Evaluation of light-logging geolocators to study mottled duck nesting ecology

Geolocators are small devices that record and store time-stamped light-levels, and are typically used to approximate the latitude and longitude of small birds across the annual cycle.  However, when geolocators are placed on leg bands of larger-bodied birds, the daily pattern of light and darkness is interrupted while females are incubating a nest.  Thus, geolocators can provide information on nesting propensity, nest success, and renesting intensity; these demographic parameters are both difficult to measure unobtrusively and are critically important in determining population dynamics of birds, especially ducks.  Here, we deployed 240 geolocators on mottled ducks Anas fulvigula in Louisiana and Texas 2018­–2019 to evaluate their utility in providing nesting data.  From July 2018–January 2022, we recovered 16 geolocators from hunter-harvested birds, and were made aware of 6 other unreported recoveries, yielding a realized recovery rate of 7.1% (9.1% unrealized).  Three of the recovered units provided breeding season data.  Two of these clearly indicated a single nest initiation in the early spring of 2019, and one of the units also logged an attempt in spring of 2020.  All three nests were incubated for approximately a month, suggesting that they all successfully hatched.  The final geolocator logged five putative nest attempts over the course of two years.  In 2019, both attempts were unsuccessful (incubated ≤10 days).  In 2020, we documented three attempts spanning 20 February–10 June, all of which appeared to have failed.  For all failed attempts, the hen left the nest at dusk or overnight and did not return, which is suggestive of mammalian predation.  Geolocators successfully provided information on breeding season activities of mottled ducks, and we documented renesting rates following nest depredation.  However, we achieved a smaller sample size than anticipated (three usable returns), resulting in an effective cost of $11,800 per usable return.  Where possible in other species, capturing birds immediately prior to the breeding season, and improvements to geolocator attachment have the potential to improve recovery rates and increase cost effectiveness of the technique.