Opportunity costs influence food selection and giving‐up density of dabbling ducks

Abstract

The interaction of animals with their food can yield insights into habitat characteristics, such as perceived predation risk and relative quality. We deployed experimental foraging patches in wetlands used by migrating dabbling ducks Anas spp. in the central Illinois River Valley to estimate variation in seed removal and giving‐up density (GUD; i.e. density of food remaining in patches following abandonment) with respect to seed density, seed size, seed depth in the substrate, substrate firmness, perceived predation risk, and an energetic profitability threshold (i.e. critical food density). Seed depth and the density of naturally‐occurring seeds outside of experimental plots affected seed removal and GUD in experimental patches more than perceived predation risk, seed density, seed size or substrate firmness. The greatest seed removal and lowest GUDs in experimental patches occurred when food resources in alternative foraging locations outside of plots (i.e. opportunity costs) appeared to be near or below a critical food density (i.e. 119–181 kg ha–1). Giving‐up densities varied substantially from a critical food density across a range of food densities in alternative foraging locations suggesting that fixed GUDs should not be used as surrogates for critical food densities in energetic carrying capacity models. Foraging and resting rates in and near experimental foraging patches did not reflect patterns of seed removal and were poor predictors of GUD and foraging habitat quality. Our results demonstrated the usefulness of GUDs as indicators of habitat quality for subsurface, benthic foragers relative to other available foraging patches and suggested that food may be limited for dabbling ducks during spring migration in some years in the midwestern USA.

Mallard survival, movements, and habitat use during autumn in Illinois

“…However, our observations were in contrast to the explanation provided by Legagneux et al (), who suggested that small roosting areas, low duck densities (<6.5 ducks/ha), and high resource availability led to shorter foraging flight distances than studies from North America. Waterbird and duck densities during autumn migration at large wetlands in our study area exceeded densities documented in France (Legagneux et al ; Hagy et al , ), and we posit that food and habitat availability were apparently abundant in portions of our study area outside of designated refuges (Stafford et al ; Hagy et al , ). Additionally, the landscape of central Illinois was dominated by agriculture and harvested crop fields were abundant during our study.…”

“…Flooding and high river levels reduced wetland management capabilities and limited production of high‐quality moist‐soil vegetation in much of the Illinois River floodplain in both 2009 and 2010 that limited subsequent foraging habitat quality for ducks during autumn migration (Hagy et al ). In contrast, an unusually high water table in the central portion of our study area (i.e., Mason County) outside of the river floodplain in both years resulted in an abundance of privately owned wetlands in agricultural fields that likely were of high quality to dabbling ducks and received extensive use (Hagy et al ). Thus, foraging habitat quality on waterfowl refuges in the IRV was relatively low and ducks were likely forced to move to other areas, which could have exposed them to higher mortality during open hunting periods.…”

“…Additionally, the landscape of central Illinois was dominated by agriculture and harvested crop fields were abundant during our study. Despite evidence that waste grain in agricultural fields has declined since the 1970s (Warner et al , Krapu et al , Foster et al ), the relatively short distances traveled on evening foraging flights in the IRV may suggest that adequate evening foraging opportunities existed near wetlands used during daytime, or a combination of factors (e.g., hunting and disturbance) or site‐specific influences, such as foraging thresholds due to predation risks, opportunity costs, or forager condition (Hagy and Kaminski , Hagy et al ), may have limited the distance traveled by mallards during our study.…”

Habitat selection and activities of dabbling ducks during non‐breeding periods

“…Throughout most of winter, open water, wooded, and aquatic bed wetlands likely provide less food energy to most species of dabbling ducks compared to moist‐soil wetlands, and primarily served other habitat functions (e.g., sanctuary, pair bonding, thermal cover). Conversely, food resources in moist‐soil and mudflat wetlands were apparently used during November through January until mean levels were likely too low to provide substantial energy to foraging birds (Hagy and Kaminski , Hagy et al ). Although densities of most species were greatest in moist soil where food densities were also greatest, this trend was inconsistent across other wetland types and patterns of habitat use and selection were likely influenced by many factors within our study area.…”

“…Gordon et al () and Cox and Afton () also observed associations between habitat use of dabbling ducks and the area of wetlands containing abundant foods. Research is limited that formally tests the hypothesis of food limitation on waterfowl during non‐breeding periods (Hagy and Kaminski , Hagy et al ). Food limitation during non‐breeding periods may reduce body condition and have carry‐over effects on subsequent survival and recruitment during the breeding season, but the relationship between waterfowl and food densities is complex (Hagy et al , Williams et al ).…”