Second North American Sea Duck Conference - Patuxent Wildlife ...

SECOND NORTH AMERICAN SEA DUCK CONFERENCE
EVALUATING VALIDITY OF USING UNTESTED INDICES OF BODY CONDITION
Jason L. Schamber, Daniel Esler, and Paul L. Flint
US Geological Survey, Alaska Science Center; jschamber@usgs.gov
Body condition is presumably linked to survival and reproduction, and subsequently population
dynamics in many birds, including sea ducks. Because direct measurement of body composition is
lethal, thereby limiting application to retrospective analyses, condition indices are widely used as a
means of relating individual body condition to future fitness. Most indices include body mass scaled
by structural size indicators calculated as simple ratios or multiple regressions. Scaled indices are
often applied arbitrarily with little or no justification; which we term “blind” use. We tested the
underlying assumption of these indices that there is a direct, consistent relationship between scaled
indices and body condition. We examined this assumption using two approaches across five species
of waterfowl: wintering Barrow’s goldeneyes (Bucephala islandica), molting harlequin ducks
(Histrionicus histrionicus) and breeding greater scaup (Aythya marila), northern pintails (Anas acuta)
and American wigeon (Anas Americana). First, we developed predictive equations to evaluate the
assumed relationship between observable condition and size-adjusted body mass. Second, we applied
various scaled indices to compare their ability to predict observed values of fat and protein. Body mass
was a moderate to good predictor of protein across all species examined (R 2 =0.58-0.84); however, body
mass predicted total fat with considerably more variability (R 2 =0.27-0.81). Inclusion of structural
measures in predictive equations improved precision of estimates ≤ 0.10 percentage points for protein
and fat across species, with the exception of fat in wigeon (0.21). Model precision differed for each
gender and between adult and juvenile goldeneye. For breeding birds, a breeding status covariate was
the most important component of model structure. Across species, scaled indices predicted protein less
precisely or equally well as body mass. Conversely, in most species the ability of scaled body mass to
predict fat was improved, although the best index varied with species. Further, by scaling body mass
to predict fat the gain in precision among species was ≤ 0.13 percentage points and reduced model fit
in many cases. The underlying logic of improving condition estimation using size-adjusted body mass
is generally valid, although does not translate into recommendations for globally applicable indices
of live birds. The improvement in precision is highly variable between tissues and among species, as
well as within species between sex/age classes. We strongly discourage the use of unverified indices;
subjectively selecting indices likely does little to improve precision and often may inflate estimated
variance. The benefit to using a scaled index was negligible as compared to just using field body
mass. Thus, we recommend that investigators use body mass alone to estimate condition rather than
“blindly” selected scaled indices. Investigators desiring increased precision may sacrifice a sub-sample
of birds to build equations appropriate for their species, sex, age, annual cycle stage, etc.; however,
they should recognize that this may provide little improvement over body mass alone.
NOV. 7-11, 2005 ANNAPOLIS, MARYLAND, USA
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