deer browsing and songbirds

December 12, 2007

in Science

The latest issue of the Ibis, the journal of the British Ornithologists’ Union, had several excellent papers. One was an overview of the impacts that deer browse has on habitat quality and subsequently bird life. I’d like to highlight some salient points, because they illustrate the interconnectedness of ecosystems, and can serve to help the average person understand that too many deer don’t just mean fewer wildflowers in the woods.

We’ll start with that simple point. Deer browsing changes under- and mid-story vegetation not only by changing the species composition (skewing it towards unpalatable species) but by reducing the abundance and density of trees, shrubs, and vines. For birds, this can mean a loss in available nest sites and roost sites, and an increased vulnerability to nest predation. This seems fairly intuitive. The impacts of deer browse on the food supply of birds is often less direct.

Deer prefer to eat growing shoots of plants, which not only affects the growth of the plant, but may also delay or prevent flowering. This can reduce the number of pollen-seeking insects on which birds may feed during the breeding season. No flowers means no fruits or seeds, which birds consume later in the season, often fueling migratory flights or providing winter forage. Of course, deer also eat fruit and seeds, putting them in direct competition with birds.

Many insects, such as lepidoptera larvae which are so important to birds feeding nestlings, also prefer to feed on actively growing plant tissue just as the deer do. This paper notes a number of studies have found that these invertebrates can be reduced by deer browse in the vegetation layers that can be reached by deer.

Leaf litter thickness often also changes in forests with high densities of deer. Their browsing can alter the amount of sunlight that reaches the forest floor, and decreased density of trees and shrubs, as well as direct grazing on herbaceous plants, often coincides with more grasses or bare ground. This can all result in reduced number of some types of leaf litter invertebrates that are important to ground foraging bird species.

The New York Times just had a short article on the stupendous increase in White-tailed Deer in the United States — doubling in population the last twenty years to an estimated 32 million animals. That’s 12 million more than were here prior to European settlement, in far less space. The Times graphic shows that here in Michigan, I have a 1 in 86 chance of hitting a deer with my car in the next year, the second-highest odds in the nation.

The Ibis paper (and indeed the entire issue) is available for free at the journal home page.

Gill, R. M. A. and R. J. Fuller. The effects of deer browsing on woodland structure and songbirds in lowland Britain. Ibis 149:119-127. doi:10.1111/j.1474-919X.2007.00731.x

Here are some links to previous posts I’ve done that included the impact of deer overpopulation:


{ 9 comments }

John December 12, 2007 at 8:47 pm

I'm surprised New Jersey is not higher on the deer accident list with the density of both drivers and deer here.

Clare December 12, 2007 at 11:33 pm

I'm surprised that there are only 12 Million more deer now than pre-European settlement. I had always thought it to be much higher. White-tail deer didn't make it to Manitoba until the end of the 1800's, beginning of 1900's and spread with the spread of agriculture. When I was still living there had pretty much completely supplanted Mule Deer in the province.

The good news for birds though is they are tasty.

Nuthatch December 13, 2007 at 6:47 am

I think their estimate might be a tad low. In urban counties such as the Detroit metro region, they have no good way to count them — or more importantly control them since they can't be hunted. I was sent an astounding photo by a cop last year of 7 enormous bucks (all over 6 points) in someone's yard that backs up against a woodlot a couple miles away…very urban. Kinda scary.

Clare December 13, 2007 at 1:04 pm

One of the largest bucks I ever saw (in terms of antler size) lived about a half a kilometre away from my apartment in Winnipeg, right near the airport. The first time I saw it, while driving home from work one night, my first thought that ran through my head was "What's an elk doing here".

I had heard that it was poached a couple of years after I saw it, possibly by someone with a crossbow.

MartyL December 14, 2007 at 2:22 pm

Where I live deer are hampering reforestation projects. They seem to be attracted to hardwood plantings in old fields, it doesn't necessarily kill the trees, but every spring they chew off every single bud. We have acres of hardwoods over ten years old that are still only 3 to 5 feet tall.

Aydin December 18, 2007 at 1:47 pm

How on earth do they know there were 20 million deer before the Europeans came? That's a wild guees!

Nuthatch December 18, 2007 at 6:12 pm

I don't know where the NYT got their 20 million number, but it's my understanding that estimates of pre-settlement deer populations are based on archeological middens, and models based on the age/class structure and sex ratios of remains found in the middens, and/or available appropriate habitat using pre-settlement vegetation maps. I think most estimates I've read actually give the number as 10 to 20 per square mile.

Wild Flora December 19, 2007 at 6:54 pm

Have you read Richard Nelson's Heart and Bood: Living With Deer in America? He explains the "deer problem" in a way that will break your heart regardless of whether you love them or think they are "rats with antlers."

As it happens, I found your item about deer right after posting something to my own blog about how contemporary landscaping practices encourage high populations of deer in the suburbs. I just added a note about your post as a followup — but I don't understand the whole trackback URL thing, so this comment may be the only way you'll find out about it. (Sorry.)

My recipe for roast venison is available on request. Unfortunately, however, I read somewhere (in Nelson's book perhaps) that killing the males, as hunters usually do, doesn't do anything to reduce deer populations.

Nuthatch December 20, 2007 at 12:45 pm

I've not checked that book out, but have added it (and your blog) to my to-do list! Thanks for the note.

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