temporal overlap of nesting duck and aquatic invertebrate ... - BioOne

WETLANDS, Vol. 23, No. 4, December 2003, pp. 739–749 q 2003, The Society of Wetland Scientists

TEMPORAL OVERLAP OF NESTING DUCK AND AQUATIC INVERTEBRATE ABUNDANCES IN THE GRASSLANDS ECOLOGICAL AREA, CALIFORNIA, USA Ferenc A. de Szalay1, L. Chantelle Carroll2, John A. Beam3, and Vincent H. Resh4 1 Department of Biological Sciences Kent State University Kent, Ohio, USA 44242 E-mail: [email protected] 2

3

4

Department of Wildlife Management Humboldt State University Arcata, California, USA 95521

California Department of Fish and Game 18110 West Henry Miller Avenue Los Banos, California, USA 93635

Department of Environmental Science, Policy and Management University of California Berkeley, California, USA 94720

Abstract: Aquatic invertebrates are essential components of duckling diets, but little is known about temporal changes of invertebrate populations in different types of brood habitats. In spring and summer 1996 and 1997, we conducted searches for duck nests in upland fields in the Grasslands Ecological Area in California’s Central Valley to determine timing of nest initiation and hatching. We also sampled aquatic invertebrate populations in adjacent permanent wetlands, semi-permanent borrow areas within seasonal wetlands that were drawn down in spring, and reverse-cycle wetlands (i.e., wetlands flooded from spring to summer) to estimate invertebrate food resources available to ducklings. Abundances of many invertebrates important in duckling diets (Gastropoda, Cladocera, Ostracoda, Amphipoda, Corixidae, Dytiscidae, Hydrophilidae) were greater in borrow areas and reverse-cycle wetlands than in permanent wetlands. Peak macroinvertebrate densities in borrow areas occurred immediately after adjacent wetlands are drawn down in March–April. Peak densities in reverse-cycle wetlands and permanent wetlands occur in May. Although total numbers of microinvertebrates (,1 mm size) and macroinvertebrates ($1 mm size) in all wetlands decreased after May, most mallard (Anas platyrhynchos) and cinnamon teal (A. cyanoptera) eggs hatched in May. Therefore, these ducklings hatch when abundant invertebrate food resources were most available in reversecycle wetlands. In contrast, most gadwall (A. strepera) eggs hatched in June after invertebrate numbers started to decrease. In areas where hydrology is controlled, managing for reverse-cycle wetlands may be a useful strategy to provide abundant invertebrate food resources during the waterfowl breeding season. Key Words: Anas cyanoptera, Anas platyrhynchos, Anas strepera, insects, invertebrates, California, crustaceans, nesting waterfowl, snails, wetlands

INTRODUCTION

1987, Miller 1987). Invertebrates are also essential for ducklings in spring and summer because they provide much of the necessary protein and energy during the pre-fledged period (Sedinger 1992). Although no studies have examined duckling diets in California, diets in other regions are up to 95% aquatic invertebrates such as snails, insects, and crustaceans (Chura 1961, Sugden 1973, Street 1977, Swanson 1988). Ducklings feed almost entirely on surface-dwelling aquatic and terrestrial invertebrates during the first two weeks after

Although most wetlands in California’s Central Valley are managed to provide migration and winter habitat for waterfowl (Heitmeyer et al. 1989), McLandress et al. (1996) found that the region is also an important breeding area for some species of dabbling ducks. Fall migrating and wintering ducks in the Central Valley feed mostly on plant foods, but diets shift to more invertebrate consumption in the late winter and spring (Connelly and Chesemore 1980, Euliss and Harris 739

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WETLANDS, Volume 23, No. 4, 2003

hatching, and later, they also feed on epiphytic and benthic invertebrates (Sugden 1973). Hens with broods select habitats with high densities of invertebrates (Talent et al. 1982, Cooper and Anderson 1996, Poysa et al. 2000), and abundant invertebrate food resources will promote rapid growth that may increase duckling survival (Street 1978, Hill et al. 1987, Cox et al. 1998). Although duck broods that hatch late in the nesting season usually have lower survival rates than those hatching earlier (Rotella and Ratti 1992, Grand and Flint 1996, Dzus and Clark 1998, Guyn and Clark 1999, Krapu et al. 2000), we have found no studies that compared temporal changes of duckling success or numbers with invertebrate abundances during the nesting season. Wetlands in the Central Valley of California are a mosaic of habitat types with varying hydrology, vegetation, and invertebrate communities (Heitmeyer et al. 1989, de Szalay et al. 1999). Wetland management techniques (e.g., burning, mowing, timing of flooding) can influence numbers of invertebrates in wetlands (Kaminski and Prince 1981, Batzer et al. 1997, de Szalay and Resh 1997, Anderson and Smith 2000). However, little is known about relative amounts of invertebrate food resources in Central Valley wetlands during the waterfowl breeding season. In this study, we sampled invertebrates in potential waterfowl brood habitat and concurrently determined timing of duck nesting in nearby upland fields. Our objectives were to 1) examine differences in invertebrate numbers among wetland types, 2) compare temporal changes in nesting duck and invertebrate numbers, and 3) suggest implications for management of these wetlands as habitat for breeding waterfowl. METHODS Study Area and Habitats Sampled This study was conducted in the Grasslands Ecological Area (hereafter, the Grasslands) of the northern San Joaquin Valley, California, USA. The Grasslands encompasses 47,000 ha of mostly seasonal wetland and upland habitats. We sampled nesting waterfowl and invertebrates from March through August 1996 and 1997 at Los Banos Wildlife Management Area (Los Banos WMA) and North Grasslands Wildlife Management Area (North Grasslands WMA). These areas are managed by the California Department of Fish and Game for wintering and breeding waterfowl and include a diversity of wetland and upland habitats. A detailed description of the study area is found in Fredrickson and Laubhan (1995). Historically, the Grasslands were flooded by winter rains and spring snowmelt from the Sierra Nevada

mountains. Little precipitation occurs in summer, and most wetlands are seasonally flooded from late-winter to early summer. Currently, most wetlands have been impounded, and managers can flood these during the dry summer season using canals and water-control structures. Precipitation patterns in the Grasslands during winter 1995–1996 were typical, with frequent storms from October 1995 through May 1996 that produced 28 cm of precipitation. Seventeen cm of rain fell after February 1996, which promoted abundant plant growth in uplands in spring. In winter 1996– 1997, heavy fall and winter storms produced 29 cm of rain from October 1996 through January 1997, but only 2 cm of precipitation fell from February through May 1997. The drought in spring 1997 reduced plant growth and density in upland fields. However, water levels in managed wetlands were similar between in 1996 and 1997. We sampled aquatic invertebrates in permanently flooded wetlands, semi-permanently flooded borrow areas, and reverse-cycle wetlands. Permanent wetlands were surrounded by upland fields dominated by crop plants (e.g., safflower [Carthamus tinctorius L.]) or pasture vegetation including grasses (Bromus sp., Hordeum sp., Distichlis sp.), clover (Trifolium sp.) and vetch (Vicia sp.). The dominant plants along the edges of the permanent wetlands were bulrush (Scirpus acutus Muhl.) and cattail (Typha spp.); deeper areas (.1.0 m) were open water with sparse submergent macrophytes. Although water levels fluctuate within these habitats, all permanent wetlands had been flooded throughout the three years prior to 1996 (pers. observ., J. Beam). Borrow areas were 10–20 m wide, ,1.5 m deep, and present along the levee at the lowest end of seasonally flooded wetlands. The water-control structures used to drain these wetlands were adjacent to the borrow area, and during drawdown, all water drained through the borrow area. Seasonal wetlands were drawn down from 29 March to 9 May in 1996 and from 23 March to 4 May in 1997, and borrow areas remained flooded through early-August in both years. The entire wetlands were re-flooded between September and October each year. Seasonal wetlands were managed for moist-soil plants such as nodding smartweed (Polygonum lapathifolium L.), water grass (Echinochloa crusgalli (L.)), and swamp timothy (Heleochloa schoenoides (L.)), and the wetlands were sometimes disced in fall to control perennial species (e.g., cattail or willow (Salix sp.). Stands of cattails and bulrush grew along the edges of borrow areas, and deeper regions were open water. Reverse-cycle wetlands were surrounded by upland fields managed for pasture vegetation or crop plants. In 1996 and 1997, reverse-cycle wetlands were flooded

de Szalay et al., NESTING DUCKS AND INVERTEBRATES from late-February to early-March and drained in August until the following spring. These are called reverse-cycle wetlands because they have water regimes that are reverse from natural cycles (i.e., they are initially flooded in spring when natural wetlands in California are beginning to dry up) (de Szalay et al. 1999). Most Central Valley wetlands are managed for wintering waterfowl, but Los Banos WMA and North Grasslands WMA have created some reverse-cycle wetlands as summer brood water for resident nesting waterfowl and shorebirds. In 1996, vegetation in reverse-cycle wetlands was a diverse mixture of hydrophytes such as watergrass, cattail, smartweed, and upland plants such as curly dock (Rumex crispus L.) and rabbitfoot grass (Polypogon monospeliensis (L.)). Vegetation in the reverse-cycle wetlands was disced just before flooding to create open water for waterbirds and to stimulate invertebrate production. After flooding, cattail quickly colonized from the seed bank because water depths were shallow. Vegetation in these wetlands was not manipulated after they were drawn down in August 1996. In 1997, these wetlands had dense cattail stands with smaller patches of other hydrophytes and upland plants. Replicate sample sites for each habitat type were selected from available wetland impoundments on Los Banos WMA and North Grasslands WMA, and all sample sites were located in different wetlands. Sites were not random because we needed replicates of each habitat type that had similar plant communities and were accessible to allow frequent sampling. In 1996, we sampled seven sites of each of the three habitat types. In 1997, we chose eight sites of each habitat type; 19 of the 21 sites sampled in 1996 were also sampled in 1997. Wetlands of each type ranged in size from 0.5 to 11 hectares. Waterfowl nesting was sampled in upland fields that were interspersed among the sampled wetlands. Nest Searching We located duck nests in upland fields from March to June in 1996 and 1997. Twenty fields on Los Banos WMA and four fields on North Grasslands WMA (198 hectares total area) were sampled in 1996, and 20 fields at Los Banos WMA and two fields at North Grasslands WMA (174 hectares total area) were sampled in 1997. We searched fields once every three weeks using two all-terrain vehicles (ATVs) pulling a 45-m rope with attached tin cans weighted with rocks for noisemakers. When a duck flushed, the nest was located and marked with a 0.5-m wooden stake placed directly at the nest and a 2-m stake placed 3 m to the North. At each nest site, we estimated egg embryonic stag-

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es and expected hatch dates by candling (Weller 1956, Klett et al. 1986). We calculated nest initiation date by subtracting the total number of eggs found in the bowl plus the number of days the eggs had been incubated (assuming that one egg is laid per day) (Klett et al. 1986). We revisited nests once weekly until the nest fate was determined (i.e., hatched, abandoned, destroyed). Abandonment was attributed to investigator influence if a nest appeared to have been abandoned on the day of discovery, and we censored these data from nest success analyses. We examined destroyed nests and damaged eggs to identify predator species with methods described in Sargeant et al. (1998). When we observed eggs hatching on dates different than the estimated hatch dates, we used the actual nest hatch date in our data. We estimated nest success following Mayfield techniques (Mayfield 1961, 1975, Klett et al. 1986) modified for waterfowl (Johnson 1979). The central span of initiation, the period between the 10th and 90th percentiles of initiation dates (Hammond and Johnson 1984), and median initiation date were used to express the length and timing of the nesting season. The timing of when eggs hatched and ducklings entered the wetlands was estimated by determining the central span and median date of hatch dates. We included expected hatch dates from destroyed nests to increase our sample size and avoid any potential bias caused by restricting analyses to only successful nests. Invertebrate Sampling We sampled aquatic invertebrates with activity traps in all wetlands. In 1996, wetlands were sampled every two weeks from 29 March through 2 August. Invertebrate numbers were already relatively large in some reverse cycle wetlands on the first sampling date because the wetlands had been flooded for 2–3 weeks. Therefore, we initiated sampling in 1997 immediately after reverse cycle wetlands were flooded and sampled from 3 March through 28 July. We sampled invertebrates every three weeks in 1997 to compensate for the longer sampling period and the greater number of wetlands sampled in 1997 than in 1996. Activity traps were modified from Murkin et al. (1983) and were constructed from 1-L plastic bottles with a funnel (small opening: 2 cm; large opening: 11 cm) attached in a hole cut into the bottle lid and a second funnel attached in a hole cut into the bottom of the bottle. Traps were attached to wooden posts at mid-water column (20–100 cm water depth). On each sampling date, we randomly located two traps in each wetland along 50-m transects in open water areas ,1 m from stands of emergent plants. Traps were collected after 24 hours, and captured invertebrates were

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Table 1. Duck nesting results in 1996 and 1997. N 5 number of nests sampled in each year. % nest success was calculated with the Mayfield method as modified by Johnson (1979). The central span and median of nest initiation and hatching dates are given in Julian days.

Species Mallard Cinnamon Teal Gadwall

Year

N

Nests/km2

1996 1997 1996 1997 1996 1997

228 122 52 16 127 52

115 70 26 9 64 30

preserved with 95% ethanol. Samples were sorted under dissection microscopes, and invertebrates were identified to the lowest taxonomic level possible (i.e., usually family or genus) using keys in Usinger (1956), Pennak (1989), Thorp and Covich (1991), and Merritt and Cummins (1996). Taxa were also classified as microinvertebrates (those taxa ,1 mm in size throughout their lifecycle) and macroinvertebrates (.1 mm in size). Statistical Analysis Invertebrate data were log (x11) transformed to equalize variances after checking for homoscedasticity. All statistical procedures were conducted with SYSTAT software (SYSTAT 1992), and we set our alpha error at 0.05 for all tests. We compared invertebrate numbers among habitat types (permanent wetlands, borrow areas, reverse-cycle wetlands), and we analyzed data from 1996 and 1997 separately because we sampled different wetlands each year. We used a repeated-measures ANOVA model with interactions to test effects of habitat type and date (Zar 1999). When habitat type was significant and habitat type x date interaction was not significant, we used Tukey’s tests to compare total means (i.e., summed over all dates) among habitat types. When interactions were significant, we used Tukey’s tests to compare habitat type means within each date. We calculated Tukey’s statistics with the subjects within habitat types mean square error to maintain a procedure-wise alpha error of P#0.05 (Zar 1999). This method may result in a high beta error because it is conservative. Because it is difficult to analyze data from rare species, we did not compare taxa that were ,2.5% of total number of microinvertebrates or macroinvertebrates collected. The repeated-measures ANOVA procedure in SYSTAT eliminates any replicates that are missing data on one or more sampling dates. In 1996, one borrow area and one reverse cycle wetland on 5 July and one borrow area on 2 August were partially drawn down for

% Nest Success (95% C.I.) 17.4 8.2 18.3 7.0 19.0 12.5

(13.0–23.3) (4.7–13.9) (9.5–35.0) (1.3–33.5) (13.0–27.8) (6.1–25.4)

Initiation Central Span Hatching Central Span (median) (median) 75–136 (96) 76–135 (103) 83–139 (114) 88–126 (110) 103–150 (125) 109–144 (127)

110–168 (130) 110–166 (137) 115–171 (144) 121–156 (143) 139–182 (158) 144–176 (160)

1–3 days, which dewatered our sampling sites. Although the aquatic invertebrate communities in the remainder of the wetland were not eliminated, we could not collect invertebrates at our sampling site. Losing these replicates would have greatly reduced the power of the statistical tests, and therefore, we excluded these dates from all analyses. We compared temporal changes in invertebrate numbers with waterfowl nesting and hatching. Invertebrate numbers and waterfowl nesting changed in nonlinear patterns. Therefore, we graphed invertebrate data with the Distance Weighted Least Squares function of SYSTAT, which is useful to develop hypotheses about trends when the shape of the function is not known (SYSTAT 1992). This method calculates a quadratic multiple regression for each point and produces a true, locally weighted curve that best fits the data (McLain 1974). RESULTS Waterfowl Nesting In 1996, we found 408 duck nests on Los Banos WMA and North Grasslands WMA (206 nests/km2) (Table 1). Mallard (Anas platyrhynchos L.) were the most abundant species, followed by gadwall (A. strepera L.) and cinnamon teal (A. cyanoptera Vieillot). Two additional nests (one blue-winged teal [A. discors L.], one northern shoveler [A. clypeata L.]) were found but not included in Table 1. In 1997, we found 190 duck nests (109 nests/km2), with mallards being most abundant, followed by gadwall and cinnamon teal (Table 1). Mayfield nest success ranged from 7 to 19% and was greater in 1996 than in 1997 for all species (Table 1). Predation (mostly coyotes [Canis latrans Say], skunks [Mephitis mephitis (Schreber)], and Franklin’s ground squirrels [Spermophilus beecheyi (Richardson)]) accounted for 79.7% of all destroyed nests in 1996 and 81.8% in 1997. The remaining causes of failure were abandonment (15.9%, 6.8%), addled

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Table 2. Invertebrate communities in all wetlands. N 5 total number of microinvertebrates or macroinvertebrates collected in 1996 and 1997. % of Total Collected 5 (number collected of each taxa/N) 3 100. % of Total Collected Taxa MICROINVERTEBRATES CRUSTACEANS Cladocera (water fleas) Copepoda (copepods) Ostracoda (seed shrimp) Other TOTAL MACROINVERTEBRATES

Figure 1. Timing of mallard, cinnamon teal, and gadwall nesting. Graphs show number of nests initiated (Initiation) or with a hatched clutch (Hatch) on the 14-day (1996) or 21-day (1997) period ending on the Julian day.

clutches (3.3%, 5.3%), or unknown causes (1.1%, 6.1%) for 1996 and 1997, respectively. Timing of waterfowl nesting was similar in 1996 and 1997, with a unimodal distribution for all species (Figure 1). All nests were initiated between Julian day 61 and 160 (1 March and 8 June, respectively) in 1996 and Julian day 63 and 153 in 1997. Mallards initiated nesting earliest; their median hatch date was in midMay (Table 1). Cinnamon teal nested slightly later than mallards, with a median hatch date of late-May. Gadwall nested later than other species and were more than three weeks later than mallards. Invertebrates in Permanent Wetlands, Borrow Areas, and Reverse-cycle Wetlands Similar aquatic invertebrate communities were collected in 1996 and 1997 (Table 2). Most common taxa ($2.5% of total macroinvertebrates and microinvertebrates collected) were found in all three habitat types in both years; these included gastropods, chironomids,

1996

1997

N 5 142,038 N 5 120,409 23.4 72.9 1.7 2.0 100% N 5 33,826

39.1 49.7 10.2 1.0 100% N 5 26,435

SNAILS Gastropoda (snails)

2.6

2.2

CRUSTACEANS Amphipoda (scuds) Gammaridae

7.5

3.4

INSECTS Hemiptera (true bugs) Corixidae

65.1

71.7

Coleoptera (beetles) Dytiscidae Hydrophilidae

10.8 4.2

8.5 3.4

5.7 4.1 100%

8.0 2.8 100%

Diptera (true flies) Chironomidae Other TOTAL

corixids, dytiscid and hydrophilid beetles, cladocerans, and amphipods. Among microinvertebrates, cladoceran and ostracod numbers were greater in reverse-cycle wetlands than in permanent wetlands (Table 3). Borrow areas had intermediate numbers of these invertebrates. These patterns were consistent through most of the season. Copepod numbers were not different among habitats in either year (Table 3). Among macroinvertebrates, gastropod, hydrophilid, and dytiscid numbers were usually greatest in reversecycle wetlands, intermediate in borrow areas, and lowest in permanent wetlands (Table 3). These patterns persisted on most sample dates in both years, although means were not always statistically different. Populations of these taxa were greatest during April–June. Amphipod numbers were greatest in borrow areas and lowest in reverse-cycle and permanent wetlands, and populations were greatest during May–June. Corixid populations showed a brief, large peak in borrow areas in April 1996 and March 1997, which coincided with drawdowns of the seasonal wetlands where they were

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Table 3. Invertebrate abundance in permanent wetlands, borrow areas and reverse-cycle wetlands in 1996 and 1997. Type indicates habitat type; P are permanent wetlands; B are borrow areas; R are reverse-cycle wetlands. Total 5 mean number of invertebrates 6 1 SE combined over all dates. Means with different letters (A,B) are significantly (P , 0.05) different (Repeated measures ANOVA with non-significant Habitat 3 Date interaction, significant Habitat effect, followed by Tukeys tests). 1996 and 1997 Julian Day 5 mean number invertebrates 6 1 SE on each sample date. Means with different letters (A,B) are significantly (P , 0.05) different (Repeated measures ANOVA with significant Habitat 3 Date interaction, significant Habitat effect, followed by Tukeys tests). Note: 1996 data from Julian days 187 and 215 are not shown because several wetlands were drawn down on these dates. 1996 Julian Day Taxa Cladocera

Copepoda

Gastropoda

Amphipoda

Corixidae

Dytiscidae

Hydrophilidae

Chironomidae

Taxa Cladocera

Copepoda

Ostracoda

Gastropoda

Amphipoda

Corixidae

Dytiscidae

Hydrophilidae

Chironomidae

Type P B R P B R P B R P B R P B R P S R P B R P B R

Total 5.6 101.5 179.1 303.5 192.0 296.7 0.2 1.3 4.9 0.1 19.6 0 16.2 57.4 123.0 0.3 5.7 20.8 0.1 1.5 9.2 5.5 4.5 4.9

Type P B R P B R P B R P B R P B R P B R P B R P B R P B R

6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6

2.0A 26.1B 45.3B 46.1 37.1 42.4 0.1 0.3 1.2 0.1 5.8 0 6.4 25.2 36.5 0.1A 2.0A,B 4.7B 0.1A 0.6A,B 2.3B 2.2 0.8 1.1

89 23.1 132.6 355.4 231.7 398.9 345.0 0.1 0.8 0.4 0.1 0.9 0 1.4 25.5 173.6 0 0.1 8.9 0 0.1 7.1 15.3 3.7 11.5

6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6

7.0A 29.4A 65.5B 23.4 17.3 22.0 0.2 0.9 47.6 0.1 0.1 1.8 0.6 1.5 0.0 5.5 27.6 20.7 0.7 1.5 3.0 0.7 1.9 0.5 0.5 1.2 1.9

103 10.5 86.1 142.4 67.2 198.8 144.8 0.1 0.3 0.4 0.1 0.4 0 1.1 14.0 89.1 0 0.1 4.5 0 0.1 5.4 14.9 0.9 7.7

19.6 19.6 470.6 210.3 140.8 186.9 1.9 1.5 394.8 0.1 0.2 0.3 0.1 0.1 0.1 0.7 6.9 2.0 0 0 3.5 0 0 0.6 4.5 2.1 5.3

6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6

117

10.6 6 9.6 186.0 6 113.1 50.5 6 18.3 202.3 6 37.1 228.1 6 83.0 273.5 6 73.9 0.1 6 0.1 3.6 6 1.5 0.4 6 0.3 060 2.4 6 0.7 060 9.1 6 7.3 228.7 6 177.7 129.3 6 71.7 0.1 6 0.1 1.4 6 0.9 10.0 6 6.4 060 1.5 6 0.8 5.4 6 3.3 3.4 6 2.6 6.1 6 2.5 3.9 6 1.3 1997 Julian Day

62

Total 25.0 82.2 234.8 153.9 115.1 145.8 0.3 1.7 88.2 0.2 0.4 3.9 1.8 5.1 0.1 14.2 64.9 51.9 1.4 4.2 10.7 1.5 3.2 1.9 2.7 5.5 7.7

6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6

83 7.3 6.0 283.3 81.0 75.6 103.0 1.9A 1.4A 346.4B 0.1 0.2 0.3 0.1 0.1 0.1 0.2 3.8 1.0 0 0 3.0 0 0 0.3 2.8 0.8 2.0

59.1 288.3 571.3 169.4 226.3 153.9 0.2 4.9 164.1 0.2 0.1 0.5 0.4 3.2 0 3.7 288.4 11.9 0.1 0.6 9.3 0.2 0.1 3.9 2.1 3.9 22.6

6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6

3.8 257.3 159.9 219.7 194.6 412.7 0.4 2.1 5.1 0.3 9.1 0 6.4 32.6 442.4 0.1 2.0 32.1 0.1 0.2 14.8 5.2 3.2 6.9

6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6

3.1 111.4 75.9 54.4 79.8 120.4 0.1 0.7 3.9 0.3A 5.2B 0A 3.4 8.8 227.5 0.1 0.9 19.7 0.1 0.2 10.2 2.5 1.2 2.8

104 34.9 90.9 345.0 39.9 60.0 45.6 0.1A 4.4A 106.9B 0.2 0.1 0.5 0.2 1.5 0 2.7A 202.5B 6.2A,B 0.1 0.3 5.1 0.2 0.1 2.2 0.8 1.0 10.4

73.9 155.9 345.3 222.8 95.9 185.9 0.3 6.0 126.7 0 0.8 3.1 0.2 5.0 0 37.6 94.1 94.9 0.3 2.8 8.3 0.2 0.9 3.8 2.1 3.6 12.9

6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6

31.7 112.3 244.5 93.9 51.6 37.2 0.1A 5.9A,B 108.0B 0 0.5 1.9 0.1A,B 2.9B 0A 27.1 52.8 53.9 0.3A 2.5A,B 3.1B 0.1 0.9 1.4 0.8 0.8 4.8

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Table 3. Extended.

1996 Julian Day 131 3.7 103.5 199.6 597.9 149.5 218.8 0.1 1.8 5.9 0.2 53.4 0 48.6 60.6 115.4 0.1 9.6 20.9 0 1.0 17.1 3.9 2.7 6.0

6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6

21.9 178.9 178.0 153.5 138.4 316.9 0.1 1.2 19.4 0.2 0.4 3.8 1.2 5.0 0.1 31.6 24.3 239.1 3.0 1.3 31.8 2.1 0.1 1.7 2.6 2.5 4.1

6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6

3.1 49.7 123.6 182.5 52.4 86.4 0.1A 1.1A,B 3.0B 0.2A 29.7B 0A 33.5 37.0 74.1 0.1 7.2 14.3 0 0.7 10.2 2.0 1.1 2.6

145 2.6 69.4 119.7 307.3 125.7 249.2 0.1 1.0 10.0 0 44.5 0 46.8 12.6 38.4 0.2 0.9 18.7 0.2 0.9 6.2 1.7 3.1 4.5

6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6

21.9 7.4 71.7 220.1 34.8 103.3 0 0.1 0.5 0.4 0.2 3.4 0.8 4.5 0 24.5 34.6 48.8 0.8 1.3 21.2 2.9 0.2 3.4 1.9 4.3 4.1

6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6

125 10.4 170.5 50.7 26.6 26.3 100.4 0.1 0.8 18.0 0.1 0.2 2.7 0.7A,B 2.0B 0.1A 25.5 10.7 141.8 2.9A 0.9A 16.5B 2.1 0.1 0.9 1.1 0.8 1.6

1.9 62.6 69.9 121.7 39.1 88.8 0.1A 0.4A,B 4.7B 0A 27.0B 0A 34.1 8.5 16.0 0.2 0.6 9.3 0.2 0.8 2.6 0.7 0.8 2.3

159 0.2 6 0.1 44.8 6 41.0 277.6 6 145.8 245.6 6 81.9 103.3 6 37.7 376.3 6 180.8 0.1 6 0.1A 0.4 6 0.2A,B 6.1 6 2.9B 0.1 6 0.1A 23.1 6 13.2B 0 6 0A 14.4 6 12.0 13.8 6 8.2 69.9 6 57.3 0.6 6 0.5 20.7 6 12.4 37.6 6 20.6 0.3 6 0.3 6.7 6 4.0 12.1 6 8.6 10.9 6 9.2 7.9 6 3.5 3.4 6 1.4 1997 Julian Day

146 18.7 6.0 17.5 122.5 9.9 22.9 0 0.1 0.5 0.4 0.1 2.5 0.5A,B 1.9B 0A 22.2 32.0 29.2 0.7 0.9 13.3 2.9 0.1 2.6 1.0 1.5 1.8

173 0.4 15.9 262.9 467.8 240.1 239.6 0.3 0.2 10.1 0.4 17.1 0 2.6 82.9 13.8 0.9 6.2 32.6 0.1 0.4 9.3 2.3 8.6 2.6

167 3.3 6.8 92.0 107.8 38.7 83.8 0 0 0 0.1 0.8 17.3 6.9 20.0 0.1 10.4 17.7 12.1 4.5 9.1 7.9 4.5 2.7 0.6 2.3 4.4 5.8

6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6

2.5 6.5 44.5 33.5 10.5 38.0 0 0 0 0.1 0.4 13.2 3.1A 10.2B 0.1A 9.9 15.5 8.4 4.5 7.5 4.0 4.4 1.9 0.3 1.4 1.5 5.0

6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6

0.3 13.2 257.9 251.3 170.5 94.7 0.2A 0.1A 4.8B 0.2A 12.4B 0A 2.2 81.9 8.8 0.8 5.1 16.7 0.1 0.3 6.0 0.9 3.5 1.3

201 0.1 2.3 7.4 155.9 96.1 258.5 0 0.2 1.5 0 6.1 0 0.1 2.2 0.8 0.1 4.4 5.2 0.1 1.2 1.4 1.6 1.1 0.8

188 0.1 0 97.8 47.9 102.6 85.8 0 0 0 0.4 0.9 1.6 1.9 2.4 0.1 3.8 25.3 5.7 1.6 12.7 3.3 2.0 3.7 1.0 1.9 9.3 6.1

6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6

0.1 0 46.4 21.3 40.7 32.9 0 0 0 0.2 0.5 1.0 1.4 1.1 0.1 2.7 12.1 4.4 1.6 8.3 2.5 2.0 2.0 1.0 1.2 2.6 5.8

6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6

0.1 1.5 3.2 43.5 39.2 167.1 0 0.2 1.2 0A 3.3B 0A 0.1 0.7 0.5 0.1 1.8 2.1 0.1 0.4 0.5 0.7 0.5 0.5

209 0.1 0.9 51.9 99.3 143.1 49.9 0 0 0 0.3 0.1 0.8 2.8 0.6 0.1 1.3 27.8 1.0 1.1 5.6 0.7 0.3 17.9 0.2 4.3 14.4 1.1

6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6

0.1 0.9 49.8 32.7 55.5 16.0 0 0 0 0.2 0.1 0.8 2.5 0.6 0.1 0.9 22.3 0.6 0.9 2.6 0.5 0.3 15.0 0.2 2.5 8.3 0.5

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located. In March 1997, corixid numbers were greater in borrow areas than in permanent wetlands. Chironomid numbers were not different among wetland types. Microinvertebrate populations in permanent wetlands peaked mid-May in 1996 and mid-April in 1997 and decreased thereafter (Figure 2). Microinvertebrate numbers in borrow areas decreased throughout 1996 and had a sigmoidal distribution in 1997 that peaked late-March. Microinvertebrate numbers in reverse-cycle wetlands decreased from March through July in 1996 and 1997. Macroinvertebrate populations had a unimodal distribution in all habitats in both years, except that numbers in borrow areas had a sigmoidal distribution in 1997 (Figure 2). Macroinvertebrates peaked in permanent wetlands in mid-May in 1996 and early May in 1997. Numbers in borrow areas peaked mid-April and late March in 1996 and 1997, respectively. These peaks coincided with drawdowns in the adjoining seasonal wetlands. Numbers in reverse-cycle wetlands peaked in late April in 1996 and early May in 1997. DISCUSSION Nesting Effort Nest densities on our study area in the Grasslands were greater than some other waterfowl production areas in North America. For example, studies in the northern prairie regions of the United States and Canada report mallard densities of 3–35 nests/km2 (Greenwood et al. 1987, Fleskes and Klaas 1991, Higgins et al. 1992), compared to mallard densities of 70–115 nests/km2 in this study. Our nesting data probably are typical for these WMAs because they are similar to data collected in previous years at these areas (J. Beam, unpubl. data) and to a recent study of mallard productivity in several regions in the Central Valley of California (McLandress et al. 1996). Mallard, cinnamon teal, and gadwall had greater nest densities and nest success in 1996 than in 1997. In both years, invertebrate species assemblages and temporal changes in abundance were similar. Therefore, changes in breeding effort and success did not seem to be related to invertebrate food resources. Waterfowl nesting can be affected by spring rainfall and ambient temperatures (Hammond and Johnson 1984, Greenwood et al. 1995, McLandress et al. 1996), and we suspect that lower nest densities in 1997 probably were related to drought conditions that occurred that spring. Invertebrate Food Resources in Wetlands Corixids, beetles, gastropods, chironomid midges, cladocerans, and amphipods were abundant in wet-

Figure 2. Seasonal changes of invertebrates in 1996 and 1997. Values are mean number of microinvertebrates or macroinvertebrates in permanent wetlands (P), borrow areas (B), or reverse-cycle wetlands (R) in 1996 (m) or 1997 (V).

lands we sampled; all are important in diets of prefledged ducklings (Chura 1961, Sugden 1973, Bengston 1975, Krapu and Swanson 1977, Street 1977, Swanson 1988). It is important to note that duckling growth is probably more affected by the quantity of invertebrate biomass in their diets than the numbers of invertebrate prey items consumed. However, many studies have found that duckling survival and feeding behavior are closely correlated with aquatic invertebrate numbers in broodwater habitats (Sjoberg and Danell 1982, Hill et al. 1986, Gardarsson and Einarsson 1994, Cox et al. 1998), which indicates that areas with large numbers of invertebrates provide abundant food resources for ducklings. We found greater numbers of many invertebrate taxa in reverse-cycle wetlands than in permanent wetlands, and intermediate numbers were present in borrow areas. Consequently, it seems that reverse-cycle wetlands provided more food resources for juvenile waterfowl than other habitats. Wetlands with short hydroperiods typically have greater invertebrate productivity than wetlands with long hydroperiods (Neckles et al. 1990). Many factors influence this relationship, including low numbers of predatory fish in intermittently dewatered habitats (Hill et al.

de Szalay et al., NESTING DUCKS AND INVERTEBRATES 1987). Furthermore, plant litter in seasonal wetlands will decompose rapidly when it is oxidized during non-flooded periods, and the released nutrients enhance algae, fungi, and bacteria food resources for aquatic invertebrates when the wetlands are re-flooded (Brinson et al. 1981). In a concurrent study in the Grasslands, mallard broods preferred reverse-cycle and semi-permanent borrow areas over permanent wetlands and duckling survival was highest in reverse-cycle wetlands (Chouinard 2000). Female ducks with broods may select wetlands with high invertebrate productivity in at least some habitats (Talent et al. 1982), and our data indicate that availability of invertebrate food resources may influence brood distribution in the Grasslands. Temporal Changes of Invertebrates and Ducklings Peak invertebrate populations occurred at different times among habitat types in this study. In reversecycle and permanent wetlands, peak macroinvertebrate populations were present in May. In borrow areas, macroinvertebrate densities peaked in March–April when the seasonal impoundments were drawn down through the borrow areas. We believe that many macroinvertebrates were carried into the borrow areas from the adjacent impoundments because we observed an immediate rise in numbers of invertebrates during drawdown. Furthermore, there was an increase in active swimmers (i.e., Amphipoda, Cladocera, Corixididae) in the borrow areas immediately after drawdown, but numbers of sessile taxa (e.g., Chironomidae) did not change. However, the large invertebrate abundance in borrow areas following drawdowns was relatively short-lived; most populations decreased within three weeks after drawdown. Median hatch dates for mallard and cinnamon teal were early to late May. Gadwall median hatch dates were early June. Abundant invertebrate food resources should be available to most mallard and cinnamon teal broods on our study sites because numbers of many taxa (e.g., Gastropoda, Amphipoda, Dytiscidae, and Hydrophilidae) are large when ducks hatch. Although other researchers have suggested that the period of waterfowl nesting generally overlaps with peak invertebrate productivity (Bataille and Baldassarre 1993, Cox and Kadlec 1995), it is important to note that we found that temporal patterns of invertebrate abundance are different among wetland types. Of the three habitat types sampled, reverse-cycle wetlands had the greatest invertebrate densities when most mallard and teal eggs hatched. In contrast, peak macroinvertebrate populations in borrow areas occurred before most eggs hatch, and permanent wetlands had relatively low invertebrate productivity throughout the spring and summer.

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Broods from late-nesting mallards and cinnamon teal will have lower food resources than broods that hatch earlier because invertebrate numbers decreased in most wetlands by late summer. Most gadwall eggs also hatch when invertebrate numbers are decreasing, but gadwall ducklings feed more on plant foods and are not as dependent on invertebrate food resources as other dabbling ducks (Sugden 1973). Management Implications Our results have implications for areas in the Grasslands managed for breeding waterfowl. Because duck nesting occurs over several months and females also require invertebrate foods during egg formation (Swanson et al. 1979), adequate invertebrate food resources are needed over broad time periods in the breeding season. Borrow areas have high invertebrate densities when the adjoining wetlands are drawndown, and reverse-cycle wetlands have high densities in early summer. Therefore, bountiful invertebrate food resources may be available over a broad time period if managers provide a diversity of wetland habitat types. Reverse-cycle wetlands can be especially important brood habitat in the Grasslands because these are highly productive when many waterfowl broods are present. Wetland managers can further maximize invertebrate availability for nesting ducks by manipulating flooding regimes in controlled wetlands, (e.g., drawdowns in some wetlands can be delayed until early summer). However, late drawdowns may delay germination of some moist-soil plants that are food for migratory waterfowl in the winter. Therefore, tradeoffs between creating breeding habitats and wintering habitat for waterfowl will limit where these strategies can be applied. ACKNOWLEDGMENTS We are grateful to the California Department of Fish and Game and the Merced County Mosquito Abatement District, especially P. Cherny, B. Cook, L. Howard, and R. Wilbur. Invaluable field assistance was provided by M. Brasher, M. Chouinard, T. Menges Chouinard, K. Cripe, C. Fein, J. Isola, B. Jordan, R. Knoernchild, K. Sande Kwasny, S. Miyamota, and N. Nelson. Comments by J. Anderson, M. Heitmeyer, H. Murkin, F. Reid, G. Rollins, D. Smith, and an anonymous reviewer improved earlier drafts of the manuscript. Funding was provided by the California DFG Comprehensive Wetland Habitat Program, State Duck Stamp funds, Ducks Unlimited, Kent State University Office of Research and Graduate Studies, and the University of California’s University-wide Mosquito Research Program.

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WETLANDS, Volume 23, No. 4, 2003 LITERATURE CITED

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