Body mass in overwintering waterfowl is an important fitness attribute as it affects winter survival, timing of spring migration, and subsequent reproductive success. Recent research in Europe and the western United States indicates body mass of mallards (Anas platyrhynchos) has increased from the late 1960s to early 2000s. The underlying mechanism is currently unknown; however, researchers hypothesize that increases are due to a more benign winter climate, increased food availability through natural and artificial flooding, introgression of wild mallard populations by game-farm mallards, or shifting of wintering distributions northward. Further investigation of factors related to winter mallard body mass increases and whether this phenomenon is occurring in other major flyways could increase understanding of intrinsic and extrinsic variables influencing waterfowl fitness. We collected and analyzed mallard body mass data in the Lower Mississippi Alluvial Valley from 1979 to 2021 to determine sources of temporal variation. We measured hunter-harvested mallards from private hunting clubs, public hunting areas, and duck-plucking businesses. Mallard body mass increased by approximately 6% among all age-sex classes from 1979 to 2021. We also compiled weather data (rainfall [cm], weather severity index information [WSI], river gage discharge [cfs] and height [m]) to relate to mallard body mass measurements.

To analyze age and sex differences among and within years from 1979 to 2021, we categorized each mallard into one of four classes comprised of adult males, adult females, juvenile males, and juvenile females, referred to as AgeSex. To explore how mallard body mass has changed from 1979 to 2021, we used year as a fixed effect within our models. Because study years (or duck hunting seasons) span calendar years (often Nov-Feb), for clarity our use of the term “year” refers to the duck hunting season initiating in November of that year and spanning to February of the next calendar year. Days refer to chronological days within hunting seasons. Because hunting season dates varied among years, we represented days within seasons as modified Julian days, with the earliest date that a mallard’s mass was measured across the study labeled as day 1 (November 19th) and each subsequent day numbered sequentially until day 83 (Feb 13th), the latest date a bird was measured.

To assess the relationship of cumulative rainfall and cold weather severity (or Weather Severity Index developed by Schummer et al., 2010; WSI) with mallard body mass, we compiled climate data from representative National Oceanic and Atmospheric Administration (NOAA) weather stations. The variables we used were daily cumulative precipitation (cm) and minimum and maximum daily temperature (°C). We obtained data from Yazoo City, Yazoo County, Mississippi (station name: Yazoo City 5 NNE) for winters 1979–1980 through 1982–1983 and from Arkansas (station names: Stuttgart 9 ESE, Des Arc, Searcy, Georgetown, Pine Bluff, Augusta, Wynne, Alicia, Keiser, Eudora, Monticello Municipal Airport, Marianna, Arkansas Post, Rohwer, Paragould, and Pocahontas) for winters 1990–1991, 1999–2000 through 2003–2004, 2015–2016, 2016–2017, 2019–2020, and 2020–2021 based on proximity of sampling sites and weather stations. We recognize that daily rainfall on a given date may not be the best measure of how precipitation influenced body mass on the date of harvest. Because the known movement of waterfowl before measurement of body mass was unknown, and it can take waterfowl anywhere from 8 to 72 h to digest most food resources (Charalambidou et al., 2005), we calculated a 3-day cumulative rainfall before the dates of mallard measurement to more accurately represent the relationship between precipitation and mallard body mass. Similarly, we did not use in our analysis the daily average temperature from the day that a bird was measured. Instead, we calculated daily average temperatures for each day and season and used these values to calculate a 3-day mean of daily average temperatures before mallards were measured. Finally, we calculated WSI using our 3-day mean temperatures (by modifying the WSI equation from Schummer et al., 2010) to evaluate the relationship of weather severity and mallard body mass. We modified the WSI equation to use three-day means rather than two-week means because we wanted to represent cumulative of temperature experienced by ducks more recently before measurement (See Eq. (1) in Veon et al. 2023).

Finally, we used river gage height (m) data as a function of flooding. River gage height values were identified using associated discharge (cfs) values from rate tables obtained from the USGS Lower Mississippi-Gulf Water Science Center for Mississippi winters 1979–1983 (gage name: Big Black River near Bovina) and for Arkansas winters 1990–1991,1999–2000 through 2003–2004, 2015–2016, 2016–2017, and 2019–2020, and 2020–2021 (gage names: Black River near Corning, Black River at Pocahontas, Black River at Black Rock, Cache River at Egypt, White River at Newport, White River at Georgetown, Cache River near Cotton Plant White River at DeValls Bluff, L′Anguille River near Colt, L′Anguille River near Palestine, Bayou Meto near Lonoke, Bayou Bartholomew at Garrett Bridge, and Bayou Bartholomew near McGehee). We collected data from river gages nearest our sample sites to examine the relationship of daily river height to mallard body mass. Similar to rainfall, we expected mallard body mass would be greater when river levels were higher because of increased foraging habitat. Importantly, we found rainfall and river gage height were not highly correlated (Pearson Correlation [r] = 0.25). This result may indicate that rivers can fluctuate from rainfall upstream of areas absent of local rainfall or by human control (e.g., locks, dams, levees) (Junk et al., 1989; MRC, 2007). Additionally, forage availability may reflect different combinations of rainfall and river flooding. Rainfall may be more influential in flooding habitat not connected to or near river systems (e.g., puddling or ponding), whereas river flooding more likely to provides access to foraging habitat in riparian and adjacent overbank habitats (Smith and Callahan, 1983; Galat et al., 1998, Heitmeyer, 2006).