What's happened to all the songbirds? As a generalist surrounded by subject matter experts, I recently went to one of my science guys to pose that question I'm hearing with increasing frequency from backyard birders to avid outdoorspeople. Whether an accurate assessment or just perception, it certainly seems like we see fewer types and reduced quantities of many of the small birds once thought plentiful, and that the skies and branches are now largely hosting crows, pigeons, mourning doves and non-native finches.
My friend, a science guy used to getting questions from this generalist, at first just sighed. People like me know there's not often a simple 1 + 1 = 2 answer to such questions, as much as we might wish it were so, and he once again said something to that effect before pointing me to a series of papers and articles. To my surprise, what I then read was not about the disappearance of birds but, rather, about the alarming loss of bugs.
As appealing as a few less flying insects might sound as we approach long summer nights, that's obviously not a great thing. Even if your last biology class was back in high school it's obvious that our world fully depends upon on what Harvard biologist E.O. Wilson refers to as the littlest building blocks of life. While it may be a tragedy of operatic proportions if the American Condor finally does die off, it's death of planet Earth if we lose bees and other pollinating insects.
The articles point to insect population declines of 40% and more in both Europe and the U.S., including well studied areas where official government efforts to control exactly such problems have been underway and largely proven unsuccessful. Reasons cited include pesticide use, climate change, increase in CO2 levels, the spread of agribusiness monoculture crops (such as corn and soybeans), urban- and suburbanization and other causes of habitat destruction. (Check out "Vanishing Act: Why Insects Are Declining and Why It Matters" (http://e360.yale.edu/feature/insect_numbers_declining_why_it_matters/3012/, a fine summary which in turn links to other articles.)
Back to the question about birds: At least one recent study (http://www.ace-eco.org/vol5/iss2/art1/) speaks to the possibility that birds which feed on insects have less insects available, thereby not only impacting their population but also making room for those birds which feed on seed or are omnivores.
Is that why I see fewer robins and towhees in my backyard than in recent years but more and more House finches and crows? Again, a simple 1 + 1 does not always lead to 2 and so I can't reasonably draw such a conclusion. One simple statement that I do know to be true, however, is this: Things are horribly out of balance, and our species is to blame.