Vegetated field margins are important landscape elements in many regions of Mexico. They can be traced to the pre-Hispanic period, especially in central Mexico, where Indians planted hedgerows, living fences and terraces, mainly to prevent erosion. Today, vegetated field margins are adjacent to crop fields and also form networks similar to those seen in Europe. An example is the agricultural landscape of El Bajio, Guanajuato, Mexico, where the density of hedgerows can reach 80 m per hectare. On average, hedgerows are 7--8-m wide, 6-m tall, with approximately 13 trees and three tree species for every 100 m of length. Field margins delineate properties and provide local farmers with many useful products, e.g., wood, fruit, medicine and fodder for animals. These structures also provide habitat for many plants and animals as well as serve as movement corridors through the agricultural landscape.;To investigate the importance of field margins for birds, resident and Nearctic-Neotropical migratory birds were surveyed in 40 field margins during spring and winter 1999 and 2000. The structural and botanical characteristics of the field margins, and the characteristics of the surrounding landscape, were related to bird species richness and abundance. At the local scale, the size of the field margin, its vertical complexity, and the abundance of trees and tree species had a positive influence on bird species richness and abundance. The most important landscape-scale variable was the density of hedgerows around field margins. These observations are considered in terms of practical management for bird conservation in the area.;Finally, to examine if hedgerows in this landscape were functioning as sinks (ecological traps) of birds, artificial nests were used to compare rates of predation in two types of hedgerows with different vegetative structure (simple and complex), and a tract of scrub forest. Significantly greater predation rates were found in scrub forest and complex hedgerows than in simple hedgerows, reflecting the higher number of predator species found there. Hedgerows as well as other narrow corridors may not always be unsuitable for nesting birds. They are valuable to avifauna in regions of intensive agriculture and they need to be protected.
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