Aphelocoma Insularis Research Articles

Habitat-linked genetic variation supports microgeographic adaptive divergence in an island-endemic bird species.

We investigated the potential mechanisms driving habitat‐linked genetic divergence within a bird species endemic to a single 250‐km2 island. The island scrub‐jay (Aphelocoma insularis) exhibits microgeographic divergence in bill morphology across pine–oak ecotones on Santa Cruz Island, California (USA), similar to adaptive differences described in mainland congeners over much larger geographic scales. To test whether individuals exhibit genetic differentiation related to habitat type and divergence in bill length, we genotyped over 3000 single nucleotide polymorphisms in 123 adult island scrub‐jay males from across Santa Cruz Island using restriction site‐associated DNA sequencing. Neutral landscape genomic analyses revealed that genome‐wide genetic differentiation was primarily related to geographic distance and differences in habitat composition. We also found 168 putatively adaptive loci associated with habitat type using multivariate redundancy analysis while controlling for spatial effects. Finally, two genome‐wide association analyses revealed a polygenic basis to variation in bill length with multiple loci detected in or near genes known to affect bill morphology in other birds. Our findings support the hypothesis that divergent selection at microgeographic scales can cause adaptive divergence in the presence of ongoing gene flow.

Body size is associated with yearling breeding and extra-pair mating in the Island Scrub-Jay

Abstract Large body size is an important determinant of individual fitness in many animal species, especially in island systems where habitat saturation may result in strong intraspecific competition for mates and breeding territories. Here we show that large body size is associated with benefits to yearling breeding and extra-pair mating in the Island Scrub-Jay (Aphelocoma insularis), endemic to Santa Cruz Island, California. This species is ~20% larger than its mainland congener, consistent with the island syndrome, indicating that body size may be a trait under selection. From 2009 to 2013, we quantified the reproductive success of a marked population of Island Scrub-Jays, tracked which yearlings acquired a breeding territory and bred, and measured the occurrence of extra-pair paternity. Two potential contributors to fitness were positively related to body size. Larger yearling males were more likely to breed, possibly due to greater behavioral dominance during aggressive encounters. Larger males were also less likely to lose paternity to extra-pair males and, anecdotally, extra-pair males were larger than the social male cuckolded. This study provides evidence that larger males may have a fitness advantage over smaller males by breeding earlier and avoiding paternity loss, but estimates of lifetime reproductive success are ultimately needed for Island Scrub-Jays and other long-lived species.

Variance Propagation for Density Surface Models

Spatially explicit estimates of population density, together with appropriate estimates of uncertainty, are required in many management contexts. Density surface models (DSMs) are a two-stage approach for estimating spatially varying density from distance sampling data. First, detection probabilities—perhaps depending on covariates—are estimated based on details of individual encounters; next, local densities are estimated using a GAM, by fitting local encounter rates to location and/or spatially varying covariates while allowing for the estimated detectabilities. One criticism of DSMs has been that uncertainty from the two stages is not usually propagated correctly into the final variance estimates. We show how to reformulate a DSM so that the uncertainty in detection probability from the distance sampling stage (regardless of its complexity) is captured as an extra random effect in the GAM stage. In effect, we refit an approximation to the detection function model at the same time as fitting the spatial model. This allows straightforward computation of the overall variance via exactly the same software already needed to fit the GAM. A further extension allows for spatial variation in group size, which can be an important covariate for detectability as well as directly affecting abundance. We illustrate these models using point transect survey data of Island Scrub-Jays on Santa Cruz Island, CA, and harbour porpoise from the SCANS-II line transect survey of European waters. Supplementary materials accompanying this paper appear on-line.

Evidence of long-term declines in Island Scrub-Jay vital rates

Mosher, B. A., P. F. Doherty, Jr., J. L. Atwood, K. A. Corey, and C. T. Collins. 2021. Evidence of long-term declines in Island Scrub-Jay vital rates. Avian Conservation and Ecology 16(2):21. https://doi.org/10.5751/ACE-01997-160221

Translocation with targeted vaccination is the most effective strategy to protect an island endemic bird threatened by West Nile virus

AbstractAimInvasive pathogens are a growing conservation challenge and often occur in tandem with rapid environmental transformation, such as climate change, drought and habitat loss. Climate change appears to have facilitated the spread of West Nile virus (WNV), a cause of widespread avian mortality. WNV is considered the primary threat to island scrub‐jays (Aphelocoma insularis), endemic to Santa Cruz Island, California. Two approaches have been proposed to safeguard island scrub‐jays: (a) vaccination and (b) conservation translocation to re‐establish a second population on neighbouring Santa Rosa Island, hypothesized to have a lower risk of WNV. These alternatives operate at regional scales but exemplify global concerns with strategic implications for conservation biogeography and climate adaptation.LocationCalifornia Channel Islands, USA.MethodsWe compared the efficacy of vaccination and translocation strategies at minimizing 25‐year quasi‐extinction risk for island scrub‐jays using a stochastic population model.ResultsUnder current WNV‐free conditions, the predicted quasi‐extinction risk for island scrub‐jays was low (~0%) but increased to ≥22% with simulated WNV outbreaks. Vaccinating ≥60 individuals reduced risk to <5%, but risk doubled if population size declined and further increased with more frequent droughts. Translocation performed best if Santa Rosa Island had a large starting population size and habitat extent, and, more importantly, a low risk of WNV establishment; if Santa Rosa Island was inhospitable to WNV, quasi‐extinction risk dropped to near zero.Main conclusionsTranslocation with targeted vaccination during high‐risk conditions was the most effective strategy to protect island scrub‐jays from West Nile virus. Although vaccination often outperformed translocation, only scenarios that included a Santa Rosa population and vaccinations achieved acceptably low species‐wide extinction risk across all potential future conditions. Our analysis informs strategies to improve the long‐term viability of the most range‐restricted bird species in the continental United States and provides a model for assessing conservation‐translocation proposals for other species and threats.

First Documented Observation of Island Scrub-Jay (Aphelocoma insularis) Precopulatory Display

We report what we believe is the first documented observation of Island Scrub-Jay (Aphelocoma insularis) copulation behavior. We compare our observations to the behaviors of other Aphelocoma jays with the aim of identifying potential species-specific elements. There are observable differences between the precopulatory display of the Island Scrub-Jay and that of more distantly related Aphelocoma jays. The display of Island Scrub-Jay and that of the closely related California Scrub-Jay (Aphelocoma californica) are very similar despite the species being isolated from each other for approximately 1 million years.

Terminal Pleistocene–Holocene Avifauna of San Miguel and Santa Rosa Islands: Identifications of Previously Unidentified Avian Remains Recovered from Fossil Sites and Prehistoric Cave Deposits

We provide an update to the fossil avifauna of San Miguel and Santa Rosa Islands based on the identification of 3509 small bird bones recovered from fossil sites on Santa Rosa and from 3 archaeological-paleontological cave deposits and 20 fossil sites on San Miguel. This work adds 64 species to the fossil avian community of these 2 islands, increasing the previous total from 83 to 147 species, making this the richest Late Pleistocene–Holocene coastal avifaunal assemblage recorded for California. Of the 64 newly identified species, 62 are new additions to the Channel Islands fossil avifauna and 40 are new to the fossil avifauna of California. Twenty-two species were confirmed to have nested on San Miguel Island during the Holocene based on the identification of juvenile/immature bones. At least 4 land birds (Burrowing Owl [Athene cunicularia], Island Scrub-Jay [Aphelocoma insularis], Bewick's Wren [Thryomanes bewickii], and Spotted Towhee [Pipilo maculatus]) that nested prehistorically on San Miguel have disappeared from the modern record there. Island Scrub-Jay bones were identified from 3 fossil sites on San Miguel and 1 site on Santa Rosa, providing the first confirmation that this species lived on both islands during the Pleistocene and Holocene. Island Scrub-Jays disappeared from San Miguel Island sometime in the Late Holocene, but a small population may have lingered on Santa Rosa Island into historic times before dying out. Destruction of island scrub and woodland habitats following the introduction of livestock in the mid-19th century probably caused the extirpation of resident breeding populations of Bewick's Wren and Spotted Towhee from San Miguel Island and Island Scrub-Jay from Santa Rosa Island.

Oak habitat recovery on California's largest islands: Scenarios for the role of corvid seed dispersal

Abstract Seed dispersal by birds is central to the passive restoration of many tree communities. Reintroduction of extinct seed dispersers can therefore restore degraded forests and woodlands. To test this, we constructed a spatially explicit simulation model, parameterized with field data, to consider the effect of different seed dispersal scenarios on the extent of oak populations. We applied the model to two islands in California's Channel Islands National Park (USA), one of which has lost a key seed disperser. We used an ensemble modelling approach to simulate island scrub oak (Quercus pacifica) demography. The model was developed and trained to recreate known population changes over a 20‐year period on 250‐km2 Santa Cruz Island, and incorporated acorn dispersal by island scrub‐jays (Aphelocoma insularis), deer mice (Peromyscus maniculatus) and gravity, as well as seed predation. We applied the trained model to 215‐km2 Santa Rosa Island to examine how reintroducing island scrub‐jays would affect the rate and pattern of oak population expansion. Oak habitat on Santa Rosa Island has been greatly reduced from its historical extent due to past grazing by introduced ungulates, the last of which were removed by 2011. Our simulation model predicts that a seed dispersal scenario including island scrub‐jays would increase the extent of the island scrub oak population on Santa Rosa Island by 281% over 100 years, and by 544% over 200 years. Scenarios without jays would result in little expansion. Simulated long‐distance seed dispersal by jays also facilitates establishment of discontinuous patches of oaks, and increases their elevational distribution. Synthesis and applications. Scenario planning provides powerful decision support for conservation managers. We used ensemble modelling of plant demographic and seed dispersal processes to investigate whether the reintroduction of seed dispersers could provide cost‐effective means of achieving broader ecosystem restoration goals on California's second‐largest island. The simulation model, extensively parameterized with field data, suggests that re‐establishing the mutualism with seed‐hoarding jays would accelerate the expansion of island scrub oak, which could benefit myriad species of conservation concern.

Bill morphology and neutral genetic structure both predict variation in acoustic signals within a bird population

Lay Summary Bills not only influence what birds eat – they can also influence what birds sound like. Island Scrub-Jays, which only occur on one island, have bills that are adapted to food in their local habitat. Here, we report that females with longer, shallower bills (characteristic of pine habitats) produce calls that differ from females with shorter, deeper bills (characteristic of oak habitats), an association that may have aided the evolution of different bills within the island.

Partial support for the central-marginal hypothesis within a population: reduced genetic diversity but not increased differentiation at the range edge of an island endemic bird.

Large-scale population comparisons have contributed to our understanding of the evolution of geographic range limits and species boundaries, as well as the conservation value of populations at range margins. The central-marginal hypothesis (CMH) predicts a decline in genetic diversity and an increase in genetic differentiation toward the periphery of species' ranges due to spatial variation in genetic drift and gene flow. Empirical studies on a diverse array of taxa have demonstrated support for the CMH. However, nearly all such studies come from widely distributed species, and have not considered if the same processes can be scaled down to single populations. Here, we test the CMH on a species composed of a single population: the Island Scrub-Jay (Aphelocoma insularis), endemic to a 250 km2 island. We examined microsatellite data from a quarter of the total population and found that homozygosity increased toward the island's periphery. However, peripheral portions of the island did not exhibit higher genetic differentiation. Simulations revealed that highly localized dispersal and small total population size, but not spatial variation in population density, were critical for generating fine-scale variation in homozygosity. Collectively, these results demonstrate that microevolutionary processes driving spatial variation in genetic diversity among populations can also be important for generating spatial variation in genetic diversity within populations.

Spatially biased dispersal of acorns by a scatter-hoarding corvid may accelerate passive restoration of oak habitat on California's largest island.

Scatter hoarding by corvids (crows, jays, magpies, and nutcrackers) provides seed dispersal for many large-seeded plants, including oaks and pines. When hoarding seeds, corvids often choose nonrandom locations throughout the landscape, resulting in differential survival of seeds. In the context of habitat restoration, such disproportional storing of seeds in areas suitable for germination and establishment can accelerate expansion and recovery of large-seeded tree populations and their associated ecosystems. Here, we investigate the spatial preferences of island scrub jays Aphelocoma insularis during scatter hoarding of acorns (Quercus spp.) on Santa Cruz Island. We use a large behavioral data set on the birds’ behavior in combination with seedling surveys and spatial analysis to determine whether 1) island scrub jays disproportionally cache seeds in specific habitat types, and 2) whether the preferred habitat type is suitable for oak regeneration. Our results show that the jays nonrandomly cache acorns across the landscape; they use chaparral and coastal sage scrub disproportionally while avoiding open and grassy areas. The areas used most often for caching were also the areas with the highest oak seedling densities. We discuss the potential role of these findings for the recovery of Santa Cruz Island’s oak habitat since the 1980s.

Explanatory unification and natural selection explanations

The debate between the dynamical and the statistical interpretations of natural selection is centred on the question of whether all explanations that employ the concepts of natural selection and drift are reducible to causal explanations. The proponents of the statistical interpretation answer negatively, but insist on the fact that selection/drift arguments are explanatory. However, they remain unclear on where the explanatory power comes from. The proponents of the dynamical interpretation answer positively and try to reduce selection/drift arguments to some of the most prominent accounts of causal explanation. In turn, they face the criticism raised by statisticalists that current accounts of causation have to be violated in some of their core conditions or otherwise used in a very loose manner in order to account for selection/drift explanations. We propose a reconciliation of both interpretations by conveying evolutionary explanations within the unificationist model of scientific explanation. Therefore, we argue that the explanatory power in natural selection arguments is a result of successful unification of individual- and population-level facts. A short case study based on research on sympatric speciation will be presented as an example of how population- and individual-level facts are unified to explain the morphological mosaic of bill shape in island scrub jays (Aphelocoma insularis).

Context-dependent seed dispersal by a scatter-hoarding corvid.

Corvids (crows, jays, magpies and nutcrackers) are important dispersers of large-seeded plants. Studies on captive or supplemented birds suggest that they flexibly adjust their scatter-hoarding behaviour to the context of social dynamics and relative seed availability. Because many corvid-dispersed trees show high annual variation in seed production, context-dependent foraging can have strong effects on natural corvid scatter-hoarding behaviour. We investigated how seed availability and social dynamics affected scatter-hoarding in the island scrub jays (Aphelocoma insularis). We quantified rates of scatter-hoarding behaviour and territorial defence of 26 colour-marked birds over a three-year period with variable acorn crops. We tested whether caching parameters were correlated with variation in annual seed production of oaks as predicted by the predator dispersal hypothesis, which states that caching rates and distances should vary with seed abundance in ways that benefit tree fitness. We also tested whether antagonistic interactions with conspecifics would affect scatter-hoarding adversely, as found in experimental studies. Caching behaviour varied with acorn availability. Caching distances correlated positively with annual acorn crop size, increasing by as much as 40% between years. Caching rates declined over time in years with small acorn crops, but increased when crops were large. Acorn foraging and caching rates were also negatively correlated with rates of territorial aggression. Overall foraging rates, however, were not associated with aggression, suggesting that reduced dispersal rates were not simply due to time constraints. Our field results support laboratory findings that caching rates and distances by scatter-hoarding corvids are context-dependent. Furthermore, our results are consistent with predictions of the predator dispersal hypothesis and suggest that large seed crops and social interactions among scatter-hoarders affect dispersal benefits for oaks and other masting tree species.

The Importance of Small Change

While conducting research on Santa Cruz Island off the coast of California, Langin found differences in the beaks of a single species of bird, the endemic island scrub-jay. Though they lived in neighboring habitats, the size and shape of the birds’ beaks change depending on the types of flora available to them. The discovery is one of several recent examples of microgeographic divergence that point to greater biodiversity than biologists had come to expect.

Islands within an island: repeated adaptive divergence in a single population.

Physical barriers to gene flow were once viewed as prerequisites for adaptive evolutionary divergence. However, a growing body of theoretical and empirical work suggests that divergence can proceed within a single population. Here we document genetic structure and spatially replicated patterns of phenotypic divergence within a bird species endemic to 250 km(2) Santa Cruz Island, California, USA. Island scrub-jays (Aphelocoma insularis) in three separate stands of pine habitat had longer, shallower bills than jays in oak habitat, a pattern that mirrors adaptive differences between allopatric populations of the species' mainland congener. Variation in both bill measurements was heritable, and island scrub-jays mated nonrandomly with respect to bill morphology. The population was not panmictic; instead, we found a continuous pattern of isolation by distance across the east-west axis of the island, as well as a subtle genetic discontinuity across the boundary between the largest pine stand and adjacent oak habitat. The ecological factors that appear to have facilitated adaptive differentiation at such a fine scale--environmental heterogeneity and localized dispersal--are ubiquitous in nature. These findings support recent arguments that microgeographic patterns of adaptive divergence may be more common than currently appreciated, even in mobile taxonomic groups like birds.

An open-population hierarchical distance sampling model.

Modeling population dynamics while accounting for imperfect detection is essential to monitoring programs. Distance sampling allows estimating population size while accounting for imperfect detection, but existing methods do not allow for estimation of demographic parameters. We develop a model that uses temporal correlation in abundance arising from underlying population dynamics to estimate demographic parameters from repeated distance sampling surveys. Using a simulation study motivated by designing a monitoring program for Island Scrub-Jays (Aphelocoma insularis), we investigated the power of this model to detect population trends. We generated temporally autocorrelated abundance and distance sampling data over six surveys, using population rates of change of 0.95 and 0.90. We fit the data generating Markovian model and a mis-specified model with a log-linear time effect on abundance, and derived post hoc trend estimates from a model estimating abundance for each survey separately. We performed these analyses for varying numbers of survey points. Power to detect population changes was consistently greater under the Markov model than under the alternatives, particularly for reduced numbers of survey points. The model can readily be extended to more complex demographic processes than considered in our simulations. This novel framework can be widely adopted for wildlife population monitoring.

Linking vegetation patterns to environmental gradients and human impacts in a mediterranean-type island ecosystem

Vegetation patterns at the landscape scale are shaped by myriad processes and historical events, and understanding the relative importance of these processes aids in predicting current and future plant distributions. To quantify the influence of different environmental and anthropogenic patterns on observed vegetation patterns, we used simulta- neous autoregressive modeling to analyze data col- lected by the Carnegie Airborne Observatory over Santa Cruz Island (SCI; California, USA). SCI is a large continental island, and its limited suite of species and well documented land use history allowed us to consider many potential determinants of vegetation patterns, such as topography, substrate, and historical grazing intensity. As a metric of vegetation heteroge- neity, we used the normalized difference vegetation index (NDVI) stratified into three vegetation height classes using LiDAR (short, medium, and tall). In the SAR models topography and substrate type were important controls, together explaining 8-15 % of the total variation in NDVI, but historical grazing and spatial autocorrelation were also key components of the models, together explaining 17-21 % of the variation in NDVI. Optimal spatial autocorrelation distances in the short and medium height vegetation models (600-700 m) were similar to the home range sizes of two crucial seed dispersers on the island- the island fox (Urocyon littoralis santacruzae) and the island scrub-jay (Aphelocoma insularis)—suggesting that these animals may be important drivers of the island's vegetation patterns. This study highlights the importance of dynamic processes like dispersal lim- itation and disturbance history in determining present- day vegetation patterns.

Reproductive Ecology of the Island Scrub-Jay

Abstract. We investigated the reproductive ecology of the Island Scrub-Jay (Aphelocoma insularis), which is endemic to Santa Cruz Island, California, and is a species of conservation concern. We documented daily nest survival, probability of renesting, annual fledging success, and breeding behavior, and determined how these factors varied in relation to the habitat characteristics of the jays' home ranges. Clutch sizes, feeding rates, and nest attendance were not correlated with chaparral cover or with home-range size. In contrast, nest predation, which accounted for 92% of documented nest failures, was less likely at nests that were more concealed, at nests of breeding pairs with home ranges that were smaller and had a greater proportion of taller vegetation, and at nests that were initiated earlier in the breeding season. Researchers' activity near nests increased the likelihood of predation, although the effect diminished with a nest's age. Despite relatively low nest survival, the high frequency of re...

Avian Response to Removal of Feral Sheep on Santa Cruz Island, California

Abstract Feral herbivores can affect birds on islands indirectly, by altering habitat structure via impacts on vegetation. I censused birds in an open shrub/grassland habitat on Santa Cruz Island, California, in 1980 and again in 2008, 24 years after feral sheep were removed, in order to assess avian responses to vegetation changes resulting from sheep removal. Vegetation change was characterized by an increased density of both overstory and low-growing shrubs, along with denser herbaceous vegetation, and these characteristics were associated with substantial changes in bird abundance. Species that prefer dense shrubs, such as Spotted Towhees (Pipilo maculatus) and Orange-crowned Warblers (Oreothlypis celata), increased in abundance, whereas species such as House Finches (Carpodacus mexicanus) and Mourning Doves (Zenaida macroura) that prefer open, disturbed areas decreased. Rufous-crowned Sparrows (Aimophila ruficeps) and Island Scrub-Jays (Aphelocoma insularis), both insular endemic taxa, showed a mixed...

Hierarchical distance‐sampling models to estimate population size and habitat‐specific abundance of an island endemic

Population size and habitat-specific abundance estimates are essential for conservation management. A major impediment to obtaining such estimates is that few statistical models are able to simultaneously account for both spatial variation in abundance and heterogeneity in detection probability, and still be amenable to large-scale applications. The hierarchical distance-sampling model of J. A. Royle, D. K. Dawson, and S. Bates provides a practical solution. Here, we extend this model to estimate habitat-specific abundance and rangewide population size of a bird species of management concern, the Island Scrub-Jay (Aphelocoma insularis), which occurs solely on Santa Cruz Island, California, USA. We surveyed 307 randomly selected, 300 m diameter, point locations throughout the 250-km2 island during October 2008 and April 2009. Population size was estimated to be 2267 (95% CI 1613-3007) and 1705 (1212-2369) during the fall and spring respectively, considerably lower than a previously published but statistically problematic estimate of 12 500. This large discrepancy emphasizes the importance of proper survey design and analysis for obtaining reliable information for management decisions. Jays were most abundant in low-elevation chaparral habitat; the detection function depended primarily on the percent cover of chaparral and forest within count circles. Vegetation change on the island has been dramatic in recent decades, due to release from herbivory following the eradication of feral sheep (Ovis aries) from the majority of the island in the mid-1980s. We applied best-fit fall and spring models of habitat-specific jay abundance to a vegetation map from 1985, and estimated the population size of A. insularis was 1400-1500 at that time. The 20-30% increase in the jay population suggests that the species has benefited from the recovery of native vegetation since sheep removal. Nevertheless, this jay's tiny range and small population size make it vulnerable to natural disasters and to habitat alteration related to climate change. Our results demonstrate that hierarchical distance-sampling models hold promise for estimating population size and spatial density variation at large scales. Our statistical methods have been incorporated into the R package unmarked to facilitate their use by animal ecologists, and we provide annotated code in the Supplement.