But the most recent results from two surveys — one nationwide in France, the other limited to one region — caused scientists to sound an alarm, because the results suggest that agricultural methods are hurting birds, according to Benoit Fontaine, a conservation biologist at the National Museum of Natural History in Paris and a leader of the national survey, conducted twice a year by volunteers.

“In the agricultural land,” he said, “there is something really bad going on.”

Over the past 17 years, the numbers of birds in farming areas have dropped by a third. Some of the species have declined even more: Meadow pipit populations, for example, fell by 68 percent. Dr. Fontaine described the situation as “catastrophic.”

He suspects that pesticides used in agriculture and intensification of land use are linked to the decline, although neither survey comes to conclusions about causes or makes any policy recommendations. But he pointed to the loss of insects, the major food source for many birds, as a likely result of pesticide use.

Vincent Bretagnolle, an ecologist at the Centre for Biological Studies Chizé, led the other survey, which has been conducted by scientists in the Deux-Sevres region in the western part of the country for 24 years. “Our results are completely in agreement with national and European surveys,” he said.

The trend is long-term, he said, except that recently, particularly at the national level, the dip has been steeper, and the more generalist bird species have been affected as they weren’t in the past.