Birds, Buckthorn and Oaks

by Carl Strang

Now that the early part of the migration season is past, I can do a preliminary test of the Birds-Buckthorn-Oaks hypothesis. The idea is that migrating birds seem to prefer buckthorn infested woodlands early in the migration season because the buckthorn understory provides shelter unavailable in restored, oak-dominated woodlands because usually the oaks have not yet leafed out to provide food and shelter. Data from last year supported this idea, because late-season migrants showed a shift from the buckthorns to the newly leafing oaks. This year a further, better test was made possible by the warm early season stimulating oaks to break buds early, so that they were well in leaf for the early part of the migration season. So, what did the data show? I compiled the numbers of neotropical migrants that don’t nest on Mayslake Forest Preserve that I counted prior to May 19 (when oaks began leafing last year), comparing last year’s counts to this year’s. The results were stark: I counted only 5 of those birds in the buckthorn woodland, compared to 22 in the restored savannas (compared to 80 and 34, respectively, last year. Numbers were low this year, in part because my back strain limited my outings, but also a lot of migrants seemed to be bypassing Mayslake).

Things are moving along. This olive-sided flycatcher, a late season migrant, stopped by Mayslake on May 14, earlier than usual.

Using last year’s proportions to calculate expected values if there had been no difference between years, the values would have been 18.9 in the buckthorn-dominated woodland, 8.1 in the savanna. It should come as no surprise that the resulting chi-squared test statistic showed a highly significant difference (34.07, even with only 1 degree of freedom, is far above the threshold). With the migration progressing so rapidly, I don’t know if there will be enough observations in the late season to consider separately, but if so there will be an additional post. Either way, I am satisfied that the data support my point.

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1 Comment

  1. June 4, 2012 at 5:51 am

    […] I mentioned in my previous post on this subject, the songbird migration was nearly done by mid-May. In fact I have only 3 […]


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