Benefits of rotational grazing and dense nesting cover for island-nesting waterfowl in southern Quebec

Intensification of agricultural practices is an important factor responsibl e for the decline of duck populations throughout North America. More than 2 00 islands covering a total of 5000 ha are found in the St. Lawrence river between Montreal and Trois-Rivieres in southern Quebec. The value of these islands as duck nesting habitat, however, is often limited by cattle grazin g. The effects of two types of habitat improvements, rotational grazing and establishment of dense nesting cover (DNC), on island-nesting waterfowl wa s studied from 1992 to 1994. Four treatments were compared: idle fields wit h no vegetation improvement but exclusion of cattle, improved pastures with seeding of forage plants for cattle, DNC fields with improved cover for du cks and exclusion of cattle and unimproved pastures used after the duck nes ting season. Before habitat improvements, grazing by cattle reduced dry mas s of green vegetation by 53% relative to ungrazed plots. No difference was found in the biomass of live (green) and dead (residual) vegetation among t he islands' sections before treatments. Nest density and the number of expe cted nests based on the area covered by each habitat were also similar amon g sections before treatment. Gadwall (Anas strepera L.), mallard (Anas plat yrhynchos L.), and pintail (Anas acuta L.) were the most abundant species n esting on the islands and this was not affected by treatments. Two years af ter habitat improvements, the number of duck nests increased. Idle fields a nd 2-year old DNC had greater visual obstruction, more residual vegetation and more litter. Densities of 2.8 and 7.0 nests ha(-1) with 69 and 82% Mayf ield nest success were recorded in the idle and DNC fields, respectively. N est success was low in improved pasture where a large proportion of nests w ere trampled (33%) or depredated (28%). Fencing permitted growth of emergen t vegetation which enabled over-water nesting by ducks. These results indic ate that with appropriate management, coexistence of cattle and nesting wat erfowl is possible on islands of the St. Lawrence river. (C)2000 Elsevier S cience B.V. All rights reserved.