Understanding the functional significance of bird sounds can provide valuable insight into behavior and how birds use habitat. We show that the Common Nighthawk wing-boom display is a territorial signal associated with the nest location that can be used to identify territorial habitat use. In other words, the Common Nighthawk wing-boom display can be considered analogous to song due to its potential function in territoriality. We captured, tagged, and tracked 21 male Common Nighthawks in northeastern Alberta to confirm the functional significance of the wing-boom display and describe Common Nighthawk territoriality. Mean wing-boom use density (hereafter “area") size was 10.2 ha (SD=11.7 ha). We found minimal overlap in wing-boom area (5 of 15 neighboring male pairs, 0.2%-4.5% overlap), suggesting the wing-boom display represents an exclusive territory. Comparison of wing-boom locations and random points within the wing-boom area confirmed that male Common Nighthawks select areas near the nest to perform wing-boom displays. There was high wing-boom area overlap for the same individual between years. Differences between years reflected shifts in nest location, suggesting that the wing-boom display is a good indicator of the nest location and territory. Future Common Nighthawk surveys should record the type of acoustic signal observed to differentiate territorial and nesting habitat use. Many taxa that produce non-vocal sounds as part of breeding displays could similarly benefit from a functional classification of song to provide insight into habitat use.

Individual Tracking and Observation

We captured and fitted male Common Nighthawks with VHF tags during the breeding seasons of 2016 and 2017. We used broadcast calls and a decoy to lure individuals into 38 mm mist nets. Individual males captured in 2016 were selected based on presence at our five study sites. In 2017, we attempted to recapture the same individuals from the previous year, as well as additional males at the same study sites. Upon capture, each individual was fitted with either a 0.6 g PicoPip Ag376, 1.1 g Pip Ag392 (Lotek wireless, Newmarket, ON, Canada), 0.35 g BD-2X, or 1.2 g BD-2 (Holohil Systems Ltd., Carp, ON, Canada) VHF transmitter (0.4 – 1.7% body mass [mean=1.0% ± 0.5% SD]). All tags were attached to fall off naturally: BD-2X VHF tags were glued to the bare patch of skin between the feather tracts on the back with super glue gel (Gorilla Glue Company, Cincinnati, Ohio); Pip Ag 392 and BD-2 VHF transmitters were tied and glued to one of the central rectrices. All work was conducted under University of Alberta Animal Care and Use Approval AUP00001523.

We relocated each individual throughout the breeding season and conducted observations of their acoustic behavior. We waited at least one day following capture to allow the birds to acclimate to the presence of the VHF tags. All observations were conducted between June 1 and July 20 when Common Nighthawks are nesting and between 21:30 and 03:30 when birds were active. Following relocation of the target individual, an observer conducted 10 minutes of focal observation of the bird, marking the location of each wing-boom display on a datasheet with distance measurements. Focal observations were terminated early if the target individual was confused with other individual nighthawks. In a few instances, we also terminated the observation if the focal individual moved out of the observation area (> 200 m); we assume those individuals were not leaving the area to wing-boom in other locations. Following each focal observation, the observer walked to the estimated location of each recorded wing-boom and used a GPS device to record the coordinates.

Nest Location

We attempted to locate the nest of each tracked nighthawk. Nightjar nests are cryptic and difficult to locate because incubating or brooding individuals will not flush until disturbed (Holyoak 2001). We thus relied on eyeshine to locate each nest; nightjar retinas have highly reflective tapeta lucida that reflects a yellow-orange luminescence. We searched each study site after dark by walking transects spaced approximately 100 m apart and sweeping back and forth with a high-powered headlamp. We assumed that the identity of the male for each nest was the male performing wing-boom displays around the nest area. On several occasions, we directly observed the male provisioning the chicks at that nest or interacting with the female.

All coordinates provided are in WGS84 decimal degrees. Nest locations in "ORNITH-21-054_NestLocations.csv" link to the wingboom locations in "ORNITH-21-054_WingboomLocations.csv" via the "BirdID" field.