Thermal data loggers have been used to monitor nest activity for a variety of avian species, primarily by identifying a difference in temperature between the relatively cool environment and the nest, which is warmed by nestlings or attendant adults. Many grassland songbirds, however, nest in warm environments where ambient and nest temperatures are frequently similar, which may limit the ability to identify nesting events from temperature data. Here, we evaluate the efficacy and potential impact of monitoring nests of grassland songbirds with thermal data loggers. We focus on a grassland-obligate species, Botteri's Sparrow (Peucaea botterii), that nests in hot, semiarid grasslands. We located and monitored 225 nests in southeastern Arizona, USA, and placed data loggers below the surface of the nest lining at a subset of 28 nests. To contrast nest temperatures with ambient temperatures, we placed a second data logger in similar vegetation within 3 m of the nest. Data loggers did not affect daily survival rates of nests. We were able to identify the date the nesting attempt ended (i.e., failure or fledging) correctly for all nests based on temperature data recorded during the cool period of the daily temperature cycle when data loggers placed below the nest lining averaged 3.9 °C warmer than the environment. During the hot period of the daily cycle, we were able to identify nest cessation correctly for only 46% of nests. Our study demonstrates that thermal data loggers can be used to monitor nest survival of grassland birds successfully provided that ambient temperatures are measurably lower than nest temperatures for at least part of the daily cycle. This provides an alternative to intensive observer-based monitoring that can increase the precision of survival estimates while potentially reducing cost, effort, and risk of disturbance to this group of high conservation concern.

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