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Cerulean Warblers (Setophaga cerulea) are among the fastest declining Nearctic-Neotropical migrant wood-warblers (Parulidae) in North America. Despite ongoing conservation efforts, little is known about their non-breeding distribution. In June 2016-2018, we deployed geolocators (n = 30) on adult male Cerulean Warblers in Indiana, USA, to track annual movements of individuals. Recovered geolocators (n = 4) showed that Cerulean Warblers occurred broadly throughout northern South America. Autumn migration lasted 44-71 days (n = 4), whereas spring migration lasted 37-41 days (n = 3). The average migration distance was 5268 km. During autumn migration, Cerulean Warblers made 1-4 stopovers (i.e., ≥2 days; n = 4) and 1-2 stopovers during spring migration (n = 3). When crossing the Gulf of Mexico during autumn migration, two birds stopped over after crossing, but not beforehand. Two others navigated through the Caribbean rather than crossing the Gulf of Mexico. During spring migration, one individual stopped after crossing, one individual stopped before crossing, and one individual stopped before and after crossing the Gulf of Mexico. No birds migrated through the Caribbean Islands during spring migration. These results represent novel information describing annual movements of individual Cerulean Warblers and will inform conservation efforts for this declining species.

Site fidelity can have significant impacts on population dynamics and may be influenced by both demographic and environmental characteristics, including age, sex, and habitat quality. We examined breeding-site fidelity of Setophaga cerulea (Cerulean Warbler), a small Nearctic–Neotropical migrant that is listed as endangered in Indiana and in decline throughout its range. Our research was part of a long-term demographic study in the research units of the Hardwood Ecosystem Experiment, southern Indiana. From 2013 to 2018, we banded 128 Cerulean Warblers, 10.2% of which returned the year following initial capture and banding. We did not find significant differences in return rates between adult males (n = 92) and adult females (n = 19; P = 0.69), novice (second-year, n = 20) and experienced (after-second-year, n = 91; P = 1) birds, or successful (n = 40) and unsuccessful breeders (n = 26; P = 0.39). We have yet to detect a bird banded as a hatch-year subsequently return the following season as a second-year (n = 0/17). Our return rates are likely affected by a combination of mortality between breeding seasons, our ability to detect returning birds, and dispersal between breeding seasons. We encourage other researchers to publish rates of Cerulean Warbler site fidelity across its range for comparison of populations and habitats to determine source/sink dynamics.

Documented observations of interspecific nest usurpation are rare and have been observed mostly in cavity-nesting species, where cavities are limiting and competition for them can be strong. We report one of the few known instances of interspecific nest usurpation in the family Parulidae (North American wood-warblers) between the Cerulean Warbler (Setophaga cerulea) and American Redstart (Setophaga ruticilla). During May and June 2016, we observed a female American Redstart usurp and successfully fledge at least one of its young from a nest built by a female Cerulean Warbler in Yellowwood State Forest, Brown County, Indiana. Based on our field observations, we provide information on nest chronology and describe American Redstart adult behaviors at the usurped nest. We hypothesize that this instance of interspecific nest usurpation may have been a result of high local breeding densities of both Cerulean Warblers and American Redstarts in our study area.

Territorial male birds of many north temperate species sing prolifically before sunrise in a behaviour known as ���dawn song,��� after which males sing at lower rates, use different song types, or cease singing altogether. From May to July 2017 in southern Indiana, we recorded 100 dawn song performances from 12 paired male Cerulean Warblers (Setophaga cerulea) and we quantified the relative influences of social (breeding stage of the focal pair) and abiotic factors (Julian date and weather) on six characteristics of dawn song. We found that dawn song was influenced by both biotic and abiotic factors, but weather had the most model support. Males were more likely to sing dawn song early in the season, and most males nesting late in the season ceased dawn song altogether. Dawn song start times (in relation to sunrise) varied little over the season, but males ended dawn song earlier, and sang at lower peak rates, as the season progressed. Additionally, males initiated and ended dawn song later on windy and rainy mornings. Multiple factors apparently influence dawn singing behaviour in paired male Cerulean Warblers, yet weather may have a greater effect on dawn song than do Julian date or social behaviour.

Female song is more widespread than previously assumed, and is adaptive in many species; however, it may also be an aberrant, nonfunctional, behavior in others. We document novel vocalizations, including female song, from 2 paired female Cerulean Warblers (Setophaga cerulea) recorded in southern Indiana, USA, in June 2017. This is the first documentation of female song in this species. When assessed spectrographically, one female's vocalizations were similar to the typical zeet call of this species, but with appreciable differences in duration, composition, and frequency bandwidth. The other female's vocalizations, which we define as a song, had large frequency modulations as well as harmonics. The females' vocalizations resembled neither each other's nor the typical song of an adult male Cerulean Warbler. Unlike most other instances of infrequent female song in temperate-breeding passerines, these vocalizations only occurred during the incubation and nestling stages, rather than during mate acquisition. Based on their context, they appeared to function in intersexual communication with the male mate. We discuss potential explanations, both adaptive and nonfunctional, and urge researchers of this species to pay particular attention to the vocalizations of females in their study populations.

Hybridization events have long interested biologists because of their implications for species concepts and taxonomy. Documenting hybridization events is important because hybridization rates are commonly used as support for accepted taxonomic divisions. In July 2017, we observed a nest where a male Cerulean Warbler (Setophaga cerulea) and a female Blackthroated Blue Warbler (Setophaga caerulescens) were feeding nestlings. The Cerulean Warbler and Blackthroated Blue Warbler communicated with each other using song (male) and calls (female), similar to a nonhybrid pair of either parent species, and the pair was not observed to engage in aggressive behaviors toward each other. Interestingly, the Black-throated Blue Warbler is not known to breed in Indiana, although it is a regular migrant in the spring and fall. We describe the behaviors of the hybrid pair at the nest, provide photographs and audio recordings for documentation, and hypothesize that this instance of hybrid pairing may have occurred due to mate scarcity.