Factors affecting fat content in mottled ducks on the Upper Texas Gulf Coast

Body condition, or an individual's ability to address metabolic needs, is an important measure of organism health. For waterfowl, body condition, usually some measure of fat, provides a useful proxy for assessing energy budgets during different life history periods and potentially is a measure of response to ecosystem changes. The mottled duck (Anas fulvigula) is relatively poorly studied in respect to these dynamics and presents a unique case because its non-migratory life-history strategy releases it from metabolic costs experienced by many related migratory waterfowl species. Additionally, as a species in decline and of conservation concern in many parts of its range, traditional methods of fat content estimation that involve destructive sampling are less viable. The goal of this study was to produce an equation for estimating fat content in mottled ducks using birds (n = 24) donated at hunter-check stations or collected by law enforcement efforts on the Texas Chenier Plain National Wildlife Refuge Complex from 2005 - 2007. Morphometric measurements were taken, and ether extraction and fat removal was used to estimate percent body fat content and abdominal fat mass, respectively. A hierarchical simple linear regression modeling approach was used to determine external morphometrics that best predicted abdominal fat content. A ratio model based on body mass and a length metric (keel and wing chord length possessed equal model support) provided the best relationship with abdominal fat in sampled individuals. We then applied the regression equation to historical check station data to examine fluctuations in fat content over time; fat content or condition varied relatively little with the exception of years characterized by major disturbances. The mottled duck condition model created here can be used to better monitor population status and health without destructively sampling individuals.