Nest box-mounted PIT tag readers reveal cryptic recruits of cavity-nesting waterfowl in south Louisiana.

Black-bellied Whistling-Ducks (Dendrocygna autumnalis; BBWD) are rapidly expanding northward into the core range of the eastern Wood Duck (Aix sponsa; WODU), yet little is known about BBWD nesting ecology. Typical field methods to study cavity-nesting waterfowl (i.e., weekly nest monitoring) preclude a full understanding of important breeding information, including nest prospecting and parasitic egg laying. To address this, we used subcutaneous passive integrated transponder (PIT) tags embedded in adults and PIT tag readers mounted on nest boxes with the objective to (1) identify individuals that used nest boxes but were not physically captured on a nest, (2) quantify box visitation, and (3) quantify BBWD pair and WODU hen behaviors during the prospecting, laying, and incubation periods. We deployed RFID readers on 40 nest boxes from March-December 2022 in Louisiana with the potential to detect BBWD and WODU marked with PIT tags in 2020-2022. We detected 48 (BBWD n = 26, WODU n = 22) adults of both species via RFID readers, and 33% (n = 16) of individuals (50% of BBWD, n = 12; 14% of WODU, n = 3) were never otherwise recaptured in 2022, meaning that traditional field methods for cavity-nesting waterfowl fail to document a substantial number of birds potentially contributing to the population via parasitism. We also used Bayesian generalized linear models to determine that both species visited a similar number of "new" (< 1 year old) and "old" (> 1 year old) nest boxes (β = 0.66, CI = -0.30, 1.64). However, BBWD preferentially visited (and subsequently nested in) old boxes at a significantly higher rate than WODU (β = 1.32, CI = 0.97, 1.66). Due to the generalist nature and rapid expansion of BBWD, an apparent aversion to newly installed boxes was unexpected, especially since there were several successful WODU nests in the new boxes before BBWD began nesting in 2022. Our study is one of the first to evaluate BBWD nesting behaviors within the core WODU breeding range, and the first to use nest box-mounted PIT tag readers to observe BBWD behavior.