Forty Years of Recruitment Data for Millions of Ducks, But What About the Accuracy?
Posted on 11/22/2002
If you’re like most waterfowl hunters, you probably eagerly await the release of the results from the USFWS’ May surveys to get estimates of this year’s breeding duck numbers and breeding ground’s potential to produce ducks. You also may have been lucky enough to experience the sheer joy of shooting a banded duck and rushed home from the blind to report band number. However, hunters are probably less familiar with the USFWS’ Waterfowl Part’s Collection Survey. Each year the USFWS asks a set of hunters from across the United States to send in one wing from each duck they harvest. After the end of the hunting season, a group of state and federal biologist get together and examine each of the nearly 100,000 wings they receive and categorize them to species, sex and age (juvenile or adult). Some of the most important pieces of information that results from this process are age ratios (the number of juvenile ducks harvested per adult) for each species.
Waterfowl managers rely on the age ratios generated from the Parts Collection Survey to assess the reproductive success for North American waterfowl. Ideally, managers would like to monitor reproductive success on the breeding grounds, where duck production can vary from year to year depending on water conditions on the prairies, and rates of nest losses and duckling survival. Unfortunately, to collect such data over large portions of the breeding grounds each year would require a very intensive effort and would be prohibitively expensive. Therefore a widely accepted alternative is to sample age ratios of fall-harvested ducks to estimate production during the previous breeding season. When duck production is high, hunters tend to harvest more juvenile ducks than adults. Conversely, when duck production is low, age ratios in the harvest also tend to be lower. These age ratios have been used to measure the continental duck production for over forty years. However in recent years, these age ratios have taken on a new importance because they are used along with data generated from the May surveys and band returns as critical inputs to Adaptive Harvest Management Models (AHM) used to set and evaluate hunting seasons.
While the techniques for categorizing wings have been established for many decades, there has never been a thorough examination of the accuracy with which the wings can be classified to sex or age. However in pilot research conducted in 2000 and 2001, by Dr. Frank Rohwer (Louisiana State University) and Dr. Ken Richkus (former Delta student) found large inconsistencies in age determination of hunter-killed ducks based on examinations of internal characteristics and age categorization based on the wing alone. In their study, Drs. Rohwer and Richkus collected wings from internally aged ducks and surreptitiously included them in a sample of wings that were examined at the annual Wingbees.
Although discrepancies for some species like American Wigeon and Gadwall were relatively low, they found surprisingly high discrepancies for most species, including Mallards. For example, 15.0% of 925 Mallard wings were aged differently at the Wingbee than from internal aging. Moreover, these discrepancies were strongly directional because they typically involved adult wings based on internal aging being categorized as juveniles at the Wingbee. If the internal aging techniques were correct, then this directionality in aging error would mean that age ratios produced by the wing aging were substantially elevated and could have impacts on the AHM process.
Unfortunately, it is not known whether internal aging of Mallards and other early nesting species, like Northern Pintails, is completely reliable, especially for birds harvested in late December and January. Internal traits that characterize a juvenile, such as the status of the reproductive organs, must change at some point to adult traits, but exactly when that change occurs is unknown. So before a full-scale validation of the wingbee aging techniques can occur, we must be certain that internal aging is reliable.
Rohwer and Richkus, with the cooperation of Dr. Robert Cox (USGS, Northern Prairie Wildlife Research Center), will focus their attention toward validating internal aging techniques using a sample of hunter shot birds that are banded. They will be asking hunters that call in to report a band to save the carcasses of their banded Mallards and Pintails with the band still on the bird. Of special importance are the birds that were banded as juveniles in the August bandings. Examination of the carcasses of these birds will reveal when the internal characteristics change to those of adult birds and tell them when internal aging techniques become unreliable. Stay tuned to Delta’s web page to figure out how you can help participate in their important study that will help to provide accurate estimates of age ratios and assist biologists in managing our waterfowl resource.