation. A comparison of volumes obtained by using a weight method as opposed to length and width data indicated that only one of the latter formulae gave an accurate estimate of egg volume. Data are also given for breaking period, times involved in various stages of hatching, hatching intervals, and hatching patterns...