[CT Birds] mockingbird follow-up

Chris Elphick elphick at sbcglobal.net
Wed Mar 28 18:24:53 EDT 2012

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 

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 Elphick
Storrs, CT
elphick at sbcglobal.net

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