Tuesday, December 6, 2011

Winter of 2010-2011's toll on birds in Connecticut

The winter of 2010-2011 was one of the worst Connecticut has ever experienced, with some stations recording the greatest snow depth ever, the snowiest January ever, and one of the highest snowfall totals for an entire season. The Boxing Day Blizzard was only the beginning as the state took major hits several more times before all was said and done with a March 24 light snow event. Such a constant onslaught of snow can spell disaster for a number of bird species who succumb to the frigid conditions or perish when they cannot find sufficient food sources. Carolina Wrens, a resident species, are said to be a great indicator of how conditions impact birds as snow can take a toll on their population, preventing them from feeding on various invertebrates on the snow-covered earth. You may have seen some come to your feeders to eat primarily suet, or perhaps peanuts or peanut butter.

I wanted to see if this assertion was accurate or not, and how much their numbers changed from 2010 to 2011, so I decided to use eBird data to take a look at things. I chose to use data by year primarily because, apart from the blizzard in the last week of 2010, the snow for the season in question was focused all in 2011. I excluded December from the 2011 totals as checklists are still being submitted, and we must bear in mind some may still have yet to come in for other parts of the year as well. However, as of around 9 a.m. today, there have been 16,434 checklists submitted to eBird for Connecticut in 2011, and 14,410 for 2010, so sample size for each year does not appear to be an issue. I do acknowledge that we are dealing with data from hundreds of unknown observers of various skill levels across only two years, and it would be foolhardy to draw any certain conclusions.

Look at frequency first, the percentage of all checklists submitted reporting Carolina Wrens of any number in 2010 and 2011:

Whoa! There is a lot to look at here...
  • In 2010, there is a spike in the spring as breeding season begins, with males singing, territorial disputes, etc.
  • There is also a drop in the middle of breeding season, as parents stay quiet and are busy feeding young, before a return to more constant levels
  • See how fast the percentage falls from the last week of 2010, and the Boxing Day Blizzard, to the first week of January in 2011, though both years have falling numbers as winter approaches
  • Then they plummet during the January bombardment of snow in 2011 compared to more constant numbers in 2010
  • 2011 mimics 2010 in some regards with a slight spike in spring, and a decrease in breeding season
  • The recovery is swift and by late summer and fall the 2010 numbers touch 2010 levels briefly
  • While the current population is certainly less than 2010 at the same date, the breeding season definitely looks to have brought numbers of Carolina Wrens back up to some degree
Let's say the species is recorded on a checklist - how many birds are then seen, on average?

  • Once again, a good correlation between years with similar features
  • Notice how the average number drops quickly in January in both years and spikes around June with pairs feeding young - while the species may be seen less, they are often seen together
  • 2011's average is a bit less than 2010 overall, though not enormously
  • However, the most obvious observation is what the heck is that spike?!
The anomalous increase occurs in the December 15 period in 2010, and it does not appear to be a fluctuation that we can attribute to limited sample size. That number, about 2.5, is the average number of birds seen derived from a total of 71 checklists reporting the species between December 15 and 21 in 2010. That many checklists reporting the species in that period is actually higher than average since Carolina Wrens were seen on approximately 52.7 checklists for each of the 48 periods in 2010. If they had been reported on, say, 15 or 20 checklists, then we might have been able to explain it away by saying the data was lacking. That does not appear to be the case.

Looking back up to the frequency graph, we can also see a spike in the period after a consistent decline in December that went unnoticed when we first looked at it. Did the birds suddenly start showing up at backyard feeders in high numbers that week? Were more tallied during surveys because of sheer luck? Could some have been moving to the south a bit and through Connecticut? They are not said to be migratory. It may simply be a coincidence, but it makes one wonder what happened...and it makes my always suspicious and imaginative mind wonder if the wrens had a hint that we were about to be inundated with snow.

Finally, the total number of birds reported each period, a straight sum...

  • Basically what we expected by now with the spring spike, winter drops, and the anomaly previously discussed
  • There is a nice jump in both Octobers as well, with the highest level in 2011 outside of the rapid January decline
  • Interestingly, the highest frequency in 2011 was also in October, though in another period
  • One would figure this would occur after a successful breeding season, but why October instead of earlier in the fall or late summer?
It seems from this data that yes, Carolina Wrens are indeed quite susceptible to large snowfalls and sizable snow depths, but that they can recover relatively quickly with the right breeding conditions the next season. Numbers are down still right now, but not nearly as far as they were at the end of the brutal winter. That strange jump in numbers last December prior to the week of the Boxing Day Blizzard may still be an anomaly overall despite the fact it was sufficiently reported in the context of the period. I do not like coincidences, though, and for the most part, I do not believe they exist. It feels somewhat ridiculous to even imply that birds can predict a storm days before its formation thousands of miles away, but it makes me wonder what was going on there.

Perhaps if we start seeing a bunch of Carolina Wrens in our yards and at our feeders we will know some snow is on the way...hey, wait, I saw one looking for insects in the wheel of my car yesterday!

Scott Kruitbosch
Conservation Technician

1 comment:

  1. Scott,

    Interesting. I have been wondering about Fish Crow. I am seeing very few of them in spots where they had been regular in good numbers througout the winter in past years. I will take a look at eBird numbers for last year vs this year and see if my impression is correct.