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Fawns per 100 Does
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Fawn to doe ratios (FDRs) collected for each Deer Management Unit (DMU) in late summer give information on fawn recruitment and survival and are used as an input into the formula for annual deer herd abundance estimation.
For additional Information….
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FDRs are used for monitoring deer population status because they provide information about fawn production and survival which are driven by the nutritional condition of the population. Fawn production is strongly influenced by food availability which is in turn affected by the size of the deer population and the quality of the habitat. In addition, survival of newborn fawns is often related to predation and the nutritional status of the doe.
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Collection and analysis methods
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FDR data are collected from three sources: Summer Deer Observation (SDO) surveys, Operation Deer Watch (ODW), and Snapshot Wisconsin. Each August and September, DNR employees (SDO) and volunteers (ODW) across the state keep records of the number of does, fawns, and bucks observed in August and September. The number of does and fawns from SDO and ODW in a DMU are combined with the number of does and fawns counted in photos taken during July and August from Snapshot Wisconsin trail cameras in that DMU. Each camera site that had at least 10 photos with does in July, August, or in both months contributes data to the DMU’s FDR. To create FDR data for each DMU including DMUs that lacked this data previously, all three data types are combined to fortify each DMU’s data for use in a spatial smoothing model. The spatial smoothing model uses splines to account for correlation between neighboring DMUs and similarities by zone type (i.e., Forest, Farmland). The ratio of fawns to does in each DMU provides an index to current reproductive rates and is an essential component in the formula used to estimate herd size.
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FDRs are shown as average number of fawns per 100 does annually with a 3-year running average to assess trend once enough years of data are available. Average FDRs vary across Wisconsin, generally lower in forested regions than in farmland regions and higher after mild winters in the north. Low FDRs in some counties may reflect higher levels of predation on newborn fawns and populations that are closer to carrying capacity. This metric is an input into the formula that is used to estimate annual deer population size by DMU.
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Limitations and precautions
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No unbiased method has been developed to measure the number of fawns per doe in late summer deer populations. However, trends in roadside observations of does and fawns, especially in forested regions, have tended to match expectations based on other measures of nutritional condition of the herd and severity of winter weather. The precision and repeatability of FDRs are functions of the number of does and fawns observed. At the DMU level sample sizes have sometimes been relatively low. It may be difficult to collect adequate sample sizes in DMUs with few total square miles or a large proportion of urban and suburban land types.
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The specific methods of collecting survey data for FDRs may be evaluated as new technologies and analyses are developed. While trail camera data have boosted sample sizes in many DMUs, the statistical methods used to analyze this data continue to evolve. Determining an appropriate modeling framework for calculating FDRs from doe and fawn photos is on-going and should be periodically assessed.
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Additional background materials related to this metric
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FDR reports are available for viewing on the Wisconsin DNR website dnr.wi.gov keyword “wildlife reports”. Learn how to participate in ODW by visiting the DNR website dnr.wi.gov keywords “operation deer watch”. Learn how to participate in Snapshot Wisconsin by visiting the DNR website dnr.wi.gov keywords “Snapshot Wisconsin”.
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