The Downy Woodpecker-Gall Fly-Goldenrod System

by Carl Strang

Last fall I posted an observation of a downy woodpecker opening goldenrod ball galls to get the larvae of gall flies. I did not stop with that observation, but began looking at galls to see how many had been opened. I found that the galls are not uniformly distributed, but occur in clusters scattered among Mayslake Forest Preserve’s meadows and prairies.

Here’s an example of a cluster of ball galls in tall goldenrod stems.

I counted some proportions of opened galls in these clusters in November, and again in the past week, but very informally. I have come out with some general impressions, and want to make more systematic observations in a future season. It appears that most of the foraging on these galls was done in the autumn, as ratios don’t seem much changed since November.

The most striking apparent pattern relates to the distance of gall clusters from woodlands. For instance, yesterday I looked at the galls in the area where I made that first downy woodpecker observation. That patch of galls is practically surrounded by the savanna, and 40 of 47 galls were opened. Earlier in the week I looked at a gall cluster 200-300m from any woodland, and the ratio was much different: downy woodpeckers had opened only 9 of 50 galls. This is generally consistent with the November counts.

When I look at this system again I want to track galls by marking their lower stems with paint dots, checking periodically and determining when they are opened. It makes sense that these woodpeckers, birds of the woodlands, would venture less often to the galls most remote from the trees, but I need to confirm it and see if there is a pattern. For instance, does the proportion of opened galls decrease gradually, or is there some threshold distance where the proportion of opened galls suddenly drops? Also, when exactly are the woodpeckers exploiting this food source? Of course, while making these counts I will be looking for clues that suggest why the pattern (if any) is what it is. Also, this is a three-species system, and what if anything is the response of each (plant, insect and bird) to the others?


1 Comment

  1. April 6, 2011 at 5:59 am

    […] is to be expected. On the other hand, they also go out into open areas to forage, as I have noted in their attention to goldenrod gall flies. Do the birds show a particular pattern of preference? I have two full years of data, and have […]

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