Determinants of reproductive success and offspring sex in a turtle with environmental sex determination

Despite the importance of maternal effects in evolution, and knowledge of links among nest site choice, timing of nesting, offspring sex, and reproductive success in animals with environmental sex determination, these attributes have not been rigorously studied in a combined and natural context. To address this need we studied the relationships between three maternal traits (nest site choice, lay date, and nest depth) and two fitness‐related attributes of offspring (hatchling sex and embryonic survival) in the riverine turtle Carettochelys insculpta, a species with temperature‐dependent sex determination, for four years. Predation and flooding were the major sources of embryonic mortality in 191 nests. Embryonic survival was influenced by both lay date and nest site choice: in one year when nesting began later than average, nests laid later and at lower elevations were destroyed by early wet season river rises. In other years early nesting precluded flood mortality. However, turtles did not nest at the highest available elevations, and a field experiment confirmed that turtles were constrained to nest at lower elevations where they could construct a nest chamber. The principal determinant of hatchling sex in 140 nests was lay date, which in turn was apparently related to the magnitude of the previous wet season(s). Clutches laid earlier in the season (a female's first clutch) produced mainly males, while later clutches (her second clutch) yielded mostly females, due to seasonal increases in air temperatures. Accordingly, later nesting produced female‐biased hatchling sex ratios in 1996, while earlier nesting resulted in sex ratios near unity in the other years. However, all‐female nests were more likely to be flooded than mixed‐sex or all‐male nests in years when nesting was late. In conclusion, we found evidence that the position of two maternal trait distributions (elevation of the nest site and lay date), associated with the reproductive strategy of C. insculpta, reflect a combination of natural selection, physical constraints, and phenotypic plasticity. © 2004 The Linnean Society of London, Biological Journal of the Linnean Society, 2004, 81 , 1–16.     

Potential limitations of behavioral plasticity and the role of egg relocation in climate change mitigation for a thermally sensitive endangered species

Anthropogenic climate change is widely considered a major threat to global biodiversity, such that the ability of a species to adapt will determine its likelihood of survival. Egg‐burying reptiles that exhibit temperature‐dependent sex determination, such as critically endangered hawksbill turtles (Eretmochelys imbricata), are particularly vulnerable to changes in thermal regimes because nest temperatures affect offspring sex, fitness, and survival. It is unclear whether hawksbills possess sufficient behavioral plasticity of nesting traits (i.e., redistribution of nesting range, shift in nesting phenology, changes in nest‐site selection, and adjustment of nest depth) to persist within their climatic niche or whether accelerated changes in thermal conditions of nesting beaches will outpace phenotypic adaption and require human intervention. For these reasons, we estimated sex ratios and physical condition of hatchling hawksbills under natural and manipulated conditions and generated and analyzed thermal profiles of hawksbill nest environments within highly threatened mangrove ecosystems at Bahía de Jiquilisco, El Salvador, and Estero Padre Ramos, Nicaragua. Hawksbill clutches protected in situ at both sites incubated at higher temperatures, yielded lower hatching success, produced a higher percentage of female hatchlings, and produced less fit offspring than clutches relocated to hatcheries. We detected cooler sand temperatures in woody vegetation (i.e., coastal forest and small‐scale plantations of fruit trees) and hatcheries than in other monitored nest environments, with higher temperatures at the deeper depth. Our findings indicate that mangrove ecosystems present a number of biophysical (e.g., insular nesting beaches and shallow water table) and human‐induced (e.g., physical barriers and deforestation) constraints that, when coupled with the unique life history of hawksbills in this region, may limit behavioral compensatory responses by the species to projected temperature increases at nesting beaches. We contend that egg relocation can contribute significantly to recovery efforts in a changing climate under appropriate circumstances.     

Climate change and nesting behaviour in vertebrates: a review of the ecological threats and potential for adaptive responses

Nest building is a taxonomically widespread and diverse trait that allows animals to alter local environments to create optimal conditions for offspring development. However, there is growing evidence that climate change is adversely affecting nest‐building in animals directly, for example via sea‐level rises that flood nests, reduced availability of building materials, and suboptimal sex allocation in species exhibiting temperature‐dependent sex determination. Climate change is also affecting nesting species indirectly, via range shifts into suboptimal nesting areas, reduced quality of nest‐building environments, and changes in interactions with nest predators and parasites. The ability of animals to adapt to sustained and rapid environmental change is crucial for the long‐term persistence of many species. Many animals are known to be capable of adjusting nesting behaviour adaptively across environmental gradients and in line with seasonal changes, and this existing plasticity potentially facilitates adaptation to anthropogenic climate change. However, whilst alterations in nesting phenology, site selection and design may facilitate short‐term adaptations, the ability of nest‐building animals to adapt over longer timescales is likely to be influenced by the heritable basis of such behaviour. We urgently need to understand how the behaviour and ecology of nest‐building in animals is affected by climate change, and particularly how altered patterns of nesting behaviour affect individual fitness and population persistence. We begin our review by summarising how predictable variation in environmental conditions influences nest‐building animals, before highlighting the ecological threats facing nest‐building animals experiencing anthropogenic climate change and examining the potential for changes in nest location and/or design to provide adaptive short‐ and long‐term responses to changing environmental conditions. We end by identifying areas that we believe warrant the most urgent attention for further research.     

Does sex-ratio selection influence nest-site choice in a reptile with temperature-dependent sex determination?

Evolutionary theory predicts that dioecious species should produce a balanced primary sex ratio maintained by frequency-dependent selection. Organisms with environmental sex determination, however, are vulnerable to maladaptive sex ratios, because environmental conditions vary spatio-temporally. For reptiles with temperature-dependent sex determination, nest-site choice is a behavioural maternal effect that could respond to sex-ratio selection, as mothers could adjust offspring sex ratios by choosing nest sites that will have particular thermal properties. This theoretical prediction has generated decades of empirical research, yet convincing evidence that sex-ratio selection is influencing nesting behaviours remains absent. Here, we provide the first experimental evidence from nature that sex-ratio selection, rather than only viability selection, is probably an important component of nest-site choice in a reptile with temperature-dependent sex determination. We compare painted turtle (Chrysemys picta) neonates from maternally selected nest sites with those from randomly selected nest sites, observing no substantive difference in hatching success or survival, but finding a profound difference in offspring sex ratio in the direction expected based on historical records. Additionally, we leverage long-term data to reconstruct our sex ratio results had the experiment been repeated in multiple years. As predicted by theory, our results suggest that sex-ratio selection has shaped nesting behaviour in ways likely to enhance maternal fitness.     

Seasonal sex ratios and the evolution of temperature-dependent sex determination in oviparous lizards

Although the adaptive significance of temperature-dependent sex determination (TSD) remains a puzzle, recent models implicate a seasonal bias in offspring sex production that translates into sex-specific fitness benefits later in life. Sex-specific emergence has been linked to fitness gains in some fish, birds and reptiles, but field data supporting the occurrence of a seasonal pattern of sex ratios in oviparous lizards are lacking. We tested the hypothesis that patterns of nest site selection and seasonal temperature changes combine to inhibit the materialization of sex-biased hatching times in a population of water dragons (Intellagama lesueurii). As predicted, a seasonal increase in air and nest temperatures resulted in a sex bias by nesting date; male-producing clutches were laid 17.8 days sooner than female-producing clutches, on average. However, the seasonal ramping of nest temperatures also caused shorter relative incubation periods in the later, all-female clutches. As a consequence of this developmental ‘catch-up’, the mean hatching date for male-producing nests preceded the mean hatching date for female-producing nests by only 7.2 days. We suggest that a contracted distribution of hatching dates compared to the distribution of oviposition dates represents a general pattern for oviparous reptiles in seasonal climates, which in TSD species may largely offset the temporal disparity in nesting dates between the sexes. Although data are needed for other TSD species, such minor age differences between male and female hatchlings may not translate into fitness differences later in life, an assumption of some models for the evolution and maintenance of TSD.     

Predicting the fate of a living fossil: how will global warming affect sex determination and hatching phenology in tuatara?

How will climate change affect species'' reproduction and subsequent survival? In many egg-laying reptiles, the sex of offspring is determined by the temperature experienced during a critical period of embryonic development (temperature-dependent sex determination, TSD). Increasing air temperatures are likely to skew offspring sex ratios in the absence of evolutionary or plastic adaptation, hence we urgently require means for predicting the future distributions of species with TSD. Here we develop a mechanistic model that demonstrates how climate, soil and topography interact with physiology and nesting behaviour to determine sex ratios of tuatara, cold-climate reptiles from New Zealand with an unusual developmental biology. Under extreme regional climate change, all-male clutches would hatch at 100% of current nest sites of the rarest species, Sphenodon guntheri, by the mid-2080s. We show that tuatara could behaviourally compensate for the male-biasing effects of warmer air temperatures by nesting later in the season or selecting shaded nest sites. Later nesting is, however, an unlikely response to global warming, as many oviparous species are nesting earlier as the climate warms. Our approach allows the assessment of the thermal suitability of current reserves and future translocation sites for tuatara, and can be readily modified to predict climatic impacts on any species with TSD.     

Inheritance of nesting behaviour across natural environmental variation in a turtle with temperature-dependent sex determination

Nesting behaviour is critical for reproductive success in oviparous organisms with no parental care. In organisms where sex is determined by incubation temperature, nesting behaviour may be a prime target of selection in response to unbalanced sex ratios. To produce an evolutionary change in response to sex-ratio selection, components of nesting behaviour must be heritable. We estimated the field heritability of two key components of nesting behaviour in a population of painted turtles (Chrysemys picta) with temperature-dependent sex determination by applying the ‘animal model’ to a pedigree reconstructed from genotype data. We obtained estimates of low to non-detectable heritability using repeated records across all environments. We then determined environment-specific heritability by grouping records with similar temperatures for the winter preceding the nesting season, a variable known to be highly associated with our two traits of interest, nest vegetation cover and Julian date of nesting. The heritability estimates of nest vegetation cover and Julian date of nesting were qualitatively highest and significant, or nearly so, after hot winters. Additive genetic variance for these traits was not detectable after cold winters. Our analysis suggests that the potential for evolutionary change of nesting behaviour may be dependent on the thermal conditions of the preceding winter, a season that is predicted to be especially subject to climate change.     

Population sex ratios under differing local climates in a reptile with environmental sex determination

Populations that experience different local climates, such as those along a latitudinal gradient, must match life history traits to local environmental conditions. In species with temperature-dependent sex determination, such as many reptiles, population sex ratio is strongly influenced by local climate, yet local climate differs substantially among populations in geographically-widespread species. We studied the painted turtle at three sites across the species’ geographic range to gain a mechanistic understanding of how sex ratios are produced under different local climates. We combined data on maternal nest-site choice, nest incubation temperature, and the resultant offspring sex ratio of populations across a climatic gradient, to demonstrate how geographic variation in behavior and physiology translates into sex ratios among populations of a widely-distributed species. We found that populations across the species’ geographic range match incubation conditions with local climatic conditions through population-specific adjustment of maternal nest-site choice. Incubation temperatures during the thermosensitive period were cooler and clutches were more male-biased in the south, with populations farther north having warmer incubation temperatures and more female-biased sex ratios, yet adult sex ratios were not strongly biased in any population. Most components of maternal nest-site choice varied latitudinally among populations, suggesting that the species may have a considerable repertoire for responding to climate change through adjustment of nest-site choice.     

Social and Thermal Cues Influence Nest‐site Selection in a Nocturnal Gecko,Oedura lesueurii

In oviparous species lacking parental care, successful reproduction depends on females selecting nest sites that facilitate embryonic development. Such sites may be limited in the environment, which can lead to multiple females using the same nest site simultaneously. However, there are several alternative explanations for communal nesting, including natal homing, predator satiation, and adaptive benefits to offspring. We used laboratory experiments to evaluate three hypotheses about nest‐site selection in velvet geckos (Oedura lesueurii), which often nest communally. We investigated whether the trend to nest communally is influenced by the following: (1) evidence of previous nesting (hatched eggshells); (2) body size; and/or (3) thermal regimes. When given the choice, females laid their eggs in shelters containing hatched eggshells rather than in empty shelters, and this was not influenced by body size. Females selected nest sites that were cooler than their own mean selected body temperatures, suggesting that thermal requirements of their developing embryos could outweigh their own thermoregulatory preferences. Field observations of natal homing and high predation rates on gravid females suggest that imprinting on nest sites and/or predator swamping also play roles in communal nesting. Collectively, our results suggest that female velvet geckos use multiple cues to select appropriate nest sites, and hence that multiple mechanisms result in communal nesting behavior in this species.     

Superficial lizards in cold climates: Nest site choice along an elevational gradient

Embryonic conditions may limit the distributions of egg‐laying ectotherms, and recent research suggests that nesting mothers of wide‐ranging species may use a number of factors to compensate for differing climates. However, while variation in temporal factors across environmental gradients are common or pervasive (i.e. seasonal timing of nesting), similar evidence for spatial factors is rare (e.g. aspect, openness and depth of nest sites). I tested the idea that a wide‐ranging lizard, the Australia water dragon (Physignathus lesueurii), uses nest depth to counter climate differences along a temperature cline at their cold‐end range margin. Two measures of nest depth were significantly, inversely related to elevation across six populations spanning 700 m. Elevation explained 83–86% of the variation in nest depth. These findings support a thermal compensatory mechanism for this pattern, although soil moisture compensation is plausible. My results directly support a recent, untested prediction that the evolution of viviparity in reptiles is preceded by a behavioural shift towards increasingly superficial nest sites in cold climates, followed by selection for increased egg retention to avoid temperature extremes. However, in the present study egg desiccation rates increased with increasing elevation in a dry year, suggesting that increased egg retention may evolve in response to lethal hydric conditions, rather than lethal temperatures. When considered alongside recent research, the present study indicates that water dragons possess several mechanisms for adjusting to climate change.     

CULTURAL INHERITANCE AS A MECHANISM FOR POPULATION SEX-RATIO BIAS IN REPTILES

Abstract.— Although natural populations of most species exhibit a 1:1 sex ratio, biased sex ratios are known to be associated with non‐Mendelian inheritance, as in sex‐linked meiotic drive and cytoplasmic inheritance (Charnov 1982; Hurst 1993). We show how cultural inheritance, another type of non‐Mendelian inheritance, can favor skewed primary sex ratios and propose that it may explain the female‐biased sex ratios commonly observed in reptiles with environmental sex determination (ESD). Like cytoplasmic elements, cultural traits can be inherited through one sex. This, in turn, favors skewing the primary sex allocation in favor of the transmitting sex. Female nest‐site philopatry is a sex‐specific, culturally inherited trait in many reptiles with ESD and highly female‐biased sex ratios. We propose that the association of nest‐site selection with ESD facilitates the maternal manipulation of offspring sex ratios toward females.     

Do long-term changes in sea surface temperature at the breeding areas affect the breeding dates and reproduction performance of Mediterranean loggerhead turtles? Implications for climate change

Global climate change is likely to have an important influence on the phenology, behaviour and population dynamics of many species. We investigate climatic related changes in the breeding phenology of Mediterranean loggerhead marine turtles Caretta caretta over a 19 year period and the potential relationship between these changes and reproductive success and performance. We found that the studied population has experienced fluctuating sea surface temperatures (SST) with an increasing trend during the last century. With increasing spring SST there is a trend towards earlier nesting. However, there is no significant relationship between SST and nesting season, defined as the duration between the first recorded emergence and the last nest laid. Our analyses indicate that marine turtles display phenological changes, and thus maintain favorable thermal conditions at the nesting sites. Furthermore, increasing spring SST was correlated with decreasing clutch size and increasing hatching success that resulted in an apparent lack of correlation between SST and hatchling production. This apparent independence might be misleading since it only holds for a limited range of SST values. Thus, if we estimate the effect of climate change on loggerhead population growth as neutral, based on the apparent independence between SST and total number of hatchlings, we will be underestimating the population extinction risk.     

How rapidly can maternal behavior affecting primary sex ratio evolve in a reptile with environmental sex determination?

Theoretical models identify maternal behavior as critical for the maintenance and evolution of sex ratios in organisms with environmental sex determination (ESD). Maternal choice of nest site is generally thought to respond more rapidly to sex ratio selection than environmental sensitivity of offspring sex (threshold temperatures) in reptiles with temperature-dependent sex determination (TSD, a form of ESD). However, knowledge of the evolutionary potential for either of these traits in a field setting is limited. I developed a simulation model using local climate data and observed levels of phenotypic variation for nest-site choice and threshold temperatures in painted turtles (Chrysemys picta) with TSD. Both nest-site choice and threshold temperatures, and hence sex ratios, evolved slowly to simulated climate change scenarios. In contrast to expectations from previous models, nest-site choice evolved more slowly than threshold temperatures because of large climatic effects on nest temperatures and indirect selection on maternally expressed traits. A variant of the model, assuming inheritance of nest-site choice through natal imprinting, demonstrated that natal imprinting inhibited adaptive responses in female nest-site choice to climate change. These results predict that females have relatively low potential to adaptively adjust sex ratios through nest-site choice.     

Nesting Success in Crimson Finches: Chance or Choice?

In avian systems, nest predation is one of the most significant influences on reproductive success. Selection for mechanisms and behaviours to minimise predation rates should be favoured. To avoid predation, breeding birds can often deter predators through active nest defence or by modifying behaviours around the nest (e.g. reducing feeding rates and vocalisations). Birds might also benefit from concealing nests or placing them in inaccessible locations. The relative importance of these strategies (behaviour vs. site selection) can be difficult to disentangle and may differ according to life history. Tropical birds are thought to experience higher rates of predation than temperate birds and invest less energy in nest defence. We monitored a population of crimson finches (Neochmia phaeton), in the Australian tropics, over two breeding seasons. We found no relationship between adult nest defence behaviour (towards a model reptile predator) and the likelihood of nest success. However, nest success was strongly related to the visibility of the nest and the structure of the vegetation. We found no evidence that adult nest building decisions were influenced by predation risk; individuals that re‐nested after a predation event did not build their nest in a more concealed location. Therefore, predator avoidance, and hence nest success, appears to be largely due to chance rather than due to the behaviour of the birds or their choice of nesting sites. To escape high predation pressures, multiple nesting attempts both within and between seasons may be necessary to increase reproductive success. Alternatively, birds may be limited in their nest‐site options; that is, high‐quality individuals dominate quality nest sites.     

The role of nest‐site selection and cereal production in differential nest predation in Common Quail Coturnix coturnix and hybrid quail C. coturnix × C. japonica

In Europe, farm‐reared quail used for restocking purposes are often hybrids between the Common Quail Coturnix coturnix and the Japanese Quail Coturnix japonica. These hybrids interbreed with wild Common Quail populations and suffer higher rates of nest predation, which would suggest that the two quail types have different nest‐site selection patterns. We monitored 103 radiotagged nesting females (77 wild Common Quail over 18 breeding seasons and 26 hybrid quail over six breeding seasons) and analysed their nest placement behaviour. Our results did not provide any evidence that the quail types showed different nest‐site selection trends, as they all preferred to nest near cereal field margins. The higher nest predation rate experienced by hybrids might therefore be explained by less effective anti‐predator behaviour among hybrid females during incubation. As no edge effect on nest predation was observed, the preference for nesting close to cereal margins could be due to the greater food resources of field margins. Moreover, Common Quail nest predation decreased with a rise in barley straw production, supporting evidence that nest cover plays an important role in the prevention of nest predation in this species. No such relationship was found for hybrid quail, but this result is not conclusive, as the data on barley straw production for hybrid quail covered a much smaller range of interannual variability. Barley straw production in Catalonia decreased significantly between 1992 and 2012, possibly as a result of a change in the barley varieties used. This practice could entail a conservation threat for ground‐nesting bird species.     

Climate change and temperature-dependent sex determination: can individual plasticity in nesting phenology prevent extreme sex ratios?

Under temperature-dependent sex determination (TSD), temperatures experienced by embryos during development determine the sex of the offspring. Consequently, populations of organisms with TSD have the potential to be strongly impacted by climatic warming that could bias offspring sex ratio, a fundamental demographic parameter involved in population dynamics. Moreover, many taxa with TSD are imperiled, so research on this phenomenon, particularly long-term field study, has assumed great urgency. Recently, turtles with TSD have joined the diverse list of taxa that have demonstrated population-level changes in breeding phenology in response to recent climate change. This raises the possibility that any adverse impacts of climate change on populations may be alleviated by individual plasticity in nesting phenology. Here, we examine data from a long-term study on a population of painted turtles (Chrysemys picta) to determine whether changes in phenology are due to individual plasticity and whether individual plasticity in the timing of nesting has the capacity to offset the sex ratio effects of a rise in climatic temperature. We find that individual females show plasticity in the date of first nesting each year, and that this plasticity depends on the climate from the previous winter. First nesting date is not repeatable within individuals, suggesting that it would not respond to selection. Sex ratios of hatchlings within a nest declined nonsignificantly over the nesting season. However, small increases in summer temperature had a much stronger effect on nest sex ratios than did laying nests earlier in the season. For this and other reasons, it seems unlikely that individual plasticity in the timing of nesting will offset the effects of climate change on sex ratios in this population, and we hypothesize that this conclusion applies to other populations with TSD.     

Arboreal nesting as anti-predator adaptation by savanna chimpanzees (Pan troglodytes verus) in southeastern Senegal

Chimpanzees (Pan troglodytes) make nests for resting and sleeping, which is unusual for anthropoid primates but common to all great apes. Arboreal nesting has been linked to predation pressure, but few studies have tested the adaptive nature of this behavior. We collected data at two chimpanzee study sites in southeastern Senegal that differed in predator presence to test the hypothesis that elevated sleeping platforms are adaptations for predator defense. At Assirik in the Parc National du Niokolo-Koba, chimpanzees face four species of large carnivore, whereas at Fongoli, outside national park boundaries, humans have exterminated almost all natural predators. We quantified the availability of vegetation at the two sites to test the alternative hypothesis that differences in nesting reflect differences in habitat structure. We also examined possible sex differences in nesting behavior, community demographic differences, seasonality and nest age differences as variables also potentially affecting nest characteristics and nesting behavior between the two sites. Chimpanzees at Fongoli nested at lower heights and farther apart than did chimpanzees at Assirik and sometimes made nests on the ground. The absence of predators outside of the national park may account for the differences in nest characteristics at the two sites, given the similarities in habitat structure between Fongoli and Assirik. However, Fongoli chimpanzees regularly build arboreal nests for sleeping, even under minimal predation pressure, and this requires explanation.     

Spring temperature, clutch initiation date and duck nest success: a test of the mismatch hypothesis

1. Increases in average global temperature during the twentieth century have prompted calls for research on the effect of temperature variation on avian population dynamics. Particular attention has been paid to the hypothesis that increased temperatures may affect a species' ability to shift their breeding efforts to match the phenology of their prey, and thus result in reduced reproductive success (the 'mismatch hypothesis'). 2. We used data from a long-term study of breeding ducks to investigate how duck nest success varied with clutch initiation date, and to test whether spring temperature affected this relationship in a manner consistent with the mismatch hypothesis. We modelled five possible functional forms of how nest success might vary with clutch initiation date and spring temperature, and used an information-theoretic approach to determine which model best described the nesting outcomes of five dabbling duck species nesting in Saskatchewan, Canada. 3. Probability of nest success for the five species did not vary strongly with clutch initiation date, and we found evidence consistent with the mismatch hypothesis for one species, northern pintail Anas acuta, although weight of evidence was weak. 4. Overall nest success of all five species was positively associated with spring temperature. These results suggest that increasing spring temperature alone (within the range observed in this study) may not affect nest success in a manner that would result in lower populations of breeding ducks.     

Kea Nestor notabilis mothers produce nest-specific calls with low amplitude and high entropy

Vocal behaviour of nesting altricial birds is subject to selection pressure from several sources. Offspring beg to attract parents’ attention, thus increasing the chances of being fed, but also increasing the chances of being detected by predators. Research on passerines has shown that parents may reduce the risk of nest predation by alarm calling to warn nestlings to be quiet, and by producing food calls which solicit begging when parents are present to defend the nestlings. Both nestlings and parents may reduce the risk of predator detection by producing calls of low amplitude and high entropy which are acoustically difficult to locate. Although extensive research has been undertaken on nesting passerine vocalizations, little is known about parrots in this regard, and studies are needed to determine whether parrots show similar adaptations. We investigated the calling behaviour of Kea Nestor notabilis mothers during the nesting period to determine whether maternal vocalizations were adapted in a way that could increase the chance of brood success. A microphone was installed inside the nest to record calls produced both inside the nest and in the direct vicinity. Our prediction was that calls outside the nest would be easy to locate and could function as alarm calls to alert conspecifics or distract the predator, whereas calls inside the nest would be difficult to locate and could serve to communicate with nestlings without alerting predators. Our results accorded with these predictions. Calls produced outside the nest were loud and tonal, and corresponded to previously described Kea alarm calls. Calls produced inside the nest, however, were high-entropy and low-amplitude calls, and formed a distinct structural category. We thus provide the first evidence that a parrot species has a vocal category for communication inside the nest, and that calls within this category are structured in a way that could reduce the risk of nest predation.     

Nest reuse in arctic‐breeding shorebirds: an analysis of potential benefits and factors affecting the occurrence of this rare behavior

During a 13‐yr study near Utqia?vik (formerly Barrow), Alaska, we documented the prevalence of nest reuse in eight arctic‐breeding shorebirds. We evaluated whether nest reuse saved individuals time and energy, enhanced nest survival, or was related to nest density. We documented 208 (6.2%) cases of nest reuse among 3336 nesting attempts. Nest reuse occurred in all years but the first and in all species, with greatest reuse in semipalmated sandpiper (10.9%) and American golden‐plover (10.0%). While most cases of nest reuse occurred with conspecifics, many cases of heterospecific nest reuse were also observed, indicating high niche overlap in nest site preferences among species. We found that individuals reusing old nests may have benefited by nesting earlier, but nest reuse did not generally enhance nest survival. A significant positive relationship was also found between nest reuse and nest density at the community level and for four of the eight species, suggesting high inter‐ or intraspecific competition combined with limited suitable nest sites may force individuals to reuse old nests. Our observations also suggest that upland nesting species may be the most dependent on old nest sites. Preferential development of these sites may therefore have a previously unknown detrimental effect on these species, although further study is needed to better determine the impact of such habitat loss.