Insectivorous birds like the chimney swift show among the most precipitous declines in numbers, and this has been occurring for many decades. A recent study done on a 50-year deposit of swift guano in a chimney at Queen's University shows a correlation between DDT in the 1950s and a loss of beetles in swift diets. This implies that this pernicious insecticide had effects that were much broader than originally thought and this study published in the Proceedings of the Royal Society provides some clues as to the diminution of populations of insect-eating birds.
Full citation: Joseph J. Nocera, Jules M. Blais, David V. Beresford, Leah K. Finity, Christopher Grooms, Lynda E. Kimpe, Kurt Kyser, Neal Michelutti, Matthew W. Reudink, and John P. Smol. Historical pesticide applications coincided with an altered diet of aerially foraging insectivorous chimney swifts Proc. R. Soc. B rspb20120445; published ahead of print April 18, 2012, doi:10.1098/rspb.2012.0445.
Photo captions and credits: Top - Chimney swifts (Chaetura pelagica) in flight have cigar-shaped bodies attached to swept-back wings. Note the spine-tipped tails. Swifts catch flying insects at altitudes higher on average than other insect eating birds like swallows and flycatchers. Credit: Mike Veltri. Bottom - After excavating into the chimney and up through the guano deposit, samples were taken from the face of the deposit carved out like a soil test pit. A 1cm high stainless steel tray was used to "peel" off each layer. Protective breathing gear was used against dust and pathogens. The work was done before swifts returned from migration. Credit: Chris Grooms, Queen's University.