In cooperatively breeding animals, genetic relatedness among group members often determines the extent of reproductive sharing, cooperation and competition within a group. Studies of species for which cooperative behaviour is not entirely based on kinship are key for understanding the benefits favouring the evolution and maintenance of cooperative breeding among nonrelatives. In the cooperatively breeding chestnut-crested yuhina, Yuhina everetti, a songbird endemic to Borneo, we tested whether unrelated helpers are more likely to gain parentage than are related helpers consistent with the hypothesis that inbreeding risk constrains reproduction by related helpers. We also examined whether related or unrelated helpers provision broods more because of differences in their potential indirect or direct fitness benefits of helping. Kin structure of breeding groups (breeding pair and up to eight helpers of both sexes, median = 2 helpers, 96% of 57 pairs had helpers) based on genetic analysis was mixed; 48% of 76 breeder/helper dyads were first-order (26%) or second-order (22%) relatives of one or both members of the breeding pair, and 52% were nonrelatives. Only unrelated male and female helpers gained parentage, and helpers did not differ in their provisioning rate according to their relatedness to the broods. We documented quasi-parasitism or co-breeding by female helpers in 14% of 29 broods and extrapair paternity by male helpers in 21% of 47 broods. This rate of extrapair paternity is relatively high among the few tropical species examined but fit with predictions for mixed-kin groups where inbreeding is avoided. These findings support the emerging pattern for cooperative breeding in birds with mixed-kin groups, wherein unrelated helpers are more likely to gain parentage than are related helpers and helping effort is not necessarily predicted by kinship. (C) 2019 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.

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.

Many tropical birds have slow-paced life history strategies, exhibiting lower metabolic rates, reduced annual investment in reproduction, and longer lifespans relative to birds at higher latitudes. Life history strategies have been relatively well documented in adult individuals in the tropics, but we know comparatively little about the immature life history stage. Here we examine strategies of feather replacement (molt) and fattening in immature Rufous-collared Sparrows (Zonotrichia capensis) in a high elevation equatorial population, following a parallel, previous study on an arctic congener, the White-crowned Sparrow (Zonotrichia leucophrys gambelii). In captivity, Rufouscollared Sparrows incurred energetic costs of experimentally induced feather growth, similar to those previously described for Zonotrichia at higher latitudes. In contrast, free-ranging immature Rufous-collared Sparrows in natural molt had fat stores that declined over time, opposite to patterns evident in arctic Zonotrichia that fatten before migration. Equatorial birds in good condition molted more heavily (controlling for fat stores), suggesting that body condition limits the intensity of molt. Heavily molting equatorial sparrows also had lower amounts of fat (controlling for body condition), suggesting a trade-off between allocation of resources to fat stores versus feather growth. Molt progressed slowly in Rufous-collared Sparrows relative to previously described patterns in their arctic congener, which is concordant with a slower pace-of-life syndrome in tropical, as compared with high latitude, birds.

Anthropogenic noise imposes novel selection pressures, especially on species that communicate acoustically. Many animals—including insects, frogs, whales and birds—produce sounds at higher frequencies in areas with low-frequency noise pollution. Although there is support for animals changing their vocalizations in real time in response to noise (i.e. immediate flexibility), other evolutionary mechanisms for animals that learn their vocalizations remain largely unexplored. We hypothesize that cultural selection for signal structures less masked by noise is a mechanism of acoustic adaptation to anthropogenic noise. We test this hypothesis by presenting nestling white-crowned sparrows (Zonotrichia leucophyrs) with less-masked (higher-frequency) and more-masked (lower-frequency) tutor songs either during playback of anthropogenic noise (noise-tutored treatment) or at a different time from noise playback (control treatment). As predicted, we find that noise-tutored males learn less-masked songs significantly more often, whereas control males show no copying preference, providing strong experimental support for cultural selection in response to anthropogenic noise. Further, noise-tutored males reproduce songs at higher frequencies than their tutor, indicating a distinct mechanism to increase signal transmission in a noisy environment. Notably, noise-tutored males achieve lower performance songs than their tutors, suggesting potential costs in a sexual selection framework.

SPCC, trill frequency and vocal performance data

Female preference for local cultural traits has been proposed as a barrier to breeding among animal populations. As such, several studies have found correlations between male bird song dialects and population genetics over relatively large distances. To investigate whether female choice for local dialects could act as a barrier to breeding between nearby and contiguous populations, we tested whether variation in male song dialects explains genetic structure among eight populations of rufous-collared sparrows (Zonotrichia capensis) in Ecuador. Our study sites lay along a transect, and adjacent study sites were separated by approximately 25 km, an order of magnitude less than previously examined for this and most other species. This transect crossed an Andean ridge and through the Quijos River Valley, both of which may be barriers to gene flow. Using a variance partitioning approach, we show that song dialect is important in explaining population genetics, independent of the geographic variables: distance, the river valley and the Andean Ridge. This result is consistent with the hypothesis that song acts as a barrier to breeding among populations in close proximity. In addition, songs of contiguous populations differed by the same degree or more than between two populations previously shown to exhibit female preference for local dialect, suggesting that birds from these populations would also breed preferentially with locals. As expected, all geographic variables (distance, the river valley and the Andean Ridge) also predicted population genetic structure. Our results have important implications for the understanding whether, and at what spatial scale, culture can affect population divergence.

Background noise can interfere with acoustic communication. Signal modifications have the potential to increase signal-to-noise ratios and reduce the masking effect of noise. Immediate signaling flexibility, a type of vocal plasticity, allows animals to modify their signal to optimize transmission depending on ambient noise conditions. Results from previous studies provide conflicting evidence about whether expression of immediate signaling flexibility is dependent upon the signaler having prior experience with noisy environments. To improve our understanding of vocal plasticity, we examined immediate signaling flexibility in white-crowned sparrows Zonotrichia leucophrys nuttalli in urban and rural locations. We experimentally broadcast noise with three different spectral profiles—city (low frequency), inverse of city (high frequency), and white (equal intensity at all frequencies)—successfully to 107 males and measured multiple features of songs produced prior to and during the experiment. We predicted that all males would adjust their song structure during the noise broadcast, but that the extent of plasticity would be greater in noisier territories and the manner of adjustment would depend on the type of experimental noise broadcast. Instead, we found that only urban males exhibit immediate signaling flexibility, through which bandwidth decreases in response to experimental noise. Signal modification did not change with type of noise broadcast or territory background noise. Although songs were less flexible than predicted, the relative reduction in signal bandwidth in urban birds could improve their signal-to-noise ratio during the noise broadcast.

AbstractThe soundscape acts as a selective agent on organisms that use acoustic signals to communicate. A number of studies document variation in structure, amplitude, or timing of signal production in correspondence with environmental noise levels thus supporting the hypothesis that organisms are changing their signaling behaviors to avoid masking. The time scale at which organisms respond is of particular interest. Signal structure may evolve across generations through processes such as cultural or genetic transmission. Individuals may also change their behavior during development (ontogenetic change) or in real time (i.e., immediate flexibility). These are not mutually exclusive mechanisms, and all must be investigated to understand how organisms respond to selection pressures from the soundscape. Previous work on white‐crowned sparrows (Zonotrichia leucophrys) found that males holding territories in louder areas tend to sing higher frequency songs and that both noise levels and song frequency have increased over time (30 years) in urban areas. These previous findings suggest that songs are changing across generations; however, it is not known if this species also exhibits immediate flexibility. Here, we conducted an exploratory, observational study to ask whether males change the minimum frequency of their song in response to immediate changes in noise levels. We also ask whether males sing louder, as increased minimum frequency may be physiologically linked to producing sound at higher amplitudes, in response to immediate changes in environmental noise. We found that territorial males adjust song amplitude but not minimum frequency in response to changes in environmental noise levels. Our results suggest that males do not show immediate flexibility in song minimum frequency, although experimental manipulations are needed to test this hypothesis further. Our work highlights the need to investigate multiple mechanisms of adaptive response to soundscapes.

Soundscapes pose both evolutionarily recent and long-standing sources of selection on acoustic communication. We currently know more about the impact of evolutionarily recent human-generated noise on communication than we do about how natural sounds such as pounding surf have shaped communication signals over evolutionary time. Based on signal detection theory, we hypothesized that acoustic phenotypes will vary with both anthropogenic and natural background noise levels and that similar mechanisms of cultural evolution and/or behavioral flexibility may underlie this variation. We studied song characteristics of white-crowned sparrows (Zonotrichia leucophrys nuttalli) across a noise gradient that includes both anthropogenic and natural sources of noise in San Francisco and Marin counties, California, USA. Both anthropogenic and natural soundscapes contain high amplitude low frequency noise (traffic or surf, respectively), so we predicted that birds would produce songs with higher minimum frequencies in areas with higher amplitude background noise to avoid auditory masking. We also anticipated that song minimum frequencies would be higher than the projected lower frequency limit of hearing based on site-specific masking profiles. Background noise was a strong predictor of song minimum frequency, both within a local noise gradient of three urban sites with the same song dialect and cultural evolutionary history, and across the regional noise gradient, which encompasses 11 urban and rural sites, several dialects, and several anthropogenic and natural sources of noise. Among rural sites alone, background noise tended to predict song minimum frequency, indicating that urban sites were not solely responsible for driving the regional pattern. These findings support the hypothesis that songs vary with local and regional soundscapes regardless of the source of noise. Song minimum frequency from five core study sites was also higher than the lower frequency limit of hearing at each site, further supporting the hypothesis that songs vary to transmit through noise in local soundscapes. Minimum frequencies leveled off at noisier sites, suggesting that minimum frequencies are constrained to an upper limit, possibly to retain the information content of wider bandwidths. We found evidence that site noise was a better predictor of song minimum frequency than territory noise in both anthropogenic and natural soundscapes, suggesting that cultural evolution rather than immediate behavioral flexibility is responsible for local song variation. Taken together, these results indicate that soundscapes shape song phenotype across both evolutionarily recent and long-standing soundscapes.

Tables of linear models ranked based on AICc values.

Anthropogenic noise presents a problem for acoustic communication in animal taxa around the world. Many animals respond by modifying their acoustic signals, sometimes along multiple axes, such as song structure, redundancy, or amplitude. To date, no study has assayed the relative response of animals to multiple axes of signal variation, such as song structure and song amplitude, associated with anthropogenic noise levels. To investigate the impact of multiple potential adaptations to anthropogenic noise on targeted receivers, we manipulated song amplitude and song minimum frequency of white-crowned sparrow (Zonotrichia leucophrys) songs. We used a 2 x 2 factorial design of playback experiments to measure male territorial responses to songs that were relatively quiet or loud in relation to typical Z. leucophrys songs and with lower or higher minimum frequencies within the range of natural Z. leucophrys songs. Males responded more strongly to louder songs than to quieter songs and more strongly to lower than to higher minimum frequency songs, with the strongest responses to louder songs with relatively lower minimum frequencies. These results indicate that whether or not increasing amplitude or increasing minimum frequency is more effective at increasing signal transmission distance in anthropogenic noise, increases in signal amplitude increase signal salience in male-male interactions. Thus in the context of territoriality and sexual selection, an increase in song amplitude can compensate for losses in signal salience due to higher minimum frequency. An increase in only song minimum frequency in the context of low frequency anthropogenic noise could be maladaptive.

Summary Moult is critical for fitness for many organisms for several reasons: it allows growth and maintains the function of the integument for protection, thermoregulation and communication. Feather moult in birds is costly and therefore typically does not overlap with migration or reproduction. In spring, the rapid succession of pre‐alternate moult, migration (if a migrant) and breeding suggests that timing of moult could constrain the initiation of breeding. A trade‐off between time spent moulting and breeding might also limit moult quality. The proximate basis for the timing of pre‐alternate moult initiation is not well known, although it typically occurs during a resource poor time of year. Food limitation combined with fitness consequences of moulting earlier suggests that plasticity in timing of pre‐alternate moult in response to food abundance should be advantageous. We experimentally tested, for the first time, if food abundance influences the timing of moult in the wild. We conducted a controlled food supplementation experiment on free‐living swamp sparrows (Melospiza georgiana) preceding and during the time of natural pre‐alternate moult (January–March 2009, 2010). Supplemented birds began moulting the body, face and crown earlier than control birds (11, 14, and 8 days earlier, respectively), indicating that food abundance limits the initiation of moult. Along with interannual variation, these results indicate that photoperiod is not the sole cue for initiation of moult. Both control and supplemented birds moulted in sequence, starting with the body, followed by the crown 9 days later, and the face 11 days after the body. The presence of a sequence further suggests energetic limitation of moult or possibly a strategy to moult‐specific regions first to ensure completion or growth at an optimal time. This study provides novel experimental evidence that food abundance can: (i) limit pre‐alternate moult timing and (ii) limit moult timing in the wild. Food limitation of moult timing could allow earlier breeding or production of higher quality feathers and thus cascade through other life‐history stages and affect reproductive success. These results indicate that food availability is a cue for moult initiation, possibly acting secondarily to photostimulation.

The Winter Food Limitation Hypothesis (WFLH) states that winter food abundance is a dominant source of population limitation of migratory birds. Evidence is accumulating that long-distance migratory birds wintering in tropical climates have high overwinter survival probabilities and that winter food limitation mainly affects their fitness nonlethally by limiting energetic reserves necessary for successful reproduction. In contrast, the relative roles of direct mortality vs. indirect effects caused by food limitation have not been investigated thoroughly on short-distance migratory birds wintering in temperate zones, where they face thermal challenges. We performed the first test of the WFLH for a temperate migratory bird in the wild on the Swamp Sparrow (Melospiza georgiana), with a replicated plot-wide food supplementation experiment. In contrast to tropical, but consistent with other temperate-wintering migrants, Swamp Sparrows on unmanipulated plots showed relatively low apparent survival across the winter. Following food addition, birds (1) immigrated to experimental plots, which subsequently supported approximately 50% higher abundances, (2) experienced increases of within-season apparent survival of 8-10%, depending on age/sex class, and (3) had higher-scaled mass index values, all supporting winter food limitation. The last two findings are interrelated because birds with higher scaled mass had higher survival probabilities, further supporting direct effects of winter food limitation. Food limitation of fat reserves might also have indirect effects on reproductive success by limiting migration timing and survival during migration. Increases in scaled mass were higher in females, suggesting that they are disproportionately affected by food limitation, possibly through competition. Based on Robust Design Modeling, we found no support for emigration prior to food addition, indicating that our estimates of mortality are unbiased.

In addition to the observed high diversity of species in the tropics, divergence among populations of the same species exists over short geographic distances in both phenotypic traits and neutral genetic markers. Divergence among populations suggests great potential for the evolution of reproductive isolation and eventual speciation. In birds, song can evolve quickly through cultural transmission and result in regional dialects, which can be a critical component of reproductive isolation through variation in female preference. We examined female and male behavioral responses to local and nonlocal dialects in two allopatric populations of rufous-collared sparrows (Zonotrichia capensis) in the Andes Mountains of Ecuador. Here we show that female sparrows prefer their natal song dialect to the dialect of an allopatric population that is just 25 km away and separated by an unsuitable higher-elevation habitat (pass of 4,200 m), thus providing evidence of prezygotic reproductive isolation among populations. Males showed similar territorial responses to all conspecific dialects with no consistent difference with respect to distance, making male territoriality uninformative for estimating reproductive isolation. This study provides novel evidence for culturally based prezygotic isolation over very short distances in a tropical bird.