Grazing Cows And Bird Disturbance
In today’s food system, the image of grazing cows is a powerful one. Being outside and living in a field is a far more natural life for cows than most confinement systems provide. While allowing cows to graze may be more humane, it’s not as simple as we think. Grazing lands also support habitat for other animals, including grassland birds. Many of these species are experiencing population declines due to habitat loss and degradation. Where do grazing cows fit into this puzzle? Does grazing benefit birds by modifying vegetation in a natural way, or do grazing cows negatively impact grassland birds through trampling of nests or other disturbances. There is little information about nest destruction by cows and existing studies have small sample sizes. However, research with artificial nests has shown destruction rates as high as 90%.
This study of nine different regions in Canada assessed the destruction of grassland bird nests by cows. They also evaluated the impact of management and geography. The review looked at 18 studies, examining everything from “grazing system (rotational or season-long), grazing pressure (stocking rate or grazing intensity), ecoregion, grassland type (native or seeded), avian species, and nest fate” to draw a comprehensive picture. Importantly, they only defined nest destruction as a loss that was attributed directly to cows. The study covered a total of 9,132 nests and found that only 38% were successful, while 53% failed. Cows accounted for less than 3% of nest failures while the vast majority (87%) were caused by predation. The proportion of nest failure depends on habitat, with what the authors describe as “substantial variation.”
This study concludes that the risk of nest destruction by grazing cows is very low. However, the authors note that their estimates are based on studies that have methodological inconsistencies and flaws. While they note that the rates of nest destruction may be too low to have a substantial impact, the holes in the data suggest that more study may be needed before we draw solid conclusions about the relationship between grazing and nest disturbance.