Here are some facts that explain why keeping a cat indoors may not be natural, but how too many outdoor cats are hurting our bird population. (Being outdoors also shortens cats' lifespans to an average of four years due to cars, dogs, injuries, etc.)
DOMESTIC CATS who stay outside or are let out at night are an often overlooked negative influence on fauna in urban, suburban, and exurban landscapes, but they have a significant impact on native rodents and birds (Hawkins et al. 2004). Rodents are missed by few people, but do we want to lose more birds?
In one study in southeastern Michigan, 25% of landowners owned outdoor cats, which killed ≤1.4 birds/cat/week. Over 23 species were killed including species of conservation concern (Lepczyk et al. 2004). There were ≤3,100 cats across the landscape that killed ≤47,000 birds during the breeding season (~1 bird killed/km/day). Even taken conservatively, these data (Lepczyk et al. 2004) suggest that cat predation plays an important role in fluctuations of bird populations.
Devices have been developed that can reduce the efficiency of domestic cats as predators (i.e., collar-mounted pounce protectors like the Cat-Bib™). However, they need to be worn consistently by cats, and most cat owners are reluctant to impose this cat apparel on them (Calver et al. 2007).
Overabundant cats in urbanized landscapes are clearly an increasing problem. More effective spay/neuter programs have to be developed for their welfare. (DeStefano and DeGraaf 2003).