Female song has been significantly understudied compared with male song, and our understanding of how this elaborate signal trait may function has been limited as a result. Reconstruction of ancestral singing behaviors indicate that Orchard Orioles ( Icterus spurius ) have reduced female song compared with tropical ancestors. In this study, we found that male and female orioles on their breeding territories responded more strongly by all metrics to male song playback than to female song playback. Given the high degree of qualitative variation in female song and the reduced response to female playback, our results suggest that female song in Orchard Orioles may be under relaxed selection pressure. Sexual dimorphism in the complexity and frequency of songs in many lineages likely evolved as a result of a reduction or loss of female song. However, little research has investigated the functional significance of these reduced or infrequent female songs. More research is required to investigate the specific selection pressures that may be acting on female singing behavior in songbirds.

Monitoring populations is critical to understanding habitat use, especially for endangered species and is important for determining the effectiveness of land management strategies. The Florida Grasshopper Sparrow (Ammodramus savannarum floridanus) is a critically endangered non-migratory grassland bird that has been monitored since the 1990s. It resides primarily in dry prairie habitat managed by frequent (2–3 years) prescribed fires. Monitoring is confounded by the presence of the migratory and wintering eastern Grasshopper Sparrow (A. s. pratensis), that has vocal and morphological similarities. These similarities could lead to misclassifications and erroneous conclusions about land management. Our goal was to determine the impact of fires on Florida Grasshopper Sparrow habitat use at two spatial scales, 100 m and 400 m buffers surrounding point count locations, while controlling for the presence of the eastern Grasshopper Sparrow. We combined point count data (1996–2011), external sources of data (eBird), and Bayesian dynamic occupancy models that accounted for misclassifications to evaluate habitat use and dynamics in response to prescribed fires. The probability of misclassifying a Florida Grasshopper Sparrow peaked in April and then decreased to near zero after May 1. The probability of persistence at point count stations decreased as years-since-fire (maximum of pixels within 400 m buffer) increased and was greatest for recent burns (< 1 year). Seasonality of the most recent burn (i.e., ordinal date of burn for pixels within 100 m buffer) also influenced probability of persistence with Florida Grasshopper Sparrows having greater-than-average persistence when burns occurred during January and July, peaking in early April. The probability of colonization peaked approximately two months after a fire and rapidly decreased to negligible values. This is the first long-term study to examine the effect of prescribed burn on Florida Grasshopper Sparrow occupancy that accounts for the presence of eastern Grasshopper Sparrows. This modeling framework may also provide utility for other species and subspecies that are difficult to distinguish during surveys.

The introduction of nonnative grazers and plants, as well as alterations to native fire regimes in grassland systems, can directly and indirectly affect the abundance of arthropods. In this study, we used sweep-net sampling at two sites occupied by the critically endangered Florida grasshopper sparrow (Ammodramus savannarum floridanus Mearns) in Osceola County, Florida, to assess how the abundance of all arthropods (orders pooled) and the four most common arthropod orders varied between native dry prairie and semi-improved cattle pasture habitat (2015-2016). We also examined the role of time since fire and fire season on arthropod abundance in both habitat types. The abundance of all arthropods, Hemiptera, Orthoptera, and Coleoptera was higher in pasture than dry prairie, but the mean body length of Orthoptera was smaller in pasture. Fire year and fire season did not affect arthropod abundance when data from both sites were combined. However, when examined separately, Orthoptera at the dry prairie site were more abundant in habitat burned in the current year than previous years, supporting the hypothesis that Florida grasshopper sparrows preferentially select recently burned prairies because they support greater resource availability. Orthoptera at the pasture site showed no significant response to fire year or season. This study is important because it is the first to compare arthropod prey abundance for Florida grasshopper sparrows across dry prairie and pasture habitat and has important implications for prescribed fire management.

Female birds in many temperate species are thought to sing reduced or quieter songs and appear to sing less often than their male counterparts. Therefore, female song may be easily overlooked. Increasingly, researchers are recording female song in well-studied species previously assumed to have little or no female song. In this study, we document the extensive use of female song in Orchard Orioles (Icterus spurius), a species with delayed plumage maturation where female song had not been well-documented. Based on observations of females singing in the early breeding season, we hypothesized that female song may function for mate attraction. To formally investigate whether females sing specifically early in the season, we assessed singing rates of each sex throughout the breeding season. We also performed detailed acoustic analyses comparing male and female song structure. Females sang significantly less often than males, and female and male songs were statistically different for five of eight variables investigated, indicating that the two sexes sing acoustically distinct songs. However, females also sang more often than initially assumed, suggesting that researchers may be missing female song in other species if they are not directly searching for it, particularly in species in which yearling males and females have similar coloration. Therefore, this study highlights the need to re-explore well-studied systems. Further research is needed to determine if and how female song may function in this species.

ABSTRACT Nest microclimate can be affected by wind and solar radiation, which can influence offspring survival. The endangered Florida Grasshopper Sparrow (Ammodramus savannarum floridanus) constructs a domed nest on the ground in dry prairie and pasture habitats of central Florida. We measured the nest opening orientation for 116 Florida Grasshopper Sparrow nests across 2 sites with different habitats (2014–2016). Nest openings were primarily oriented to the northeast and orientation did not differ by site. Nest opening orientation did not influence the probability that a nest would successfully fledge at least 1 nestling. It is possible that Florida Grasshopper Sparrows may orient nests to the northeast to avoid wind-blown rain from summer storms and solar radiation during the hottest part of the day, but high local rates of nest predation documented at both sites mask the selective advantage of nest orientation on nest survival.

Most of our knowledge about the function, evolution, neurobiology and ontogeny of complex vocalizations in birds comes from previous work on male song. However, the recent increase in focus on female bird song over the last few decades has helped bridge that gap in our knowledge and has vastly changed our understanding of the function, phylogeny and evolution of bird song. Here we present evidence that female song may be modifiable based on signal function. Eastern bluebirds primarily use female song for within-pair communication with their social mate. We examined the female songs of eastern bluebirds sung during within-pair communication and compared the acoustic structure to female songs used in other contexts. Within-pair communication songs differed significantly from other female songs that we measured by six of the eight acoustic variables we measured. Additionally, we examined the song structure of within-pair communication songs at variable distances between mates and between neighbour females. Song structure did not vary over distance. However, we found significant differences between neighbouring females and the number of syllables and frequency modulations each female included in her within-pair communication songs. We propose that female songs have the potential to indicate individual identity, through acoustic structure modification. Our findings further highlight the need for additional study of female song and functional acoustic plasticity in bird song. Animal signals may be stereotyped to convey honest information, they may be functionally plastic to convey additional information, or elements of a signal may fall in both categories. Historically, bird song has been primarily studied as a stereotyped male trait. Whereas male bird song may have variable acoustic structure in some instances, such as extreme aggression, the functional plasticity of female song is largely unknown. We examined female song plasticity for within-pair communication songs of female eastern bluebirds. We present some of the first evidence that female song structure may be variable between functional contexts and that females may alter their song structure to encode additional information, such as individual identity. Our results inform future work on the function and ontogeny of female bird song. Additionally, our findings support the need for further study on female bird song and the functional plasticity of complex acoustic signals.

Vegetation characteristics can influence nest-site selection and nest survival of birds. The Florida Grasshopper Sparrow (Ammodramus savannarum floridanus) is a critically endangered ground nesting grassland bird endemic to central Florida. Currently, the two largest remaining populations are found on sites with differently managed habitats. One site is burned regularly to maintain native dry prairie habitat and the other is a cattle pasture that is mowed and burned to optimize cattle forage. Little is known about how vegetation influences Florida Grasshopper Sparrow nest-site selection and nest success in these different habitats. We measured microhabitat characteristics (percent vegetation cover and grass height) at Florida Grasshopper Sparrow nests and paired random plots at both sites for three breeding seasons (2014-2016). Percent cover differed significantly between the sites for seven of eight vegetation types, with the cattle pasture characterized primarily by grasses and dry prairie characterized by more diverse native vegetation. Despite these vegetative differences, grass height did not differ at the two locations, suggesting that plant height, rather than plant species composition, may be more important for nest site selection in this bird. Microhabitats around nests at the dry prairie site had 31% less bare ground and 32% more grass than non-nest plots. No variables predicted the placement of nests at the cattle pasture, possibly because of the more homogenous habitat at that site. We did not find a vegetative component of nest success, which suggests that other nonvegetative factors may influence nest predation. Understanding the vegetation characteristics associated with Florida Grasshopper Sparrow nests will help inform habitat management strategies for maintaining vegetation height for nest-site habitat of this critically endangered subspecies.

The incidence of extra-pair paternity (EPP) is highly variable across bird taxa. While EPP is known to affect reproductive variance, the causes of temporal variation in rates of EPP are poorly studied. Breeding density has often been proposed as an important factor influencing EPP variation, but it has received mixed support. Over a 5-year period we examined the rate of EPP in a socially monogamous Grasshopper Sparrow (Ammodramus savannarum) population on a restored grassland in Maryland. Of 124 broods, 62.1% had extra-pair young (EPY), and of 448 nestlings, 36.8% were the result of extra-pair fertilizations. Over the 5-year study, percent EPY ranged from 26% to 60%. EPP rates were positively correlated with the local density of territorial males, supporting the density hypothesis, and 81% of the EPY were from neighboring males. Male within-pair mating success was related to extra-pair mating, suggesting an active behavioral tradeoff between within-and extra-pair matings, especially at high densities. By examining males at the individual level, we found males that were not cuckolded also had the most genetic offspring (within-pair and extra-pair), whereas males that were cuckolded also failed to sire many extra-pair offspring. Here we show that density plays an important role in the availability of mates, EPP, and the opportunity for sexual selection.

Vocalizing birds are ubiquitous and often prominent in areas that are reached by noisy human activities. Birds have therefore been studied for the effects of man-made sound on song production and perception, physiological stress, distribution range, breeding density, and reproductive success. There are examples of birds that sing louder, higher, and longer when ambient-noise levels are elevated due to human activities. This may lead to perceptual advantages through masking release, although song modifications may also lead to a functional compromise. Fitness benefits of noise-dependent modifications have not been proven yet. Masking effects are reported for outdoor and indoor studies, but data on physiological consequences are not widespread yet. There are also still only few experimental studies on more long-term consequences of man-made sound on development, maturation, and fitness. Observational data on species distributions and densities show that there are birds that persist at noisy sites but also that artificially elevated noise levels can have detrimental consequences for particular species. Birds in noisy localities may move away or stay and fare less well. Furthermore, the effects of noise pollution can go beyond single species because all species may be more or less negatively affected, but the effect on one species may also positively or negatively affect another. The variety in sensitivity among species and the diversity in impact and counterstrategies have made birds both cases of concern and popular model species for fundamental and applied research.

Understanding complex natural systems requires approaches with minimal statistical limitations and biases. Previously, however, the fields of behaviour, ecology and evolution generally have been restricted to evaluating differences between statistical distributions. This framework can be very powerful. Unfortunately, it has also created a bias in these, and many other, fields towards publications focusing only on phenomena for which there are statistically significant differences. However, there is a wide range of questions that would benefit from reversing the existing paradigm and testing, instead, for equivalence. We have adapted the two one-sided test (TOST), also known as equivalence testing, from pharmaceutical science to be applicable to behavioural and ecological studies. We created a repeated measures analysis that allows researchers to statistically examine similarities between distributions. We compared song structure in male and female eastern bluebirds, Sialia sialis, as a case study for this method. We failed to find significant differences between male and female songs via a more traditional test, repeated measures ANOVA. Therefore, no definitive conclusion could be drawn about the similarities or differences in song structure. However, our repeated measures equivalence test showed that, based on five standard measures of song variation, male and female eastern bluebirds sing statistically equivalent songs. Our study highlights the presence of complex female song in a temperate songbird species. Additionally, we provide a new statistical test useful for expanding the statistical toolbox to assess new behavioural and ecological questions, and to help counteract publication bias in our fields. (C) 2018 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.

Geographic variation in acoustic signals can be important in species divergence, especially the maintenance of prezygotic barriers to gene flow. Furthermore, selective pressures on acoustic signals likely vary both across geographic distances and among vocalizations used in different behavioral contexts. We described the call repertoire of 5 subspecies of Marsh Wren (Cistothorus palustris) in eastern North America and tested for variation in both the acoustic structure and likelihood of production of each call type at 3 functional-ecological levels: subspecies identity, migratory pattern, and habitat type. Three of the 7 described call types exhibited acoustic variation best explained by either migratory pattern or habitat type. These calls were used principally in courtship-territorial patrol contexts, whereas 4 calls that did not exhibit geographic variation were used in agonistic interactions. How often a call is used in a population may be indicative of the behavior or breeding phenology associated with that vocalization. We found that 4 calls varied in how commonly they were produced among the subspecies and/or habitat types. We also described and quantified the degree to which these 5 subspecies produce calls in association with song-a little reported, but possibly more widespread, behavior in birds. Marsh Wrens commonly embedded 3 call types into song in a nonrandom pattern. This behavior was more common in freshwater-marsh populations than in saltmarsh populations, and we discuss several possible functions for call-song associations. Overall, when geographic variation in call structure occurred, it was most commonly explained by differences in habitat type and, therefore, may be indicative of local adaptations that could limit gene flow across environments.

Grasshopper Sparrows are specialists in open grassland habitats and face acoustic challenges normally associated with that habitat type. They produce several types of calls and two distinct types of song, all of which are high-pitched for songbirds (6—10 kHz). The primary territorial song, also known as the “buzz” song, consists of 3 or 4 brief introductory notes followed by a high-pitched, rapidly modulated trill. The function of the secondary song type, or “warble” song, remains unknown, but data from autonomous recording units demonstrates a correlation with pairing status and breeding cycle timing. Operant discrimination tests with Grasshopper Sparrows show a broader audiogram and extended high frequency auditory limit when compared with other small songbirds, suggesting that these birds, and potentially related species as well, have evolved a wider spectral range of auditory sensitivity in this habitat type. Auditory detection and discrimination thresholds were used to explore the active space and communication distances of this species' vocalizations using a habitat bioacoustics model incorporating its normal territorial behavior. Results suggest that songs can be detected up to three territories away from the singer, but that birds may have difficulty discriminating between different conspecific songs more than two territories away.

There are many ways for signalling systems to be complex, one of which is a large signal repertoire. However, it is frequently challenging to estimate larger repertoires accurately. We present the first description of repertoire size for a subspecies of marsh wren on the Gulf Coast of North America, Cistothorus palustris thryophilus, using four repertoire estimation techniques ( simple enumeration, Wildenthal curve-fitting, Davidson & Wilkinson curve-fitting, and the Coupon Collector model). We demonstrate that the repertoire estimation methods in question perform with varying degrees of accuracy under natural conditions, and propose that rather than establishing an absolute repertoire size for a given individual it may instead be necessary to use multiple techniques to establish a repertoire range. We find that C. p. thryophilus has a song repertoire size falling well within the range of repertoires produced by other marsh wren subspecies in eastern North America, although it may be on the larger end of this range. We find no evidence supporting a correlation between migratory distance and increased repertoire size in this species, as might be predicted under a sexual selection hypothesis.

Geographic variation in birdsong and differential responses of territorial males to local and non-local song variants have been documented in a number of songbird species in which males learn their songs through imitation. Here, we investigated geographic song variation and responses to local and non-local song in the grasshopper sparrow (Ammodramus savannarum), a species in which males develop song by improvisation rather than imitation, as a first step toward understanding how the extent and salience of geographic song variation is related to the mode of song development. To describe the geographic variation in song, we compared songs from populations in eastern Maryland and central Ohio, USA, using multiple acoustic analysis techniques. We then conducted a playback experiment in Maryland using local and non-local (Ohio) songs to test how territorial males responded to this geographic variation. We found acoustic differences between songs from the two sites. However, males responded similarly to playback of these songs, suggesting that this geographic variation is not behaviorally salient in a territorial context. Together with previous studies, our results suggest that across species, geographic song variation and the extent to which this variation functions in communication may be correlated with the accuracy with which song models are imitated during song development.