Age and Gender Classification of Merriam’s Turkeys from Foot Measurements

1. What they did

Wildlife managers want to know the proportion of young (subadult) to adult hens in a turkey population, because young and adult hens nest at different rates. Sex and age can be read from feathers, spurs, beards and droppings, but most of these methods need the bird in the hand or close views, and trapping is costly and may give a biased sample. Tracks can be found without catching birds, and earlier studies had used track size for eastern turkeys, but nobody knew how accurate this was.

The authors tested whether foot size can tell the sex and age of Merriam’s turkeys:

• Measuring. They trapped 202 Merriam’s turkeys in Arizona (112) and South Dakota (90) and measured two things on each foot: the length of the middle toe pads, and the length from the middle toenail to the heel pad. They recorded each bird’s sex and age (subadult or adult).
• Building the classification. They compared the two states, then used discriminant function analysis to work out which sex and age class each measurement most likely belongs to, with a jackknife method to estimate accuracy.
• Testing it. They applied the classification to 82 Merriam’s turkeys trapped near Grand Junction, Colorado.

2. What they found

Arizona and South Dakota birds

• Middle toe pad lengths were similar in both states. Toenail to heel pad lengths were similar for females, but Arizona males measured longer than South Dakota males. Separate classifications for each state did not consistently do better, so the birds were pooled.
• Males and females overlapped, yet both measurements predicted sex well: about 92% correct from middle toe pads and 98% from toenail to heel pad. Birds with middle toe pads over 5.8 cm and toenail to heel pad over 10.4 cm were probably males.
• The paper gives tables of probabilities for each 0.1 cm step of both measurements, for sex and for sex and age together.

Sex and age together

• Middle toe pads were less reliable. Young and adult females were hard to separate: 24% of large subadult females were classed as adults and 18% of small adult females as subadults. 16% of subadult males were classed as adult males, but only 5% of adult males as subadults. No females were classed as adult males. Overall about 78% were correct (an error rate of 22%).
• Toenail to heel pad did much better: 7% of larger subadult females and 8% of smaller adult females were misclassified, and all adult males were correct. Overall 94% were correct.

The Colorado test

• Middle toe pad length gave the right sex for 99% of the Colorado birds.
• Age was much less reliable: 56% of subadult females were classed as adult females and 55% of adult females as subadult females; for males the errors were 27% and 33%. In the abstract, sex and age together were right only 50% of the time.
• The toenail to heel pad method could not be tested, because the Colorado measurements had not been taken the same way.
• Colorado birds had longer and more variable feet. Some fed year-round on waste grain in barnyards, which may have boosted their growth, including foot size.

Comparison with eastern turkeys

• The largest adult females had middle toe pads under 6.1 cm and the smallest adult male over 6.3 cm. No errors occurred in sexing adult birds with either measurement.
• Adult hens of both eastern and Merriam’s turkeys measured under 10.8 cm from toenail to heel pad.

3. What they concluded

• Middle toe pad and toenail to heel pad lengths classify the sex of Merriam’s turkeys with high accuracy in late winter or early spring; for adult birds, nearly 100%.
• The authors expect that measurements from turkey tracks can be used to predict both sex and age. Early comparisons of eastern turkey tracks suggest the same could work for other subspecies.
• The toenail to heel pad measurement gives better age classes, but it may be hard to use in the field, because toenail marks do not always show in turkey tracks. Adding a correction for the toenail would be difficult, as toenails vary in length and shape and are molted every year. The middle toe pads to heel pad may be a better measure.
• The method still needs to be tested on tracks of marked wild birds of known age and sex.
• Field measurements should be taken between winter and early spring, when turkeys are hardly growing.

4. How tracking was or was not used

No tracks were measured in this study. The measurements were taken from the feet of trapped birds, as a stand-in for the tracks they leave. The study’s purpose was to see whether track measurements could tell trackers and managers the sex and age of turkeys, and the authors discuss what would and would not show in real tracks (toenail marks are not always visible; earlier work found web to middle toe pad markings hard to see without snow).

7. Implications for conservation (from the paper)

• Managers need better ways to estimate and monitor populations. Knowing the share of young to adult hens matters because their nesting success differs between populations.
• At present there is no reliable way to tell sex and age of wild turkeys in the field without capture; track measurements could provide one, after validation on tracks of known birds.
• Foot size may differ between regions and with diet (as with the grain-fed Colorado birds), so a classification built in one area may not transfer well to another, especially for age.