North Coast Diaries

Is it a population trend or is it something else?

I always feel obliged to quote Mark Twain before launching into a discussion on statistics: "Figures often beguile me, particularly when I have the arranging of them to myself; in which case the remark attributed to Disraeli would often apply with justice and force: 'There are three kinds of lies: lies, damned lies, and statistics.'"

Some have argued, in recent days, that wintering White-throated Sparrow numbers are on the increase and Oregon Christmas Bird Count data would seem to back this up.

This is a graph of the number of White-throated Sparrows reported on Oregon Christmas Bird Counts between 1971 and 2006 and there's no arguing the fact that White-throated Sparrow detections have increased. A birds/party*hr graph shows a similar trend, though with a lower coefficient of correlation.

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But an increase in detections of White-throated Sparrows is not necessarily an increase in the actual, on the ground, number of White-throated Sparrows. We're dealing with two very different things here: an actual, documented increase in the number of detections AND an hypothesis for explaining why the number of detections has happened.

A change in the over-wintering population of White-throated Sparrows may seem like the simplest explanation, but we can actually form two different hypotheses for the observed change in detections. Both are equally valid on their faces; both are, to some degree, testable:
1. The observed change in White-throated Sparrow detections is due to a shift (change) in the population of White-throated Sparrows in Oregon.
2. The observed change in White-throated Sparrow detections is due to a change in the efficiency of detections by observers.

In other words, the change in detections could be due to changes in White-throated Sparrow populations or it could be due to a change in the population of observers (skill level and/or total effort by observers).

Is the observed change in detection rates unique to White-throated Sparrow?

No, it isn't. Two other sparrow species show similar increases in detection rates and they show them in approximately the same time frame as White-throated Sparrow.

Lincoln's Sparrows actually show a greater increase over time and Swamp Sparrow shows what can only be called a punctuated increase, a lower steady state, an abrupt increase, a higher steady state. It is certainly possible that all three species increased in populations at about the same time, for similar reasons. The problem is that Lincoln's and Swamp Sparrows are not particularly common birds at feeders. They are, however, very habitat specific species that observers deliberately seek out.

A check of Fox and Golden-crowned Sparrows also shows a statistically significant increase over time. Song Sparrows detections, on the other hand, seem to have remained steady.

I would argue that the population change hypothesis is weakened by detection changes in other sparrow species, all at about the same time.

The White-throated increase may also be punctuated

We can actually take the White-throated Sparrow data and divide it quite neatly into pre-1989 and post-1989 data sets and when we do, we get something that looks suspiciously like the Swamp Sparrow graph. In fact, we can (with much less confidence) sort the Lincoln's Sparrow data into two pieces, separated by a punctuated increase.

Something happened in the late-1980's and early-1990's, that seems to have precipitated the observed increase in detections of 3 relatively uncommon sparrow species and whatever it was seems to have happened in a single jump. A punctuated change in the detection numbers for three different species, all around the same time, suggests a change in observer behavior, not species behavior.

All counts do not show the same trends

Looking at individual counts, it turns out that some counts show no change in White-throated Sparrow numbers (Portland, Tillamook Bay, Medford), some show a linear increase (Corvallis) and some show a punctuated increase (Salem,Eugene, Roseburg) and the punctuated change in Coos Bay numbers may be directly attributable to a change in compilers (circa 1998) and, possibly, the institution of "sparrow chumming".

How Christmas Counts have changed

As birding has become more popular, the demographics of Christmas bird count participation have also changed. It turns out that the number of participants in counts has increased.

And the amount of time observers spend in the field during Christmas counts has changed.

So, there are more eyes, spending more time looking for stuff on Christmas counts. Observers are predisposed to maximize their species list. They've probably read ID articles in Birding magazine and absorbed the wisdom contained in Oregon Birds articles like: Irons, D and D. Fix. 1990. How to Search for Passerines More Effectively in Winter: notes on winter habitat microsites. Oregon Birds 16(4): 251-254. There are more CBC participants and they are motivated to put in the extra time to run up those species totals.

There are also demonstrated increases in the number of people maintaining bird feeders (see: La Rouche, G.P. 2001. Birding in the United States: a demographic and economic analysis. USFW Report 2001-1). Field guides have improved the ability for novice birders to correctly identify birds. And the internet has changed the way we communicate with one another through direct email and online listserves like Oregon Birders on Line, Tweeters and many more localized groups.

The best we can do when we look at data and test hypotheses, is to assign the more likely cause(s) and less likely. It seems to me that the more likely cause for the observed increase in detections of White-throated, Lincoln's and Swamp Sparrows is an increase in observer skill and effort. The number of birds now being detected more accurately reflects the numbers that were there all along, but going undetected.

But we have not disproven the change in wintering populations hypothesis. Christmas Bird Count data is a very blunt instrument, rife with biases that allow the user to build all sorts of inferred relationships. They may be real, maybe not.

Real White-throated Sparrow numbers may be on the increase, maybe not. The unequivocal changes in observer demographics for Christmas Counts biases the sample in an inextricable way. We cannot get to real changes in White-throated Sparrow number using these data and the apparent increase in numbers of other sparrows requires that we either assume all sparrow populations are increasing or there is a bias, unrelated to sparrow numbers, at work. Birder demographics is the most obvious external bias and the most likely explanation for what we see in the CBC data.

Christmas Bird Count data, with all its caveats and disclaimers is the only source of comprehensive, winter population data we have. We need a different, long-term data set (that includes more than one person's backyard) to test these hypotheses further.