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Deer and Rain: Take 2

Posted: May 1, 2015

When it rains did deer movement follow the same patterns in 2013 and 2014?

Almost 1,700 readers responded to a survey about deer movements. One of the survey questions asked: How does rain affect deer movements?

Here were your responses:

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In October 2013, we found that temperature had no effect on deer movements because there were no days that were super cold and rainy. 

Also, rain did NOT affect the movement of female deer. There was some evidence that females moved a little less in rain, but there was very little difference. However, there was a large difference in male deer movement. 

So, how did deer move in October 2014?

I performed the same analysis of deer movement in October 2014 as we did for the 2013 data.

I calculated the total miles traveled by deer during the month of October both during the day and at night.

Then, I accessed NOAA weather data for temperature and rainfall. Total daily rainfall was used to define “steady” rain as <1.0 inch and >0.1 inch (because we had no information on how long the rain fell each day).

In October 2014, there were 8 days of rain. Maximum temps ranged from 44 - 77⁰F, while minimum temps ranged from 25 - 58⁰F.

I analyzed the movement data for these four factors:

1) Difference between day and night
2) With or without rain
3) Minimum temperature (> or <45⁰F)
4) Male or female

And now, for the 2014 results:

Temperature did not appear to have any effect on the total distance traveled by both males and females. Only 4 days in October saw temperatures dip below 40⁰F while it was raining. This is too few days to detect an effect.

For females, there were no significant effects of rain or daylight on the average distance traveled. There is little variability, suggesting that in October most female deer move about the same amount during the day as they do at night.

The graph below represents the distances moved by females in October 2014. The squares are the average distance traveled, while the vertical lines (95% confidence intervals) are a measure of the variability of deer movements.

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In 2014, throughout the month of October, females traveled just under ½ a mile per day – no matter the weather.

But males are a different story. Rain had a significant effect on movement, but daylight did not. On average, males moved less when it was raining, regardless of whether it was day or night.

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When it rained, males traveled about 1/2 a mile – essentially the same as females – whether it was day or night. When it was not raining, males moved about 8/10 of a mile.

Comparing 2013 and 2014

The scientific process usually starts by observing some phenomenon, such as patterns in deer movements. The next step is to explain what caused the pattern.

However, scientists first like to see the pattern repeated.

In the case of deer movements and rain, we now have 2 years of data that show (in October!) that females do not seem to be affected by rain and males move less when it rains.

So why are males affected but not females? We don’t know!

But, to those of you who agreed deer would move less with rain, you are an expert when it comes to bucks, but the does are still a bit tricky!

-Kate Williams

Kate is a graduating senior in Wildlife and Fisheries Science in the Department of Ecosystem Science and Management. After graduation she is headed to northeastern Washington to work on a study of mule deer.

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