Please consider the negative impact of cats before you release them. You like the birds, right? Well, despite what the feral-cat advocates and their websites say, cats absolutely kill wildlife, including birds, and destroy nests. This is well-documented in the peer-reviewed literature (see some references below) and the denial by feral-cat advocates is completely unsupported by science. To quote Daniel Patrick Moynihan, "Everyone is entitled to their own opinion, but not their own facts." If you're just an animal rights activist, why would you value the cats' lives over that of the rats anyway?
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By comparison, the 2 rat species you are seeing are non-native and don't belong here, from an evolutionary perspective.
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Since people here will attack me, I'll let you know that I'm a biologist who has been working on songbird population dynamics in developed landscapes for nearly 20 years. I don't make things up. Wouldn't know how to :).
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Current lit is reviewed here:
Longcore, T., C. Rich, and L. M. Sullivan. 2009. Critical assessment of claims regarding management of feral cats by trap–neuter–return. Conservation Biology 23(4):887-894
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and local literature samples here:
Marzluff, J.M., R. Bowman, and R. Donnelly, editors. 2001. Avian Conservation and Ecology in an Urbanizing World. Kluwar Academic Press, Norwell, MA. 585pp.
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Marzluff, J.M and K. Ewing. 2001. Restoration of fragmented landscapes for the conservation of birds: a general framework and specific recommendations for urbanizing landscapes. Restor. Ecol. 9:280-292.