Oology is a branch of ornithology studying bird eggs, nests and breeding behaviour. The word is derived from the Greek "oion", meaning egg. Oology can also refer to the hobby of collecting wild birds' eggs, sometimes called egg collecting, bird nesting or egging, which is now illegal in many jurisdictions. Oology became increasingly popular in Britain and the United States during the 1800s. Observing birds from afar was difficult because high quality binoculars were not readily available. Thus it was often more practical to shoot the birds, or collect their eggs. While the collection of the eggs of wild birds by amateurs was considered a respectable scientific pursuit in the 19th Century and early 20th Century, from the mid-20th Century onwards it was increasingly regarded as being a hobby rather than a scientific discipline. In the 1960s, the naturalist Derek Ratcliffe compared peregrine falcon eggs from historical collections with more recent egg-shell samples and was able to demonstrate a decline in shell thickness. This was found to cause the link between the use by farmers of pesticides such as DDT and dieldrin, and the decline of British populations of birds of prey.