We know that nearly all groups of birds have been declining over the past 50 years. Solid estimates exist of a 30% decline in bird populations in the United States and Canada since 1970.

The American Bird Conservancy recently published its estimates of the annual number of bird deaths by source, all of which are associated with direct or indirect human impacts. Nine threats are identified that each result in the death of at least 500,000 each year.

Mitigating some of these causes of avian death are systemic problems and individual efforts are not very effective. A million birds a year die each year by being trapped in mines and wastewater pits, while 680,000 birds perish from collisions with wind turbines.

Lead poisoning takes the lives of 1.2 million birds a year. Many of these are waterfowl that ingest lead pellets from the sediment of water bodies to use in their gizzards to grind food. Replacement of steel shot with lead shot in shotgun shells is helpful.

Birds are disoriented by the lights on tall communication towers, particularly during nocturnal migration. Those collisions kill 6.8 million birds a year. Collisions with and electrocution by power lines kill 36.5 million birds a year. Again, these deaths are a consequence of the ways humans alter our environment for our own comfort.

Although we have moved away from some harmful pesticides like DDT, chemicals to control agricultural insect pests pose dangers to other animals. The bird toll of pesticides is 72 million birds a year. These deaths are unintended but still add to the toll.

Collisions with cars take the lives of 200 million birds a year. Those are hard to reduce with our lifestyles dependent on car transportation.

An estimated 600 million birds die each year from collisions with windows. Birds either see a reflection of their habitat in the windows during the day or perceive a lighted room beyond a window as a tunnel at night. Significant improvements in reducing reflectivity have been made. We can make a difference here as individuals.

The most devastating source of bird mortality is cat predation. Cats kill 2.4 billion birds each year in the United States. That is twice the mortality of the other eight mortality sources combined.

We need to realize that cats are not part of the native fauna of North America. Rather, they represent domesticated cats that arose by artificial selection in western Asia, likely derived from the African wildcat.

Cats accompanied Columbus and were on the Mayflower. However, they did not become popular pets until the end of World War I. Today, our pet cat population is estimated at 85 million, about half of which spend some time outdoors.

To reduce the impact of cat predation, cats should be kept as indoor-only pets. Cats are simply too efficient at capturing birds. Sure, cats enjoy being outside but a risk-reward analysis to me comes down clearly on the indoor cat solution. People who insist on letting their cats go outside should at least put a bell or a cat bib (a Google search will yield lots of cat bib hits) on their cat. Bird feeders and bird baths should be maintained high enough to deter a jumping cat.

Pet cats are only part of the problem. There are 60-100 million feral cats on the loose. On average, a feral cat kills three times as many birds a year as a free-ranging pet cat. Some animal rights groups that are capturing feral cats to neuter them in an effort to keep the population from growing. Other people favor exterminating feral cats. As an unabashed cat lover, I find these conversations difficult but necessary as our bird populations continue to decline. Imposing the deaths of 2.4 billion birds a year should give us all pause.

Herb Wilson taught ornithology and other biology courses at Colby College. He welcomes reader comments and questions at whwilson@colby.edu