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) [log in to unmask] ______________________________________________________________________ Ohio-birds mailing list, a service of the Ohio Ornithological Society. Our thanks to Miami University for hosting this mailing list. Additional discussions can be found in our forums, at www.ohiobirds.org/forum/. You can join or leave the list, or change your options, at: http://listserv.muohio.edu/scripts/wa.exe?LIST=OHIO-BIRDS Send questions or comments about the list to: [log in to unmask]