How many ducks is a normal amount of ducks?

2 04 2014

After independent beach walker Doug McNair raised the alarm about an increased number of White-winged Scoter carcasses on Cape Cod this winter, I felt even more drive to work up some of our data from that area. With a particular eye toward the annual counts of scoters and Common Eiders from month to month, I generated some very simple, rough and ready, back of the envelope charts for your perusal. I disclaim: I am a veterinarian and not a scientist. My capacity to analyze data is that of a commoner. Still, I find these rather interesting to look at.

By way of a bit of guidance, the y-axis here is measured in carcasses/km, which we refer to as an “encounter rate.” We know we aren’t finding every bird that washes up dead in any given month; at best, we get a snapshot. Some species turn up more commonly, and some are found more commonly when they do. Those can be two distinct issues we have to deal with. Additionally, different beaches generate different numbers of birds, which can skew things. Some of our beaches come and go from year to year as volunteers join or retire, so the year-to-year comparisons are not perfectly apples to apples in that respect either. Given those caveats, and the knowledge that there are many others, give a look at these three charts, showing encounter rates for a handful of either the most common species found (gulls, eiders, scoters) or species of particular interest given recent die-offs (alcids). Click on each chart to view a larger version.




An additional factor to consider is the scale involved. Some years, the encounter rates all remain low, barely exceeding 1.00 carcasses/km for any species. Other years, the encounter rate for a particular species may approach 1.6 carcasses/km (Common Eiders in 2011, for instance). So keep an eye not only on the lines themselves, and the peaks and valleys, but also the magnitude from one year to the next.
As for scoters, their general profile is that their numbers increase in winter (to be expected given their life history and migration patterns). But some winters are particularly bad for them, and others seem fairly mild. 2010 and 2011 didn’t see many, for instance, but January and February 2012 show higher numbers. Whether that’s actually a trend is far beyond the power of these very rudimentary charts to determine. But I will certainly be looking forward to seeing what 2013’s data show us for the scoters, given that we’ve gotten a handful of reports from beachwalkers and wildlife rehabbers that they do seem to be beaching more commonly of late.

So, Seanetters, tell me, have any questions about this stuff? Any sorts of data you’d particularly like to see? I’m at your service.



2 responses

6 04 2014

First of all, thank you for providing feedback to the beach walkers. It is important to see that the data we generate is worthwhile and scrutinized. Also, would it be helpful to compare beached bird counts to live bird counts (e.g. from ebird) to provide percent of beached to live sightings as a function of time? Perhaps more scoters are simply present but the percent which are beached doesn’t change.

10 04 2014

Completely agree! It’s a great idea to do a side by side of live birds v. carcass encounters, even just to get a rough sense.

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