I would venture to guess many of us have witnessed a cat with a dying songbird. Nature’s red domestic. But have we stopped long enough to think about the extent to which our house pets are responsible for declines of songbird populations? What other human activities reduce populations of songbirds and other birds from our (sub)urban and natural landscapes and our agro-ecosystems? There are many such direct sources of mortality including pesticide ingestion, destruction of nest or nesting hens by farm and forestry machinery, hunting the birds themselves, collisions with automobiles, buildings and cell phone towers or windmills and slicking from oil spills. But what are their single and combined impacts? Paul Smith, a research scientist recently hired at the National Widlife Research Centre, has recently been a guest editor, along with Travis Longcore, of an important series of contributed papers on this theme, in the journal Avian Conservation and Ecology.
The many contributors provide some of the first ever estimates for direct sources of mortality of birds at a national level. There are numerous potential applications in the regulatory world, but the two primary applications for conservation biologists are: i) To identify information gaps and prioritize more detailed investigations on particular industrial sectors, human behaviours, or geographic areas (e.g., at present, cats kill several orders of magnitude more songbirds than wind turbines, suggesting that studies of the former are more urgently needed than the latter); and ii) As a starting point for more detailed investigations of demographic impacts (that is, realistic population projections under conditions of continued or elevated threats of mortality from said causes).
Several authors note shortcomings relating to imprecision and inability to translate mortality into population “effects”. Many of the estimates are imprecise because the necessary pieces of information are lacking. In particular, species-specific information was often lacking, so that it is difficult to relate mortality to specific species or populations. Species differ in susceptibility, and a mortality rate of 1,000,000 per year for a suite of songbirds might be trivial or devastating, depending on how that mortality is spread across species already at risk. Similarly, 1,000,000 dead seabirds is arguably more of a concern than 1,000,000 dead songbirds, given that the latter tend to be more towards the r-selected end of the spectrum; able to withstand large natural mortality and potentially able to compensate for additional non-natural mortality through density dependent processes. Also, although not the focus of these studies, it is important to point out that clearing of forest to make way for development, for example, does more than kill birds: it removes the habitat permanently from the landscape. These legacy effects can have lasting demographic impacts, even when direct mortality is small.
Professor & student