S. Lapointe et al. Agriculture, Ecosystems and Environment 78 2000 261–272 265
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. Unsuccessful 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 andor 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 with χ
2
tests 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 χ
2
. 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
; t
47
= 3.1, P = 0.004.
In terms of cover quality, no difference among treat- ments was recorded for live F
3,46
= 2.08, P = 0.12
and dead biomass F
3,46
= 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 F
3,46
= 3.38, P = 0.03. In 1994, DNC93
had more live biomass than all other treatments F
5,78
= 5.77, P 0.0001. Finally, DNC93 and idle
field also had more residual vegetation than improved pasture, DNC94 and ploughed field F
5,78
= 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: F
5,96
= 48.9; im-
proved pastures: F
5,138
= 46.8; unimproved pastures:
F
5,54
= 10.1; DNC93 fields: F
5,24
= 11.3; DNC94
fields: F
5,71
= 19.9; ploughed fields: F
5,108
= 16.9,
266 S. Lapointe et al. Agriculture, Ecosystems and Environment 78 2000 261–272
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 n
a
1992
b
1993 1994
Live Dead
Live Dead
Live Dead
Idle field 23, 23, 17
433 ± 60 a
c
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 DNC93
d
4, 5, 5 369 ± 105 a
77 ± 13 a 830 ± 198 a
132 ± 96 a 1631 ± 700 a
222 ± 82 a DNC94
d
–, –, 13 –
– –
– 614 ± 130 b
24 ± 9 b Ploughed
–, –, 19 –
– –
– 327 ± 94 b
14 ± 6 b
a
Number of plots sampled in 1992, 1993 and 1994, respectively.
b
Vegetation was sampled before establishment of treatments.
c
Means ±1 SE followed by the same letters within a column are not significantly different P 0.05.
d
DNC93 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 F
5,82
= 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 F
5,96
= 3.38,
P = 0.007, improved pastures F
5,138
= 27.3, P
0.0001 and ploughed fields F
5,108
= 2.35, P = 0.05.
Idle fields and DNC93 had more litter than improved
S. Lapointe et al. Agriculture, Ecosystems and Environment 78 2000 261–272 267
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
Others
a
6 9
2
a
Included American black duck Anas rubripes Brewster, blue-winged teal Anas discors L., 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
− 1
during the 3 years of the study, respectively.
Gadwall Anas
strepera L., mallard Anas platyrhynchos L. and northern pintail Anas acuta L.
were the most abundant species totaling nearly 75 of the duck nesting population every year Table 2.
Species composition did not vary among years χ
2 10
= 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 chronology
a
Treatment number of nests Idle field 111
Improved pasture 19 Unimproved pasture 37
DNC93
b
35 DNC94
b
35 Ploughed 14
Early 25
26 27
46 26
64 Intermediate
31 37
27 46
23 14
Late 44
37 46
8 51
22
a
Early: 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.
b
DNC93 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 DNC93
a
DNC94
a
Ploughed field pasture
pasture 1992
b
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
a
DNC93 and DNC94 refer to the dense nesting cover treat- ments established in 1993 and 1994, respectively.
b
Nest density established before habitat improvements.
In 1994, the proportion of nests found in each treatment varied with nesting chronology Table 3;
χ
2 10
= 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
− 1
in 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; χ
2 3
= 5.47, P = 0.15. In 1993, more nests than ex-
pected were found in idle field while fewer nests were found in improved and unimproved pastures
χ
2 3
= 13.83, P 0.0005. In 1994, idle field and
268 S. Lapointe et al. Agriculture, Ecosystems and Environment 78 2000 261–272
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
χ
2 5
= 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, 2840 70 and 7199 72 hatched at least one egg in 1992 and
1993, respectively. In 1994, the fate of 233254 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
− 1
followed 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
In 1994, 16 nests were found in emergent vegeta- tion in interior marshes, mostly between Grande-Ile
and Ile-aux-Fermiers. These included eight nests of redhead Aythya americana Eyton, six of mallard and
two of unknown species. Twelve 75 of the nests hatched at least one egg.
S. Lapointe et al. Agriculture, Ecosystems and Environment 78 2000 261–272 269
Table 5 Mayfield nest success and fate of nests in six treatments on islands at Varennes, Quebec, 1994
Treatment n
a
Mayfield success 95 confidence intervals
Fate Hatched
Depredated Abandoned
Trampled Idle field
110, 105 69 a
b
58–81 81
12 7
Improved pasture 21, 18
15 b 5–46
39 28
33 Unimproved pasture
37, 34 68 a
51–89 76
6 18
DNC93
c
35, 29 82 a
67–100 86
10 4
DNC94
c
34, 32 53 a
35–81 72
16 12
Ploughed 15, 15
47 ab 21–99
73 27
Total 252, 233
63 55–71
76 14
7 3
a
Numbers 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.
b
Percentages followed by the same letter are not significantly different P 0.05.
c
DNC93 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
