Conservation value of working landscapes in Mesoamerica for Nearctic-neotropical migratory birds

Abstract

Populations of many of Nearctic-neotropical migratory birds have declined in the past several decades, recent estimates suggested a dramatic loss of 2.5 billion birds over the past 50 years in North America. Habitat loss and degradation represent a major threat in the tropics. Managed agroecosystems have the potential to mitigate some impacts of land conversion, however, little is known regarding the habitat quality provided by working landscapes in the overwintering range. In this research, we surveyed the migratory bird community in the rapidly expanding oil palm plantations in southern Mexico; and also the declining population of the Wood Thrush (Hylocichla mustelina) inhabiting forest fragments in an agricultural matrix in Costa Rica. We assessed the value of both human-modified habitats by using a combination of demographic, distributional, and individual habitat quality indicators, as well as the relationship of these indicators with environmental characteristics. In the Mexican oil palm plantations, we found that species richness of migratory birds tended to be higher in forest patches than in oil palm, that community assemblages of migratory birds differed between habitats, and that differences in migratory bird abundance were driven by vegetative structure. Specifically, when differences in indicators occurred between oil palm and native forest, most migratory species exhibited indicators of better habitat quality in the native forest. Lastly, we observed, for the first time, territoriality in oil palm plantations and estimated home range sizes for the American Redstart (Setophaga ruticilla), which tended to be smaller than in the native forest. The Wood Thrush population in Costa Rica exhibited an average territory size estimated of 0.71 ha. We were able to determine associations between fragments' characteristics and body conditions, whereby birds in young and more humid fragments exhibited better fitness. Additionally, fragment size alone is probably not the best indicator of habitat quality for Wood Thrushes in Costa Rica. Our results suggest that most species of migratory birds assessed responded positively to forest structure complexity, and that age and sex ratios combined with measures of the physiological conditions, environmental moisture and home range sizes can be used to assess habitat quality for migratory birds overwintering in working landscapes. Importantly, determining a species’ territoriality dynamics, is key when selecting a given indicator of habitat quality for each species due to distributional behavior. Our results also suggest that management strategies that promote forest-like conditions in oil palm plantations can improve the habitat quality in this agroecosystem for