Nocturnal songs in a diurnal passerine: attracting mates or repelling intruders?

Tracking Small Animals in Complex Landscapes: A Comparison of Localisation Workflows for Automated Radio Telemetry Systems

Automated radio telemetry systems (ARTS) have the potential to revolutionise our understanding of animal movement by providing a near-continuous record of individual locations in the wild. However, localisation errors in ARTS data can be very high, especially in natural landscapes with complex vegetation structure and topography. This curtails the research questions that may be addressed with this technology. We set up an ARTS grid in a valley with heterogeneous vegetation cover in the Colombian high Andes and applied an analytical pipeline to test the effectiveness of localisation methods. We performed calibration trials to simulate animal movement in high- or low-flight, or walking on the ground, and compared workflows with varying decisions related to signal cleaning, selection, smoothing, and interpretation, along with four multilateration approaches. We also quantified the influence of spatial features on the system's accuracy. Results showed large variation in localisation error, ranging between 0.4-43.4 m and 474-1929 m, depending on the localisation method used. We found that the selection of higher radio signal strengths and data smoothing based on the temporal autocorrelation are useful tools to improve accuracy. Moreover, terrain ruggedness, height of movement, vegetation type, and the location of animals inside or outside the grid area influence localisation error. In the case of our study system, thousands of location points were successfully estimated for two high-altitude hummingbird species that previously lacked movement data. Our case study on hummingbirds suggests ARTS grids can be used to estimate small animals' home ranges, associations with vegetation types, and seasonality in occurrence. We present a comparative localisation pipeline, highlighting the variety of possible decisions while processing radio signal data. Overall, this study provides guidance to improve the resolution of location estimates, broadening the application of this tracking technology in the study of the spatial ecology of wild populations.

Development of activity rates in fledgling songbirds: when do young birds begin to behave like adults?

Logistical and technological limitations have constrained the characterization of behavioural development in songbirds, particularly during the post-fledging period. Recently, advances in radio-telemetry technology — automated radio-telemetry systems (ARTS) capable of recording continuous, high-resolution spatial-temporal data on radio-tagged birds — have opened the door to more comprehensive examinations of fledgling behaviour. We examined development of activity rates (number of times a bird was determined to have moved per number of detections, per hour) in fledgling Dickcissels using ARTS established at two grassland sites in east-central Illinois, USA. Specifically, we described general patterns of fledgling activity rates and determined the age at which fledglings began exhibiting activity rates like adults. We found that juveniles decreased activity rates immediately following fledging, likely to avoid detection by snakes and other predators, but increased activity levels throughout the rest of the post-fledging period. Peak hours of fledgling activity occurred around 0700 and 1800 h for all ages, with consistently low activity rates at night. On average, fledglings began exhibiting adult-like activity rates approximately 22 days after fledging, around roughly the same time they stopped being fed by adults. While our study provides important insights into development of fledgling behaviour in the Dickcissel, it remains unclear how patterns of behavioural development vary within and among species.

Seasonal patterns of nocturnal singing by ovenbirds and white-throated sparrows

Recent research suggests that many diurnal songbirds also sing at night. The functions of nocturnal singing by diurnally active birds are not well understood. We used automated recorders to record nocturnal singing from May through July 2014. We examined how date, temperature, wind, weather, and lunar phase influenced nocturnal vocal behaviour. We found that nocturnal singing by ovenbirds and white-throated sparrows was related to date with clear seasonal patterns that did not mirror the dawn chorus. Nocturnal singing rates declined seasonally, but peaked earlier for white-throated sparrows than for ovenbirds. Both species sang less often on nights with a full moon or precipitation. Ovenbirds also sang fewer songs on cold and windy nights while white-throated sparrows sang less often on cloudy nights. We show that nocturnal song is shaped by environmental factors and shows a seasonal pattern that may be useful for gaining insights into functions of nocturnal song.