Food availability for wild animals is notoriously difficult to measure, and few of the studies citing food abundance as a cause make any attempt to measure it directly. Instead, evidence is presented in the form of proxy variables that are thought to provide an index of food abundance. For rural sparrow populations, in common with those of a variety of declining farmland birds, changes in agricultural practices are cited, including increased use of herbicide and insecticide, and changes in sowing and harvesting regimes. For urban sparrow populations, the main factors are said to be conversion of front gardens for off-road parking, development of brownfield sites, and planting of ornamental shrubs, all leading to a lack of insects to feed nestlings.

 

I disagree with the proposition that a decline in food abundance has caused House Sparrow populations to collapse. There is little or no critical evidence to suggest that this is the case, and arguably such evidence is impossible to obtain, since there are so many potential covariates of any of the proxy indices that have been used for food abundance. For urban areas in particular, it could be argued that the environment has become greener, with better air quality over the period of sparrow decline, and it is unarguable that there has been a vast increase in urban litter, with commensurate feeding opportunities for Sparrows.

 

Instead, I think the answer to the problem is quite simple, but that researchers have been looking in the wrong place. It would be unfair, however, to say that there is an elephant in the room. The culprit is far more stealthy and elusive.