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Subject:

Carolina Wrens Shifting Range (long)

From:

Frank Marenghi

Reply-To:

Frank Marenghi

Date:

Tue, 23 Mar 2010 22:03:14 -0400

Hi all,

I thought folks would be interested in this article I found on the great backyard bird count (GBYBC) website entitled Carolina Wren's Shifting Range: http://www.birdsource.org/gbbc/science-stories/past-stories/carolinawren.html 

It confirms the fact that harsh winters in the northern part of the species range can reduce it's abundance and distribution, particularly in 1977 where their numbers declined ~ 50%. It took about 10 years for the population to fully recover. The northern part of their range is considered by the GBYBC to the Great Lakes area and southern New England according to Christmas Count records. 

Since we probably got more snow than some of out friends up north this year, and it wasn't unusually cold in the northern part of the range, I am optimistic that these guys may fare pretty well this year. 

As was mentioned, feeders definitely help these birds out through tough times but according to another fact sheet I found on-line, "in periods of harsh
        weather birds move off territories in search of food." These suburban populations, while may not be directly comparable to non-suburban populations, are still a valid an important contributor to the modern day population of the species and may help buffer any ill effects the rural birds my suffer. 

When I look at the ebird data for Anne Arundel County for the last 5 years, what I see is a lot of natural variability. In fact, there seems to more variability in numbers of birds between weeks than there is between years. Therefore, safely separating any  differences between years with this data would be tenuous at best. It is natural for there to be fluctuations in their relative abundance (between 0.5 and 4 birds per hour for example for AA Co.) and doesn't necessarily signify a population decline. The abundance (which is really relative abundance) and the birds per hour metric are probably the best to use because they account for effort. Additionally comparing the same location between years would help control for location differences but not eliminate them. Among other factors that could bias the ebird data that are not accounted for are changes in habitat type over time, changes in birder ability over time, which affect detectability. For now, we can just assume these things haven't changed much over the last five years (even though they have). 

Another thing that looked interesting is that the relative abundance seems to be lowest for all years in March, which for AA Co., would correspond to the onset of pair bond formation and nesting, therefore detectability would be expect to be lower. Birds per hour was about the same in March which suggests that if you encountered birds, you were likely to encounter an average number but  that more people were submitting reports with 0 Carolina Wrens on them for the month. 

I also glanced at the Massachusetts ebird data (for the state as a whole) and saw that their numbers for this year are right in the middle of the values for the last five years. 

As was mentioned previously, it will be interesting to see what the BBS surveys pick up this year and next. 


If I haven't put you to sleep yet, Thanks! 


Frank Marenghi
Annapolis, MD


P.S. If I made any glaring errors, please let me know off-list. 



"There are lies, damn lies, and statistics." - Benjamin Disraeli






> Date: Tue, 23 Mar 2010 09:46:41 -0400
> From: 
> Subject: Re: [MDOSPREY] Carolina Wrens
> To: 
> 
> Interesting question. I can still hear two counter-singing Carolina Wrens by my
> house near Frederick. I explored eBird data. Right enough, the "abundance"
> measure for last week (beginning 3/15) was lower than in the same week in the
> previous 3 years (0.34, compared to 0.50,0.47, 0.58 in 2007-09). However, the
> "birds/hour metric" was higher than in 2007 and 2009 but lower than 2008. Quite
> surprising. These data suggest there has been a decline, but it's not
> disastrous - a level of mortality that could be compensated for in one very
> productive breeding season. BBS counts from the upcoming spring should give us
> a better handle on this though.
> 
> Andy Wilson
> Frederick
> photo galleries at: http://www.pbase.com/andywilson
 		 	   		  
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