. La Canadian field-naturaliste. L'histoire naturelle. 168 Le Canadian Field-Naturalist Vol. 117 canards nicheurs. L'objectif de cette étude était d'évaluer les effets à court terme d'améliorer la couverture végétale sur l'abondance des oiseaux chanteurs des prairies qui nichent au sol. La zone d'étude 115-ha archipel de Varennes (45° 40' N, 73° 27' W) est situé à 20 km au nord-est de Montréal (Canada) le long du fleuve Saint-Laurent. Il est composé de quatre îles d'une superficie allant de 9,4 à 59,8 ha (figure 1). Avant de couvrir les améliorations, les espèces végétales dominantes de ces îles ont été vesce jargeau (Vicia cracca), l'agrostide blanche (Agros

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. The Canadian field-naturalist. Natural history. 168 The Canadian Field-Naturalist Vol. 117 nesting ducks. The objective of this study was to assess the short-term effects of plant cover improve- ments on the abundance of ground-nesting grassland songbirds. Study Area The 115-ha Varennes archipelago (45° 40' N, 73° 27' W) is located 20 km northeast of Montreal (Canada) along the St. Lawrence River. It is composed of four islands ranging in size from 9.4 to 59.8 ha (Figure 1). Before cover improvements, the dominant plant species on these islands were Tufted Vetch (Vicia cracca), Redtop (Agrostis alba), Red Fescue (Festuca rubra) and Kentucky Bluegrass (Poa pratensis). On one island (Grande-ile), however, tall grasses such as Phalaris arundinacea and Calamagrostis canadensis were dom- inant. As a result of continuous grazing pressure, trees and shrubs are absent. Emergent marsh vegetation consists primarily of cattails (Typha angustifolia) and scattered clumps of Giant Bur-reed (Sparganium eury- carpum) and arrowheads (Sagittaria spp.). Communal pasture is a traditional agricultural practice on islands of the freshwater section of the St. Lawrence River in Quebec (Belanger and Picard 1999). Approximately 100 cows are present each year on the Varennes islands from late May-early June to mid- November. Before 1993, cows roamed freely over the four islands. In fall 1992, Ducks Unlimited Canada initiated a controlled grazing system; one 19-ha pas- ture was seeded with a mixture of Timothy (Phleum pratense), Yellow Sweet-Clover (Melilotus officinalis), Smooth Brome (Bromus inermis) and clovers (Trifolium spp.) to improve the quality of the forage (improved pasture - IMPP). A dense nesting cover (DNC93) was also established on a 5-ha plot by seeding Reed Canary Grass (Phalaris arundinacea). The remaining parts of the islands were left in their natural state with some being grazed by cattle after the duck-nesting season (27.4 ha of unimproved pastures - UIPP) or left und

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