2006 SOUTHEASTERN NATURALIST 5(2):311–316 Breeding Biology of Mottled Ducks on Agricultural Lands in Southwestern Louisiana R. Scott Durham1,2,* and Alan D. Afton3 Abstract - Breeding biology of Anas fulvigula maculosa (Mottled Ducks) has been described in coastal marsh and associated habitats, but little information is available for agricultural habitats in Louisiana. We located nests to determine nest-initiation dates and clutch sizes during the primary breeding season (February–May) in 1999 (n = 29) and 2000 (n = 37) on agricultural lands in southwestern Louisiana. In 1999, 60% of located nests were initiated between 22 March and 10 April, whereas in 2000, only 22% of nests were initiated during the same time period. Average clutch size was 0.9 eggs smaller in 2000 than in 1999. Annual differences in reproductive parameters corresponded with extremely dry conditions caused by low rainfall before the laying period in 2000. Flooded rice fields appear to be important loafing and feeding habitat of Mottled Ducks nesting in agricultural lands, especially during drought periods when other wetland types are not available or where natural wetlands have been eliminated. Introduction Anas fulvigula maculosa Ridgway (Mottled Duck) is a resident west Gulf coastal species strongly associated with coastal marsh environments and adjoining agricultural land. Because of population concerns and habitat loss across their range, Mottled Ducks are a priority species in the Gulf Coast Joint Venture of the North American Waterfowl Management Plan (Esslinger and Wilson 2001). Nesting studies of Mottled Ducks in Louisiana have focused on coastal marsh habitats, or areas associated with river deltas or islands (Allen 1981, Baker 1983, Holbrook 1997, Walters 2000). The breeding biology of Mottled Ducks in agricultural areas of Texas is well documented (Engeling 1950, Singleton 1953, Stutzenbaker 1988), but it has not been studied in Louisiana. Grand (1992) suggested that low autumn and winter rainfall before the nesting season in Texas coastal marsh habitat would reduce wetland availability and thereby limit food resources necessary to nesting Mottled Ducks. We hypothesized that variation in precipitation would influence initiation dates and clutch sizes of Mottled Ducks nesting on agricultural habitats in southwest Louisiana. We recorded and compared these reproductive parameters between two years of varying rainfall amounts. 1Sweet Lake Land and Oil Company, 358 Chalkley Road, Bell City, LA 70630. 2Current address - Louisiana Department of Wildlife and Fisheries, 2000 Quail Drive, Baton Rouge, LA 70898-9000. 3USGS-Louisiana Cooperative Fish and Wildlife Research Unit, Louisiana State University, Baton Rouge, LA 70803. *Corresponding author - sdurham@wlf.louisiana.gov. 312 Southeastern Naturalist Vol. 5, No. 2 Study Area We searched for Mottled Duck nests on 11,493 and 12,764 ha of agricultural lands owned by Sweet Lake Land and Oil Company (SLLOC) in southwest Louisiana in 1999 and 2000, respectively (Durham and Afton 2003). Sweet Lake Land and Oil Company agricultural lands were located in Cameron and Calcasieu Parishes, southeast of Lake Charles, LA, and encompassed a range of permanent, semi-native, and improved pastures, rice field complexes, and other habitats. Soils on SLLOC lands are Mowata- Vidrine-Crowley, and Morey-Leton-Mowata (Midkiff et al. 1995, Roy and Midkiff 1988). Originally, this agricultural region was part of a 10,360-km2 triangular shaped prairie (Taylor 1956). Native grass species were Andropogon gerardii Vitman (big bluestem), Tripsacum dactyloides L. (eastern gama grass), Sorghastrum nutans (L.) Nash (Indian grass), Schizachyrium scoparium (Michx.) Nash (little bluestem), and Panicum virgatum L. (switch grass) (Smeins et al. 1992). Methods From observation stations (elevated locations) selected each year near wetlands where Mottled Ducks were consistently observed, nests were located by observing “drop flights” and subsequently searching areas where females landed (Durham and Afton 2003). Upon nest discovery, we estimated initiation date using an egg-laying rate of 1 egg per day (Singleton 1953), and estimated stage of incubation by candling (Klett et al. 1986, Weller 1956). To minimize disturbance, we did not revisit nests until after estimated hatch date. We used Statistix8 software (Analytical Software 2003) to compare the distribution of nest initiation dates between years with a chi-square test for differences in probabilities as described by Conover (1971). We compared mean clutch sizes of nests between years using a ttest. The a priori level of significance for each test was P ≤ 0.05. Results The distribution of nest initiations differed between years ( χ2 = 13.35, df = 6, P = 0.04). In 1999, we determined initiation dates for 23 nests. The first egg was laid on 22 March, and 7 nests were initiated within the first 10- day period, 22 to 31 March (Table 1). The following 10-day period, 1 to 10 April, produced 7 more nests. Fewer weekly nest initiations occurred during the remaining 10-day periods. In 2000, we determined initiation dates for 32 nests. The first egg was laid on 19 March, but most nest initiations did not occur until the 10-day period of 11 to 20 April, when 9 nests were initiated (Table 1). The two following 10-day periods each produced 7 nests. Mean clutch size differed between years (t = 1.83, df = 37, P = 0.04). In 1999, clutch size for 11 incubated nests averaged 10.4 eggs (± 0.12 SE) with a range of 8 to 12 eggs, whereas in 2000, clutch size for 28 incubated nests averaged 9.5 eggs (± 0.05 SE) with a range of 4 to 11 eggs. 2006 R.S. Durham and A.D. Afton 313 Discussion Although dates of first eggs were similar between years, most nests were initiated later in 2000 than in 1999. In 1999, 60% of located nests were initiated between 22 March and 10 April, whereas in 2000, under extremely dry conditions across the landscape (Table 2), only 22% of nests were initiated during the same time period (Table 1). Combined autumn and winter rainfall before the 2000 breeding season was considerably less (49.9 cm) than that preceding the 1999 breeding season (Table 2). Fewer wetlands suitable for feeding and loafing were available in spring 2000, and we were only able to locate 22 observation stations (1/580 ha), compared to 30 observation stations (1/383 ha) in 1999, a 27% decrease. On our study area, Mottled Ducks did not lay eggs in 2000 until farmers flooded rice fields in early spring. Prior to flooding, we observed few suitable wetlands available for Mottled Ducks to loaf or feed. Lack of suitable wetlands probably reduces the amount of foraging habitat available Table 1. Number of Mottled Duck nests initiated and percentage of total nests initiated by 10- day periods on or near Sweet Lake Land and Oil Company agricultural lands in southwest Louisiana, 1999 and 2000. Number of nests Percentage of total 10-day period 1999 2000 1999 2000 22–31 March 7 1 30 3 1–10 April 7 6 30 19 11–20 April 3 9 13 28 21–30 April 2 7 9 22 1–10 May 2 7 9 22 11–20 May 2 1 9 3 ≥ 21 May 0 1A 0 3 AThis nest was initiated on 3 June 2000. Table 2. Annual rainfall (cm) by month at Lake Charles, Louisiana, 1998–2000. From NOAA climatalogical data (www.nws.noaa.gov). Month 1998 1999 2000 January 25.43 13.79 3.99 February 13.03 3.35 1.88 March 9.53 9.47 5.74 April 6.20 0.10 13.94 May 0.10 10.31 21.46 June 12.83 24.92 13.23 July 4.90 9.91 11.84 August 9.40 traceA 3.12 September 31.95 5.00 7.95 October 13.74 3.25 4.32 November 7.42 2.51 30.10 December 9.19 11.40 5.16 Total 143.71 94.03 122.73 Anot measurable 314 Southeastern Naturalist Vol. 5, No. 2 to breeding females. Aquatic macroinvertebrates (associated with wetland availability) are an important food source to mallards (Krapu and Reinecke 1992) and the closely related Mottled Ducks (Moorman and Gray 1994) during the breeding season. We believe further research is needed to examine the relationship of wetland availability and invertebrate abundance to Mottled Duck breeding biology. Mean annual clutch sizes were within the previously reported range of 8 to 10.4 eggs for this species (Singleton 1953, Stutzenbaker 1988); however, the upper range of clutch sizes (12) was slightly lower than the reported maximum of 13 in more coastal habitats (Stieglitz and Wilson 1968, Walters 2000). It is possible that the annual difference in clutch size that we observed also may have resulted from the extremely dry conditions and concomitant later nest initiations in 2000 (Table 2), perhaps caused by reduced availability of food resources. Individuals among waterfowl species that nest later within a breeding season generally produce fewer young than those that nest early (Rohwer 1992). Our observations may indicate that seasonal and permanent wetland habitats located in close proximity to quality nesting cover are important to Mottled Ducks breeding in agricultural lands. Observed nesting activity occurred near flooded rice fields and other wetlands adjacent to dense nesting cover. Because in 2000, nests were initiated only after farmers flooded fields for rice planting, flooded rice fields are likely important loafing and feeding habitats for nesting Mottled Ducks in agricultural areas, especially during drought periods when other wetland types are not available or where natural wetlands have been eliminated. Rice farming acreage in Louisiana is driven by local, continental, and world markets. United States agricultural economics, farm programs, and trade agreements with other nations that affect the amount of rice being farmed in Louisiana also may affect Mottled Duck populations using ricebased agricultural habitats in the Louisiana coastal region. Acknowledgments We appreciate the financial support of A.C. Leach and family, Sweet Lake Land and Oil Company, Region 4 of the United States Fish and Wildlife Service, Louisiana Department of Wildlife and Fisheries, and Louisiana Cooperative Fish and Wildlife Research Unit and School of Renewable Natural Resources at Louisiana State University. We are grateful to G.A. Harris, P.M. Yakupzack, T. Delaine, and Cameron Prairie NWR for providing housing and technical support. We thank M.J. Anteau for helpful comments on the manuscript and with field work. We thank E. Johnson for her assistance with field work. We thank Dr. S.H. 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