Intraspecific variation in environmental and geographic space use: insights from individual movement data

Species’ ranges arise from the interplay between environmental preferences, biotic and abiotic environmental conditions, and accessibility. Understanding of – and predictive models on – species distributions often build from the assumption that these factors apply homogenously within each species, but there is growing evidence for individual variation. Here, I use movement data to investigate individual-level decisions and compromises regarding the different costs and benefits influencing individuals’ geographic locations, and the species-level spatial patterns that emerge from these. I first developed a new method that uses tracking data to quantify individual specialisation in geographic space (site fidelity) or in environmental space (environmental specialisation). Applying it to two species of albatrosses, I found evidence of site fidelity but weak environmental specialisation. My results have implications for how limited research efforts are best-targeted: if animals are generalists, effort are best spent by understanding in depth individual patterns, i.e., better to track fewer individuals for long periods of time; whereas if animals tend to be specialists, efforts should be dedicated to tracking as many individuals as possible, even if for shorter periods. I then investigated individual migratory strategies and their drivers in nine North American bird species, using ringing/recovery data. I found latitudinal redistribution of individuals within the breeding and non-breeding ranges that generally did not follow textbook patterns (‘chain migration’ or ‘leapfrog migration’). Migratory individuals tend to trade off the benefits of migration (better tracking of climatic niche; better access to resources) and its costs (increasing with migratory distance). I found that birds are more likely to remain as residents in areas with warmer winter temperatures, higher summer resource surpluses and higher human population densities (presumably because of a buffering effect of urban areas). Overall, my results highlight the importance of considering individual variation to understanding the ecological processes underpinning species’ spatial patterns.St John's College Benefactors' Scholarship Cambridge Philosophical Societ

The depth of Sooty Shearwater Ardenna grisea burrows varies with habitat and increases with competition for space

The Sooty Shearwater Ardenna grisea, an abundant but declining petrel, is one of many seabird species that construct breeding burrows, presumably because these confer protection from predators and the elements. Little is known about the causes of variation in Sooty Shearwater burrow architecture, which can differ markedly both within and between breeding sites. We hypothesize that burrow architecture varies in response to habitat type and competition for space. To address these hypotheses, we recorded Sooty Shearwater burrow dimensions on Kidney Island, the largest Sooty Shearwater colony in the Falkland Islands, South Atlantic, and modelled these as functions of burrow density (a proxy for competition) and habitat indices. Our models suggest that Sooty Shearwaters burrow further underground in response to competition for breeding space, and that soil underlying dense tussac grass Poa flabellata is more easily excavated than other substrates, indicating how vegetation restoration could aid the conservation of this species

Integrating habitat and partial survey data to estimate the regional population of a globally declining seabird species, the sooty shearwater

Many animal populations are thought to be in flux due to anthropogenic impacts. However, censusing organisms to understand such changes is often impractical. For example, while it is thought that over half of pelagic seabird populations are declining, most breed in burrows or on cliffs, in large, remote colonies, making them difficult to count. Burrow-nesting sooty shearwaters (Ardenna grisea) are abundant but declining in their core (South Pacific) breeding range, potentially due to introduced rodents and habitat loss. In contrast, Kidney Island, their largest colony in the Falkland Islands (Southwest Atlantic), purportedly grew by several orders of magnitude since the mid-1900s. This island is rodent-free, and native tussac grass (Poa flabellata) has increased following cessation of historical exploitation. To estimate the sooty shearwater population in the Falkland Islands, and its relationship with breeding habitat availability, we sampled burrow density and occupancy on Kidney Island and modeled these as functions of habitat. Both indices responded positively to a proxy for historical increases in tussac cover. We estimate that breeding sooty shearwaters occupy ∼140,000 (95% CI: 90,000–210,000) burrows on Kidney Island. Moreover, using additional survey data and Generalized Functional Response models to account for intra-island variation in habitat availability, we estimate that 25,000 (95% CI: 20,100 - 30,500) burrows could be occupied on nearby islands from which non-native rodents have been recently eradicated. Our study shows that habitat selection functions, generalized where necessary, not only improve population estimates but provide biological insights needed to reverse declines in seabirds and other species

Habitat preferences, foraging behaviour and bycatch risk among breeding sooty shearwaters Ardenna grisea in the Southwest Atlantic.

Pelagic seabirds are important components of many marine ecosystems. The most abundant species are medium/small sized petrels (<1100 g), yet the sub-mesoscale (<10 km) distribution, habitat use and foraging behaviour of this group are not well understood. Sooty shearwaters Ardenna grisea are among the world’s most numerous pelagic seabirds. The majority inhabit the Pacific, where they have declined, partly due to bycatch and other anthropogenic impacts, but they are increasing in the Atlantic. To evaluate the sub-mesoscale habitat preferences (i.e. the disproportionality between habitat use and availability), diving behaviour and bycatch risk of Atlantic breeders, we tracked sooty shearwaters from the Falkland Islands during late incubation and early chick-rearing with GPS loggers (n = 20), geolocators (n = 10) and time-depth recorders (n = 10). These birds foraged exclusively in neritic and shelf-break waters, principally over the Burdwood Bank, ~350 km from their colony. Like New Zealand breeders, they dived mostly during daylight, especially at dawn and dusk, consistent with the exploitation of vertically migrating prey. However, Falkland birds made shorter foraging trips, shallower dives, and did not forage in oceanic waters. Their overlap with fisheries was low, and they foraged at shallower depths than those targeted by trawlers, the most frequent fishing vessels encountered, indicating that bycatch risk was low during late incubation/early chick-rearing. Although our results should be treated with caution, they indicate that Atlantic and Pacific sooty shearwaters may experience markedly differing pressures at sea. Comparative study between these populations, e.g. combining biologging and demography, is therefore warranted

Individual consistency in migration strategies of a tropical seabird, the Round Island petrel

Background: In migratory species, the extent of within- and between-individual variation in migratory strategies can influence potential rates and directions of responses to environmental changes. Quantifying this variation requires tracking of many individuals on repeated migratory journeys. At temperate and higher latitudes, low levels of within-individual variation in migratory behaviours are common and may reflect repeated use of predictable resources in these seasonally-structured environments. However, variation in migratory behaviours in the tropics, where seasonal predictability of food resources can be weaker, remains largely unknown. Methods: Round Island petrels (Pterodroma sp.) are tropical, pelagic seabirds that breed all year round and perform long-distance migrations. Using multi-year geolocator tracking data from 62 individuals between 2009 and 2018, we quantify levels of within- and between-individual variation in non-breeding distributions and timings. Results: We found striking levels of between-individual variation in at-sea movements and timings, with non-breeding migrations to different areas occurring across much of the Indian Ocean and throughout the whole year. Despite this, repeat-tracking of individual petrels revealed remarkably high levels of spatial and temporal consistency in within-individual migratory behaviour, particularly for petrels that departed at similar times in different years and for those departing in the austral summer. However, while the same areas were used by individuals in different years, they were not necessarily used at the same times during the non-breeding period. Conclusions: Even in tropical systems with huge ranges of migratory routes and timings, our results suggest benefits of consistency in individual migratory behaviours. Identifying the factors that drive and maintain between-individual variation in migratory behaviour, and the consequences for breeding success and survival, will be key to understanding the consequences of environmental change across migratory ranges

Opportunities and limitations of large open biodiversity occurrence databases in the context of a Marine Ecosystem Assessment of the Southern Ocean

The Southern Ocean is a productive and biodiverse region, but it is also threatened by anthropogenic pressures. Protecting the Southern Ocean should start with well-informed Marine Ecosystem Assessments of the Southern Ocean (MEASO) being performed, a process that will require biodiversity data. In this context, open geospatial biodiversity databases such as OBIS and GBIF provide good avenues, through aggregated geo-referenced taxon locations. However, like most aggregated databases, these might suffer from sampling biases, which may hinder their usability for a MEASO. Here, we assess the quality and distribution of OBIS and GBIF data in the context of a MEASO. We found strong spatial, temporal and taxonomic biases in these data, with several biases likely emerging from the remoteness and inaccessibility of the Southern Ocean (e.g., lack of data in the dark and ice-covered winter, most data describing charismatic or well-known taxa, and most data along ship routes between research stations and neighboring continents). Our identification of sampling biases helps us provide practical recommendations for future data collection, mobilization, and analyses

Effects of competitive pressure and habitat heterogeneity on niche partitioning between Arctic and boreal congeners

The rapidly changing climate in the Arctic is expected to have a major impact on the foraging ecology of seabirds, owing to changes in the distribution and abundance of their prey but also that of competitors (e.g. southerly species expanding their range into the Arctic). Species can respond to interspecific competition by segregating along different niche axes. Here, we studied spatial, temporal and habitat segregation between two closely related seabird species: common guillemot Uria aalge (a temperate species) and Brünnich’s guillemot Uria lomvia (a true Arctic species), at two sympatric sites in Iceland that differ in their total population sizes and the availability of marine habitats. We deployed GPS and temperature-depth recorders to describe foraging locations and behaviour of incubating and chick-rearing adults. We found similar evidence of spatial segregation at the two sites (i.e. independent of population sizes), although segregation in environmental space was only evident at the site with a strong habitat gradient. Unexpectedly, temporal (and, to a limited extent, vertical) segregation appeared only at the least populated site. Overall, our results show complex relationships between the levels of inferred competition and that of segregation

Individual consistency in migration strategies of a tropical seabird, the Round Island petrel

Background: In migratory species, the extent of within- and between-individual variation in migratory strategies can influence potential rates and directions of responses to environmental changes. Quantifying this variation requires tracking of many individuals on repeated migratory journeys. At temperate and higher latitudes, low levels of within-individual variation in migratory behaviours are common and may reflect repeated use of predictable resources in these seasonally-structured environments. However, variation in migratory behaviours in the tropics, where seasonal predictability of food resources can be weaker, remains largely unknown. Methods: Round Island petrels (Pterodroma sp.) are tropical, pelagic seabirds that breed all year round and perform long-distance migrations. Using multi-year geolocator tracking data from 62 individuals between 2009 and 2018, we quantify levels of within- and between-individual variation in non-breeding distributions and timings. Results: We found striking levels of between-individual variation in at-sea movements and timings, with non-breeding migrations to different areas occurring across much of the Indian Ocean and throughout the whole year. Despite this, repeat-tracking of individual petrels revealed remarkably high levels of spatial and temporal consistency in within-individual migratory behaviour, particularly for petrels that departed at similar times in different years and for those departing in the austral summer. However, while the same areas were used by individuals in different years, they were not necessarily used at the same times during the non-breeding period. Conclusions: Even in tropical systems with huge ranges of migratory routes and timings, our results suggest benefits of consistency in individual migratory behaviours. Identifying the factors that drive and maintain between-individual variation in migratory behaviour, and the consequences for breeding success and survival, will be key to understanding the consequences of environmental change across migratory ranges

Cold comfort: Arctic seabirds find refugia from climate change and potential competition in marginal ice zones and fjords

Climate change alters species distributions by shifting their fundamental niche in space through time. Such effects may be exacerbated by increased inter-specific competition if climate alters species dominance where competitor ranges overlap. This study used census data, telemetry and stable isotopes to examine the population and foraging ecology of a pair of Arctic and temperate congeners across an extensive zone of sympatry in Iceland, where sea temperatures varied substantially. The abundance of Arctic Brünnich’s guillemot Uria lomvia declined with sea temperature. Accessibility of refugia in cold water currents or fjords helped support higher numbers and reduce rates of population decline. Competition with temperate Common guillemots Uria aalge did not affect abundance, but similarities in foraging ecology were sufficient to cause competition when resources are limiting. Continued warming is likely to lead to further declines of Brünnich’s guillemot, with implications for conservation status and ecosystem services