The study analyzed data from 1,033 North American Breeding Bird Survey routes for 261 bird species between 1987 and 2021.[2][3] Overall, all birds on the continent have declined, with 122 species – 47% of those monitored – showing significant declines and 63 species experiencing accelerated declines.[1][2][3][4] Hotspots of accelerated decline are located in the Mid-Atlantic, Midwest, and California, areas of intensive agriculture.[1][2][3][4] Agricultural intensity, measured by the extent of arable land, fertilizer and pesticide use, is the main predictor of these hotspots.[1][2][4] Bird declines cause lower birth rates and higher mortality rates, affecting insect regulation, seed dispersal, and the food chain.[1][2] The analysis is correlational and cannot separate the influence of individual agricultural factors.[1]