This maybe one answer to your question.  The best answer I can think of 
off the top of my head.  

Arborvitae cones have a high count of energy, nutrients and food resource, 
(i.e., protein and carbohydrates).  
The Pine Siskins and birds in general need to consume high amounts of 
energy, nutrients for there food resources.  Therefore, the size of 
seeds,etc., need to have high content of energy, the bird needs to 
be able to handle the food (i.e. costs and benefits of Food Handling,) 
(According to Handbook of Bird Biology, Cornell Lab of Ornithology. 
Podulka, Sandy et al. 2004.) 
  If you have any more questions write back?
See you in the Field, 

Jeremiah W. Roth
Conservation Biology 
Applied Geography (GIS)
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