Abstract The syrinx is the unique vocal organ found in birds, enabling them to produce diverse vocalizations, such as songs and calls. Among the Passeriformes (songbirds), the suborder Tyranni (suboscines) exhibits a less structurally complex syrinx compared to the suborder Passeri (oscines). Unlike oscines, suboscines typically have innate stereotyped songs that develop without requiring external acoustic stimuli, and they lack the syrinx’s muscular complexity and flexibility that oscines rely on for song learning. Several studies suggest that in suboscines—particularly tracheophones—sound production involves vibrations of the tympaniform membranes within the syrinx, whose interaction with airflow contributes to shaping the acoustic properties of vocalizations. Despite the well-documented vocal diversity of suboscines and the existing basic understanding of their syrinx anatomy, the specific ways in which syrinx morphology shapes acoustic traits remain poorly understood in a phylogenetic framework. We measured the syrinx morphology of 112 fluid-preserved specimens from 38 tracheophone species, alongside their songs. Specifically, we aimed to determine how syrinx, beak size, and body weight influence vocal frequency in this group. Notably, our findings reveal that narrower bronchial diameters, largely independent of body mass, are associated with a higher peak frequency (i.e. the frequency of maximum energy within a selected section) of the fundamental harmonic, a pattern that is consistent with how syringeal morphological constrictions may influence airflow dynamics during song production. This novel discovery may have significant implications for understanding how morphological traits shape the mechanisms that determine the spectro-temporal components of song in suboscines and, more broadly, across passerines.
Abstract Determining variation in behavioral traits is critical for predicting how animals cope with anthropogenic climate change, including those that buffer reproductive output against changing environmental conditions. We investigated how contrasting ocean conditions influenced parental care in the Marbled Murrelet (Brachyramphus marmoratus), a secretive seabird that nests in coastal coniferous forests along the northeastern Pacific Ocean. Using >12,000 hr of video recordings, we quantified chick provisioning at 18 nests that represented 1,332 feeding events across 2 yr of poor (2018, 2019) and 2 yr of more favorable ocean conditions (2021, 2022). Adults provisioned chicks with a single forage fish during nest visits, which occurred primarily during early morning and late evening. Chick diet was consistent across ocean conditions: Pacific sand lance (Ammodytes personatus) and species in the smelt family (Osmeridae) comprised >90% of identified prey, with Pacific herring (Clupea pallasii) and Northern anchovy (Engraulis mordax) also present. Chicks received significantly fewer fish per day during poor ocean conditions (2.0 fish day−1; 95% CI: 1.6, 2.4) relative to more favorable ocean conditions (2.7 fish day−1; 95% CI: 2.4, 3.2). Fewer fish per day were also delivered as nest distance from the coast increased, independent of ocean conditions. However, estimated daily energy intake of chicks was slightly greater during more favorable ocean conditions, likely because Pacific sand lance delivered to chicks during poor ocean conditions were larger and estimated to have a greater energy density. Despite this partial compensation during poor ocean conditions, chicks took 25% longer (8 days) to fledge, increasing their risk of being depredated. Our study indicates that successful murrelet reproduction is dependent on a small number of prey species and that adults did not seek alternative prey to buffer the impacts of poor ocean conditions when rearing offspring. Therefore, climate-driven changes in prey availability are an important limitation on murrelet reproductive output.
Abstract Reproductive success in wild animals can be associated with prevailing climatic conditions across the breeding period, and these patterns may be attributed to both direct (e.g., excessive heat, freezing) and indirect (e.g., mediated by parental behavior) effects of climate. Parents experiencing inclement weather may adjust their behavior at different stages of reproduction to strike a balance between self-maintenance and investment in offspring. Understanding how weather conditions affect reproductive success is increasingly valuable in the era of global climate change which is associated with both directional (warming) and nondirectional (increasing frequency of extremes) shifts in weather patterns. Here, we investigated how weather conditions and the presence and number of extreme climate events (5th and 95th percentile) during different stages of breeding affected reproductive success in Poecile gambeli (Mountain Chickadee). Based on 13 yr of data from 892 nests at 2 elevational sites in the Sierra Nevada, USA, we show that variation in both ambient temperature and precipitation was associated with differences in reproductive success, but associations were not uniform across elevations and depended on the reproductive stage. Lower temperatures, more precipitation, more extreme cold events and more extreme precipitation events during egg laying were associated with smaller brood sizes at both elevations. More precipitation and more extreme precipitation events during incubation were also associated with decreased brood sizes, but intermediate average temperatures during incubation were associated with the largest brood sizes at both elevations. Temperature during the nestling stage was associated with differences in nestling mass, but these relationships differed between high and low elevations. Our results show that weather conditions during reproduction can have important effects on reproductive success, but such effects vary depending on reproductive stage and across environments.
Abstract As a result of its climate, isolation, and topographical variation, the Hawaiian archipelago has produced one of the greatest examples of avian adaptive radiation in the Hawaiian honeycreepers. While the foraging behaviors and ecologies of some Hawaiian honeycreepers closely reflect those of continental species, others have evolved novel and unique morphologies unrepresented among other birds. Here we describe the skeletal anatomy and functional implications of Hemignathus wilsoni (‘Akiapōlāʻau)—often referred to as the Hawaiian woodpecker. The upper and lower bills of H. wilsoni are markedly different in shape, with the stout, short lower bill used to gouge into woody substrate, and the thin, elongate, and down-curved upper bill used to probe and extract prey. Our osteological examination reveals a substantial difference in the length and shape of the upper bony rostrum and the overlying rhamphotheca. Previously unrecognized, morphological differences in these two features signal a material properties trade-off during probing with specific areas of flexion. Morphology of certain muscle insertions in the lower leg signal strengthened dorsiflexion of the ankle relative to other cardueline finches, which aids in full-body assisted movement during woody substrate gouging. Additionally, the femur is proportionally short and robust compared to closely-related continental cardueline finches—a condition similar instead to mainland woodpeckers which aids in lowering the center of mass against perches when foraging. Overall, the skeletal anatomy of H. wilsoni reflects the effects of insular evolution and highly specialized foraging behaviors, while also retaining signals of its mainland cardueline finch ancestry.
