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

Abstract

Black-bellied Whistling-Ducks (Dendrocygna autumnalis; hereafter, BBWD), are rapidly expanding northward into the core range of the eastern Wood Duck (Aix sponsa; hereafter WODU), yet little is known about their 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 duplex-style 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 would fail to document a substantial number of birds potentially contributing to the population via parasitism. We also determined that BBWD preferentially visited (and subsequently nested in) boxes that are >1 year old (β = 1.32, CI = 0.97, 1.66), while WODU did not display a preference. Due to the generalist nature and rapid expansion of BBWD, an apparent neophobia of 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 used nest box-mounted PIT tag readers to observe BBWD behavior.