Benefits of rotational grazing and dense nesting cover for
island-nesting waterfowl in southern Quebec
Stéphane Lapointe
a,1, Jean-François Giroux
a,∗, Luc Bélanger
b, Bernard Filion
c aDépartement des sciences biologiques, Université du Québec à Montréal, C.P. 8888, Succursale Centre-ville, Montréal, Québec,Canada H3C 3P8
bEnvironment Canada, Canadian Wildlife Service, 1141 route de l’Église, C.P. 10100, Ste-Foy, Québec, Canada G1V 4H5 cDucks Unlimited Canada, 710 rue Bouvier, suite 260, Québec, Canada G2J 1A7
Received 7 July 1998; received in revised form 9 August 1999; accepted 29 September 1999
Abstract
Intensification of agricultural practices is an important factor responsible for the decline of duck populations throughout North America. More than 200 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 grazing. The effects of two types of habitat improvements, rotational grazing and establishment of dense nesting cover (DNC), on island-nesting waterfowl was studied from 1992 to 1994. Four treatments were compared: idle fields with no vegetation improvement but exclusion of cattle, improved pastures with seeding of forage plants for cattle, DNC fields with improved cover for ducks and exclusion of cattle and unimproved pastures used after the duck nesting season. Before habitat improvements, grazing by cattle reduced dry mass of green vegetation by 53% relative to ungrazed plots. No difference was found in the biomass of live (green) and dead (residual) vegetation among the islands’ sections before treatments. Nest density and the number of expected nests based on the area covered by each habitat were also similar among sections before treatment. Gadwall (Anas streperaL.), mallard (Anas platyrhynchosL.), and pintail (Anas acutaL.) were the most abundant species nesting on the islands and this was not affected by treatments. Two years after habitat improvements, the number of duck nests increased. Idle fields and 2-year old DNC had greater visual obstruction, more residual vegetation and more litter. Densities of 2.8 and 7.0 nests ha−1with 69 and 82% Mayfield nest success were recorded in the idle and DNC fields, respectively. Nest success was low in improved pasture where a large proportion of nests were trampled (33%) or depredated (28%). Fencing permitted growth of emergent vegetation which enabled over-water nesting by ducks. These results indicate that with appropriate management, coexistence of cattle and nesting waterfowl is possible on islands of the St. Lawrence river. ©2000 Elsevier Science B.V. All rights reserved.
Keywords:Ducks; Nesting density; Nesting success; Rotational grazing; DNC; Quebec
∗Corresponding author. Tel.:+1-514-987-3000 (ext. 3353);
fax:+1-514-987-4647.
E-mail address:giroux.jean-francois@uqam.ca (J.-F. Giroux). 1Present address: Pharmascience inc, 6111 Royalmount Avenue, suite 100, Montr´eal, Qu´ebec, Canada H4P 2T4.
1. Introduction
Expansion of farming and changes to more intensive agricultural practices are among the most important factors responsible for the decline of duck populations throughout North America (Sugden and
gen, 1984). Ducks now have to nest in the remaining small and fragmented habitats where predation is often higher than in contiguous habitats (Clark and Nudds, 1991; Pasitschniak-Arts and Messier, 1995). Even though dabbling duck populations have less declined in eastern than in western North America, biologists are concerned with the rate at which wetlands and their surroundings are being converted into farmlands and industrial lands (Anonymous, 1986). They are there-fore seeking ways to overcome loss of natural habi-tats. Island construction is one of the most productive techniques because ducks can nest at higher densities and with better success attributed to lower mammalian predation (Giroux, 1981; Duebbert, 1982). However, island construction is expensive and could be inappro-priate where duck populations are low (Lokemoen, 1984; Bélanger and Tremblay, 1989). Managing ex-isting islands may therefore be a more cost-efficient strategy (Lokemoen and Woodward, 1992).
In Quebec, the most productive areas for nest-ing dabblnest-ing ducks are located on islands of the St. Lawrence river. More than 200 islands ranging in size from <0.1 to >2000 ha and representing nearly
5000 ha of land are found between Montreal and Trois-Rivieres. However, the value of these islands for ground-nesting birds is often limited by cattle grazing (Bélanger and Lehoux, 1995; Bélanger and Picard, 1999). This reduces the screening effect of vegetative cover, which can lower nest density and success (Lokemoen et al., 1990; Gilbert et al., 1996; Kruse and Bowen, 1996). Grazing can also affect residual vegetation, an important component of nest-ing cover for early nestnest-ing ducks (Duebbert, 1969; Kirsh, 1969; Kirsh et al., 1978). It may provide ideal temperature and humidity for better egg hatchability (Francis, 1968; Duebbert, 1969). In addition, graz-ing and tramplgraz-ing of shoreline vegetation decrease over-water nesting (Krapu et al., 1979; Kirby et al., 1992). Finally, trampling of eggs by cattle directly affects nest success (Jensen et al., 1990).
Studies have shown that it is possible to reduce the adverse effects of cattle with rotational grazing systems (Gjersing, 1975; Barker et al., 1990). Pas-tures are divided into smaller range units and cattle are periodically moved among these units (Kie et al., 1994). Rotation of cattle throughout the growing sea-son maintains plants at a vegetative stage that pro-vides the most digestible forage (Conrad and Martz,
1985). Thus, higher stocking rates are possible and calf weight gain per hectare is better than with contin-uous grazing (Barker et al., 1990). Furthermore, this leaves more undisturbed cover for nesting waterfowl because less area is required for cattle.
The establishment of dense nesting cover (DNC) has also been proposed to enhance duck productivity because it generally supports higher nest densities and success (Duebbert and Kantrud, 1974; Klett et al., 1988; but see McKinnon and Duncan, 1999 for different results). Improved nesting cover is easy and less expensive to establish and can be sown on islands where cover is inadequate (Lokemoen, 1984; Willms and Crawford, 1989). Several years, however, may be needed before cover becomes adequate for nesting (Livezey, 1981).
The effectiveness of rotational grazing systems and cover improvements on nesting waterfowl has been generally demonstrated in the mid continent of North America. This area is characterized by productive wet-lands called prairie potholes that support large num-bers of breeding ducks (Bellrose, 1979). On the other hand, little is known about the effects of these tech-niques in eastern North America where wetlands are often located along rivers or coastal shores and where breeding populations of ducks are less numerous than in the mid continent. Moreover, growing conditions are different between the dryer prairie region and east-ern North America and this could affect the estab-lishment of seeded vegetation. The objective of this study was therefore to evaluate the effectiveness of ro-tational grazing systems and cover improvements to enhance nesting habitat for waterfowl in eastern North America. More specifically, the aims were to com-pare different grassland management practices on (1) quality of nesting cover, (2) nest density of dabbling ducks and (3) their nesting success on islands of the St. Lawrence river in southern Quebec.
2. Material and methods
2.1. Study area
The study was conducted between 1992 and 1994 on four adjacent islands near Varennes, Quebec, Canada (45◦40′N, 73◦27′W), 16 km northeast of
continental temperate, with a mean annual tempera-ture of 6.1◦C and a total annual rainfall of 768 mm.
The islands ranged in size between 9.4 and 59.8 ha for a total of 111.5 ha. No trees or shrubs were present. As water levels recede following spring run-off, the islands are joined together forming small interior marshes with stands of cattail (Typha angustifoliaL.), big burreed (Sparganium eurycarpum Engelm) and arrowhead (Sagittariaspp.).
Cattle were brought to the islands by boat in late May to early June and removed in November. A to-tal of 114, 100 and 85 cows were present in 1992, 1993 and 1994, respectively. During the first year, cattle grazed everywhere on the islands (Fig. 1).
Fig. 1. Location of the treatments on islands at Varennes, Quebec, 1992–1994.
Ile-aux-Fermiers, Masta and St. Patrice islands were dominated by red-top (Agrostis alba L.), red fescue-grass (Festuca rubra L.), Kentucky bluegrass (Poa pratensis L.) and cow vetch (Vicia cracca L.). Grande-Ile was covered with reed canary grass (Phalaris arundinacea L.) and Canada reed-grass (Calamagrostis canadensis[Michx.] Beauv.).
inermisLeyss.) and clover (Trifoliumspp.). Mixtures of reed canary grass in association with timothy, tall fescue-grass (Festuca elatior L.), Orchard grass (Dactylis glomerata L.) or tall wheat grass ( Agropy-ron elongatum [Host] Beauv.) were sown on 5 ha on Grande-Ile to establish the DNC. Non-selective herbicide (glyphosate) was applied to treated fields before cultivation. Masta, St. Patrice as well as parts of Grande-Ile and Ile-aux-Fermiers were left idle with no grazing. Interior marshes were also protected from cattle by permanent or temporary fences.
In 1993, a rotational grazing system was estab-lished (Fig. 1). From the end of May to mid July, cattle were restricted to the improved pasture. After the duck nesting season, they were moved to the unimproved pasture on Ile-aux-Fermiers until mid September. Another rotation between these two pas-tures took place between mid September and the end of October. Finally, cattle were allowed to graze freely on all islands (including in DNC) for 2 weeks until they were removed in mid November. During the duck nesting period from May to July, there were four treatments: idle field (59.9 ha), improved pasture with cattle (19.2 ha), unimproved pasture without cattle (27.4 ha) and a 1-year old DNC (DNC93: 5.0 ha).
In the fall of 1993, another 9.4 and 5.6 ha of DNC were established on St. Patrice and Grande-Ile, re-spectively, by sowing Western wheat grass (Agropyron smithii Rydb.) and crested wheat grass (Agropyron cristatum[L.] Gaertn.). A portion of Ile-aux-Fermiers was ploughed so it could be converted to improved pasture but was not seeded on time. This pasture was therefore a ploughed field (17.6 ha) during the 1994 duck nesting season. The other treatments included: idle field (39.1 ha), improved pasture with cattle (19.2 ha), unimproved pasture without cattle (15.6 ha), a 1-year old DNC (DNC94: 15.0 ha) and a 2-year old DNC (DNC93: 5.0 ha). Rotation of cattle among pastures in 1994 was similar to the previous year.
2.2. Cover evaluation
In 1992, the effect of cattle grazing on the vege-tation was evaluated by establishing 50 0.5 m×1.0 m exclosures that were randomly located on the four is-lands and put in place before the arrival of cattle. In July, all the vegetation above 1 cm was clipped in a
10 cm×10 cm randomly selected quadrat within each exclosure and in grazed sites, also located randomly at more than 50 m away from the exclosures. Vegeta-tion was sorted into live biomass defined as any green plants or plant parts, and residual vegetation that con-sisted of dead biomass from the previous and current years. Vegetation was then dried to constant weight in a micro-wave oven (Bilanski and Ghate, 1978) and weighed to the nearest 0.1 g. Data from unfenced plots were also used to compare live and dead biomass of vegetation among treatments before and after the man-agement. In 1993, the same 50 plots were used while in 1994 an additional 39 plots were randomly located, for a total of 89. Sampling was conducted during the first week of July each year, using the same procedure. In 1994, visual obstruction readings were taken ev-ery 10 m along a 100 m NE–SW line in each plot using a Robel et al. (1970) pole. Readings were taken at a distance of 4 m from a height of 1 m. These readings give an index of cover quality by taking into account height and density of vegetation; it is highly corelated to biomass (Robel et al., 1970). Depth of residual veg-etation accumulated on the ground was measured at the same time with a ruler (±0.5 cm). Visual obstruc-tion and litter depth readings were taken every 2 weeks from early May to mid July.
2.3. Nest search
placed 4 m NE of the nest. Semi-rigid plastic poles were used as nest markers in pastures in 1993 and 1994 because bamboo sticks proved to be inadequate markers when cattle were present (most of them fell in 1992). Nest locations were also marked on 1 : 10 000 aerial color photographs. A nest was defined as a bowl containing at least one egg. Number of eggs, amount of down and incubation stage evaluated by floatation (Westerskov, 1950) were recorded at each visit. Nests were revisited during the following nest search or after the expected hatching date. A nest was considered suc-cessful if at least one egg hatched. Unsucsuc-cessful nests were recorded as depredated if there was evidence of broken or missing eggs, abandoned if the nest was not tended by a female and had cold eggs, and trampled if the clutch and the surrounding area were destroyed by cattle hooves. Predators were identified when possible following the criteria of Rearden (1951) based on re-maining eggshells and/or characteristics of the nests.
2.4. Costs of habitat improvements
For each treatment, costs of ground preparation (tillage, harrowing, seeding and spreading of herbi-cides and fertilizers), chemical products, seeds and fencing were evaluated. Costs for ground prepara-tion were obtained from Le Comité des références économiques en agriculture du Québec (1994) and do not represent the actual costs involved during the study because some operations were done on an ex-perimental basis and were therefore more costly than regular operations. Seed prices are from Labon. All prices are in 1997 CDN dollars.
2.5. Data analyses
Paired t-tests were used to evaluate the effect of cat-tle grazing by comparing biomass between ungrazed and grazed plots. Comparisons among treatments or sampling periods for live and dead biomass, visual obstruction and litter depth were made using analy-ses of variance (ANOVAs). Homogeneity of variances and normality were verified and logarithmic transfor-mations were applied when necessary. When a sig-nificant effect was found, Tukey multiple comparison tests were performed to determine where differences occurred.
Annual differences in the species composition of the breeding population were compared withχ2tests
as well as the proportion of early, intermediate and late nests in each treatment. Comparisons of expected and observed distributions of nests among treatments were made using χ2. The proportion of the nesting
population (expected numbers of nests) in each treat-ment was weighed proportional to the area covered by each habitat. For each year, contrast tests were used to determine where differences occurred among treat-ments. In 1994, nest success was calculated for each treatment using the Mayfield-40% method followed by pairwise comparisons of daily survival rates among treatments (Johnson, 1979). For all analyses, signifi-cance level was fixed at P=0.05 and all means are shown±1 SE.
3. Results
3.1. Nesting cover
In July 1992, before any habitat improvements, grazing by cattle reduced live biomass by 53% (ungrazed plots: 671±50 g m−2; grazed plots:
355±37 g m−2; t
47=5.88, P<0.0001). However,
there was more dead biomass (residual vegetation) in grazed than in ungrazed plots (122±81 versus 81±68 g m−2;t47=3.1,P=0.004).
In terms of cover quality, no difference among treat-ments was recorded for live (F3,46=2.08, P=0.12)
and dead biomass (F3,46=0.65, P=0.59) in 1992
before the management (Table 1). In 1993, after fencing and DNC seeding, there was significantly more live biomass in DNC93 than in improved pasture (F3,46=3.38, P=0.03). In 1994, DNC93
had more live biomass than all other treatments (F5,78=5.77, P<0.0001). Finally, DNC93 and idle
field also had more residual vegetation than improved pasture, DNC94 and ploughed field (F5,78=11.85, P<0.0001).
From May to July, the visual obstruction index increased in every treatment during the summer of 1994, except in improved pastures where it decreased after the arrival of cattle (Fig. 2; idle:F5,96=48.9;
im-proved pastures:F5,138=46.8; unimproved pastures: F5,54=10.1; DNC93 fields: F5,24=11.3; DNC94
Table 1
Live and dead aboveground biomass (g m−2 dry mass) of vegetation used as nesting cover by ducks measured in July in six treatments on islands at Varennes, Quebec, 1992–1994
Treatment na 1992b 1993 1994
Live Dead Live Dead Live Dead
Idle field 23, 23, 17 433±60 ac 126±14 a 577±85 ab 86±18 a 780±135 b 217±40 a Improved pasture 12, 12, 24 212±53 a 130±21 a 419±127 b 134±51 a 366±71 b 50±12 b Unimproved pasture 11, 10, 10 396±90 a 119±37 a 808±155 ab 207±69 a 632±70 b 103±24 ab DNC93d 4, 5, 5 369±105 a 77±13 a 830±198 a 132±96 a 1631±700 a 222±82 a
DNC94d –, –, 13 – – – – 614±130 b 24±9 b
Ploughed –, –, 19 – – – – 327±94 b 14±6 b
aNumber of plots sampled in 1992, 1993 and 1994, respectively. bVegetation was sampled before establishment of treatments.
cMeans (±1 SE) followed by the same letters within a column are not significantly different (P> 0.05). dDNC93 and DNC94 refer to the dense nesting cover treatments established in 1993 and 1994, respectively.
P<0.0001, for all tests). Idle field and DNC93 had
higher visual obstruction than improved pasture in early May (F5,82=20.79,P<0.0001) and the
differ-ence remained significant between DNC93 and im-proved pasture throughout the summer (P<0.0001).
Fig. 2. Visual obstruction index measured with the Robel et al. (1970) pole and litter depth in six treatments during the summer 1994 on islands at Varennes, Quebec. Significant differences among treatments for each period are indicated by different letters (P<0.05).
Litter depth decreased during the summer but differ-ences were only significant in idle fields (F5,96=3.38, P=0.007), improved pastures (F5,138=27.3, P<
0.0001) and ploughed fields (F5,108=2.35,P=0.05).
Table 2
Species composition (%) of duck nests initiated on the islands at Varennes, Quebec, 1992–1994
Species Year (number of nests)
1992 (139) 1993 (137) 1994 (254)
Gadwall 24 28 32
Mallard 25 20 26
Pintail 24 27 18
Shoveler 10 9 14
American Wigeon 10 7 8
Othersa 6 9 2
aIncluded American black duck (Anas rubripes Brewster), blue-winged teal (Anas discorsL.), redhead (Aythya americana) and unidentified species.
pastures throughout the summer (P<0.0001) while
DNC94 and the ploughed fields had a very thin litter (Fig. 2).
3.2. Number of nests and species composition
A total of 143, 143 and 263 duck nests were found in the upland portions of the islands during the 300, 384 and 357 person-hours of searching in 1992, 1993 and 1994, respectively. Nineteen nests found in hunter’s blinds or artificial structures (e.g., oil drums, small fenced plot within an electrical tower) were excluded from subsequent analyses. Combined nest densities for all islands were 1.3, 1.2 and 2.3 nests ha−1during the
3 years of the study, respectively.
Gadwall (Anas strepera L.), mallard (Anas platyrhynchosL.) and northern pintail (Anas acutaL.) were the most abundant species totaling nearly 75% of the duck nesting population every year (Table 2). Species composition did not vary among years (χ102 =16.6,P=0.08).
Table 3
Percentage of early, intermediate and late nests of ducks in six treatments on islands at Varennes, Quebec, 1994
Nesting chronologya Treatment (number of nests)
Idle field (111) Improved pasture (19) Unimproved pasture (37) DNC93b (35) DNC94b (35) Ploughed (14)
Early 25 26 27 46 26 64
Intermediate 31 37 27 46 23 14
Late 44 37 46 8 51 22
aEarly: before 15 May; intermediate: between 16 May and 5 June; late: after 5 June. Nests with unknown initiation date were excluded from this analysis (n=3).
bDNC93 and DNC94 refer to the dense nesting cover treatments established in 1993 and 1994, respectively. Table 4
Number of duck nests per hectare in six treatments on islands at Varennes, Quebec, 1992–1994
Year Treatment
Idle Improved Unimproved DNC93a DNC94a Ploughed field pasture pasture
1992b 1.0 0.5 1.1 1.0 – –
1993 1.5 0.7 0.8 1.8 – –
1994 2.8 1.1 2.4 7.0 2.3 0.9
aDNC93 and DNC94 refer to the dense nesting cover treat-ments established in 1993 and 1994, respectively.
bNest density established before habitat improvements.
In 1994, the proportion of nests found in each treatment varied with nesting chronology (Table 3;
χ102 =26.4, P=0.003). The DNC93 system had a
greater percentage of nests initiated early and at an intermediate date than later in the season. Ploughed field had also more early nesters (mallard and pintail), whereas DNC94 and unimproved pasture had more late nesters.
Nest density in each treatment varied from 0.5 to 1.1 nests ha−1in 1992, with improved pasture having
the lowest density (Table 4). These densities, however, are underestimated because some nests (n=34 or 0.3 nest ha−1) could not be associated to a treatment (loss
of data). In 1993 and 1994, after fencing and DNC seeding, idle field and DNC93 had more nests per hectare than any other treatments.
In 1992, the number of nests found in each treat-ment did not differ from the expected number based on the relative area covered by each treatment (Fig. 3;
χ32=5.47, P=0.15). In 1993, more nests than
Fig. 3. Observed and expected proportion of nests in six treatments on islands at Varennes, Quebec, 1992–1994. The expected numbers were based on the proportion of the area covered by each treatment. Significant differences (P<0.05) are shown with asterisks.
DNC93 had also more nests than expected while improved pasture and ploughed field had fewer (χ52=81.2,P<0.0005).
3.3. Nesting success and productivity
Nesting success was overestimated in 1992 and 1993 because inadequate markers prevented fate de-termination of some nests that have probably been trampled by cattle. Nevertheless, 28/40 (70%) and 71/99 (72%) hatched at least one egg in 1992 and 1993, respectively. In 1994, the fate of 233/254 nests (92%) was determined: 76% of them hatched, 14% were depredated, 7% were abandoned and 3% were trampled.
Mayfield nest success was 63% in 1994 (Table 5). Success was significantly lower in the improved pas-ture than in the other treatments except the ploughed
field. Lower success in the improved pasture was re-lated to higher predation rate and trampling by cattle. Predation rate in the ploughed field was as high as in the improved pasture.
Combining nest density and success indicates that the DNC93 was the most productive treatment with 5.7 hatched nests ha−1followed by the idle field (1.9),
the unimproved pasture (1.6), the DNC94 (1.2), the ploughed field (0.4) and the improved pasture (0.2).
3.4. Nesting in emergent vegetation
Table 5
Mayfield nest success and fate of nests in six treatments on islands at Varennes, Quebec, 1994
Treatment na Mayfield success (%) 95% confidence intervals Fate (%)
Hatched Depredated Abandoned Trampled
Idle field 110, 105 69 ab 58–81 81 12 7 0
Improved pasture 21, 18 15 b 5–46 39 28 0 33
Unimproved pasture 37, 34 68 a 51–89 76 6 18 0
DNC93c 35, 29 82 a 67–100 86 10 4 0
DNC94c 34, 32 53 a 35–81 72 16 12 0
Ploughed 15, 15 47 ab 21–99 73 27 0 0
Total 252, 233 63 55–71 76 14 7 3
aNumbers of nests used to calculate Mayfield success and fate of nests, respectively. Nests with unknown fate are excluded for calculations of apparent nest success (hatched).
bPercentages followed by the same letter are not significantly different (P> 0.05).
cDNC93 and DNC94 refer to the dense nesting cover treatments established in 1993 and 1994, respectively.
3.5. Costs of habitat improvements
The improved pasture was the most expensive treat-ment because of the higher price of seeds and fencing (Table 6). Fencing was considered only for pastures as-suming that cover was indirectly protected from graz-ing in DNC and idle field. When cows are present, costs associated with unimproved pasture should be added when establishing idle fields or DNC.
4. Discussion
The study lacked spatial replication because it was impossible to find other islands similar in size, cover and distance from shore. Moreover financial
Table 6
Cost per hectare (1997 $CDN) of habitat improvements on islands at Varennes, Quebec
Treatment Ground Chemical Seeds FencingcTotal preparationaproductsb
Improved pasture 33 180 192d 474 879
Unimproved pasture – – – 474 474
Dense nesting cover 33 180 135e – 348
aIncludes land tillage ($14), spreading of herbicide ($2), har-rowing ($9), seeding ($6) and spreading of fertilizers ($2).
bIncludes fertilizers ($94), lime ($76) and herbicide ($10). cPermanent fences for 1 ha (100 m×100m)=400 m. Cost in-cludes labor and material.
dSeeds: 20 kg ha−1of a mixture ofBromussp. (24%),Phleum
pratense(34%),Melilotus officinalis(34%) andTrifoliumsp. (8%). eSeeds: 16 kg ha−1 of a mixture of Phalaris arundinacea (56%) andFestuca elatior(44%).
constraints precluded the set up of a large scale repli-cated experiment. Nevertheless, temporal replication (before and after) was utilized to circumvent this problem. Furthermore, the study results are consid-ered applicable to the Varennes islands and to other nearby islands of the St. Lawrence river.
Results from this study, along with those of Barker et al. (1990), indicate that with appropriate manage-ment, use of prairies by cattle and their improvements for nesting waterfowl is possible. Even though the ro-tational grazing system and cover improvement did not result in a greater overall nest density in 1993, changes in nest distribution occurred and a greater proportion of nests were found in the idle field. Two years of rest was sufficient for the plants to recover and to estab-lish new production. In 1994, there was more live and dead biomass in the idle field than in the improved pasture. The number of duck nests increased in the idle field and was higher than expected based on the area covered by this treatment whereas it was lower than expected in the improved pasture. Before the transfer of cattle in July, unimproved pasture had more vege-tation than the improved pasture and this resulted in an increase of duck nest numbers in 1994. Restrict-ing cattle to a smaller improved pasture was clearly beneficial for the overall waterfowl production on the islands.
contrasts with results in southern Saskatchewan where only 1.1–1.4 nests ha−1with 8–26% nest success were
found in DNC plots (McKinnon and Duncan, 1999). In dryer regions such as the mid-continent prairies, DNC reaches maximum growth after 3–5 years and may become too dense for nesting when 7–8 years old (Duebbert and Kantrud, 1974; Duebbert et al., 1983). Maximum growth and a reduction in the value of DNC for nesting waterfowl could occur over a shorter time interval in Quebec where growing conditions are bet-ter. In this case, grazing by cattle during short periods as it was done in the fall 1993 on the Varennes islands, could be used as a tool to maintain cover quality of DNC.
Nest densities on islands at Varennes were similar to those on other artificial or natural islands in southern Quebec (Bélanger and Tremblay, 1989; Bélanger and Lehoux, 1995) but lower than in the Prairie pothole region, reflecting different abundance of waterfowl (Giroux, 1981; Duebbert, 1982; Willms and Craw-ford, 1989). Gadwall, mallard, and northern pintail were the most abundant species nesting at Varennes consistent with the results reported by Bélanger and Lehoux (1995) for islands in southern Quebec. Gad-wall and mallard are common island nesters but the use of islands by pintail is less frequent (Giroux, 1981; Duebbert et al., 1983). This may be attributed to re-gional variation in the abundance of this species and to the low and scarce vegetation found on some parts of islands of the St. Lawrence river, providing suit-able nesting cover for pintails (Bélanger and Trem-blay, 1989; Bélanger and Lehoux, 1995).
About one third of the nests in the improved pasture were trampled. Cow density increases with reduction of area allocated to grazing and therefore the proba-bility of nest trampling is higher (Jensen et al., 1990). Although the present study recorded a high percent-age of nests destroyed by cattle, the total proportion of trampled nests for all the islands remained low (3%) after fencing. Trampling of nests by cattle seems to be of secondary importance compare to the effects of grazing since many studies have also showed a low percentage of trampled nests with specialized grazing systems (Koerth et al., 1983; Bareiss et al., 1986).
High nest success is typical of island nesting ducks because isolation from mainland reduces mammalian predation (Lokemoen and Woodward, 1992). The absence of trees at Varennes may have also reduced
nest predation by raccoons and by birds like Ameri-can crows (Corvus brachyrhyncosBrehm), which are more important predators when they have perches. Herring gull (Larus argentatus Pont.) and Great Black-backed Gull (Larus marinusL.) are also known to prey upon duck nests but their numbers were low.
Before fencing, grazing and trampling of interior marshes by cattle was considerable and emergent veg-etation was nearly absent. Excluding cattle from marsh edges allowed over-water nesting by mallard and red-head. This record of breeding redheads at Varennes is one of the most northeastern one for that species (Gau-thier and Aubry, 1995). Emergent vegetation can pro-vide additional space for pairs as well as good brood rearing habitat both as escape cover and support for invertebrates that serve as food for ducklings (Whyte et al., 1981). A larger number of broods was recorded in marshes on and around the islands after fencing (Bélanger, L., unpublished data). Brood survival may have also been better in 1993 than in 1992, contribut-ing to the increase in the number of duck nests in 1994. Improved pasture was the most expensive manage-ment and resulted in low nest production but these ex-penditures were necessary to keep cattle in a more re-stricted area with good forage. The DNC system was expensive compared to idle fields but had three times as many successful nests per hectare than idle fields. Life expectancy of the seeded DNC is estimated to be more than 10 years (Lokemoen, 1984) with approxi-mately $1000 per year for maintenance and rotation of cattle. Long-term effects of improvements were not evaluated and costs could be amortized over the years. Benefits may then be higher if duck numbers increase with homing of successful females (Lokemeon et al., 1990).
5. Conclusions
pre-dation is more important (Kirsh, 1969). Beneficial ef-fects of cattle restriction could then be even greater. The DNC system, however, may be relatively less pro-ductive on the mainland in southern Quebec and other areas with less abundant waterfowl population because it seems to be more effective in reducing avian than mammalian predation (Clark and Nudds, 1991). Re-cent studies confirmed that prohibitively large areas of DNC would be required to have a substantial effect on regional duck population in southern Saskatchewan (McKinnon and Duncan, 1999).
In integrated land management and sustainable development strategies, these habitat improvements could also be advantageous to landowners. Increase in beef production and reduction of soil erosion, two beneficial effects of specialized grazing systems, are strong arguments to persuade farmers to use rotational grazing systems to benefit both waterfowl and cattle (Barker et al., 1990; Kirby et al., 1992). Restricting access of cattle to some sectors of the islands during the fall season may also reduce potential conflicts with waterfowl hunters. Finally, the effects of these duck-oriented management practices on other prairie bird species should also be considered since cattle grazing can reduce the abundance of these species (Bélanger and Picard, 1999).
Acknowledgements
We are thankful to C. Berthiaume, F. Blouin, A. Cossette, G. Couture, E. Desfossés, S. Goupil, J. Hamel, J. Lefebvre, C. Miqueu and F. St-Pierre for their assistance in the field and laboratory. We would also like to acknowledge the work of Ducks Unlim-ited staff for managing the islands. This study was financially supported by the Canadian Wildlife Ser-vice through the Eastern Habitat Joint Venture and the St. Lawrence Action Plan and by the Université du Québec à Montréal. We thank B. Pollard and two anonymous reviewers for their comments on the manuscript.
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