[CT Birds] mockingbird follow-up

Dennis Varza dennisvz at optonline.net
Wed Mar 28 19:22:42 EDT 2012

Here is a possible habitat change that would be real tough to quantify.

With increased subdivisions  lots become smaller and more heavily  
cultivated. By this I mean fewer neglected brushy patches.

Another interesting trend I noticed as houses become more packed the  
trees around them become smaller.
Big oaks and maples give way to birch, cherry and dogwood trees.

Dennis Varza

On Mar 28, 2012, at 6:24 PM, Chris Elphick wrote:

> Eric proposes a couple of good alternative explanations for changes in
> mockingbird numbers.  Predator-prey population cycling typically  
> occurs when
> there is a tightly-linked dependency among one (or a very few)  
> predator species
> and one (or a very few) prey species.  In contrast, mockingbirds  
> have many
> potential predators and most things that prey on mockingbirds have  
> many
> potential prey.  In these situations, cycling rapidly breaks down  
> (or never
> develops) because the predators can always switch to eating  
> something else when
> mockingbird numbers decline - hence there is no feedback to slow  
> growth of the
> predator population, which has to happen for the cycles to develop.
> It could be that there is an overall increase in predators - and  
> some of the
> potential candidates in the mockingbird case (e.g., Cooper's hawk)  
> have
> certainly increased over a similar time frame.  If that were the  
> explanation,
> though, one would expect the trends of other species vulnerable to  
> those same
> predators to all show the same pattern.  I've not gone through all  
> the trend
> data in detail, but to pick one specific example - cardinals, which  
> one might
> expect to be affected by predators in a similar way to mockingbirds  
> (similar
> habitat, similar size, similar range expansion history etc.), have  
> shown a
> significant overall popualtion increase.  More generally, woodland/ 
> suburban
> species as a group tend also to have shown an increasing trend over  
> the time
> period.
> A change in some competitor species is also possible.  For  
> competition to be an
> issue, though, there would have to be a limited food supply.  I  
> have no data,
> but I don't know of any evidence that multiflora rose (or other  
> food sources)
> have declined - except possibly as a result of overall loss of early
> successional habitat.  Sorting these questions out is tricky - and  
> one of the
> frustrating parts of life as an ecologist is that there is always  
> an alternative
> hypothesis.  The good part is that there are data sources out there  
> that can be
> used to try to tease apart some of the complexity.
> Chris
>  Chris Elphick
> Storrs, CT
> elphick at sbcglobal.net
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