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A conceptual model for migratory tundra caribou to explain and predict why shifts in spatial fidelity of breeding cows to their calving grounds are infrequent.
Abstract and Figures
ABSTRACT: Calving grounds of migratory tundra caribou (Rangifer tarandus) have two prominent characteristics. Firstly, the cows are gregarious, and secondly, the annual calving grounds spatially overlap in consecutive years (spatial fidelity). The location of consecutive annual calving grounds can gradually shift (either rotationally or un-directional) or more rarely, abruptly (non-overlapping). We propose a mechanism to interpret and predict changes in spatial fidelity. We propose that fidelity is linked to gregariousness with its advantages for individual fitness (positive density-dependence). Our argument is based on a curvilinear relationship between the density of cows on the calving ground (which we use to index gregari-ousness) and spatial fidelity. Extremely high or low densities are two different mechanisms which can lead to reduced spatial fidelity to annual calving grounds and reflect the caribou's adaptive use of its calving ranges.
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... Second, the effects of caribou on ecosystem processes and nutrient cycling via their role as grazers has been studied in depth (Stark and Grellmann 2002, Lindwall et al. 2013 and their creation of local nutrient hotspots from feces (van der Wal et al. 2004, Barthelemy et al. 2015, urine (Barthelemy et al. 2018) and carcasses (Swanson 1979) has begun to be studied, but the implication of these cumulative effects on landscape patterning has not. Caribou herds also have discrete population events in which they aggregate, such as calving events (Gunn et al. 2012, Campeau et al. 2019. Finally, many populations of caribou are in rapid decline globally (Vors and Boyce 2009); hence, it is important to understand the potential role they play in sustaining tundra and boreal ecosystems via spatial nutrient redistribution. ...
Animals can be important vectors of nutrient transfer within and across landscapes, with important implications for ecosystem productivity and composition. While it is presumed large ungulates are agents of nutrient dispersal via movement and activity, research analyzing their net effects on landscapes remains scarce. We present an individual-based model that investigates how caribou affect the distribution of nutrients on a landscape through consumption only, as well through the cumulative effects of consumption and nutrient deposition (i.e. fecal waste and carcass deposition). We explored these dynamics in simulations that altered the context of environments, either initially containing heterogeneous or homogeneous nutrient distributions, animal densities and sociality of caribou behavior. In the consumption-only simulations, caribou density and sociality created different patterns of heterogeneity at both the landscape and local scale depending on the initial landscape conditions. In these simulations, caribou populations crashed at high densities because the lack of animal deposition resulted in low resources across the landscape. This was not the case when considering the cumulative effects of consumption and deposition, indicating the return of nutrients from animals may be important for population stability. Additionally, in simulations considering the cumulative effects of caribou, increasing caribou density increased landscape heterogeneity irrespective of the initial condition (i.e. heterogeneous and homogeneous landscapes), and maintained or increased local heterogeneity in heterogeneous and homogenous landscape, respectively. Importantly, in all simulations the net impact of caribou at the individual patch level was extremely variable, suggesting that animal inputs are highly varied throughout the landscape. Our results indicate the movement of large ungulates such as caribou can increase the heterogeneity of available nutrients within a landscape and provide an important feedback for population stability. Thus, the loss of large ungulates from natural ecosystems via anthropogenic activity is likely to result in less heterogeneous natural landscapes.
... We defined fidelity as the tendency of an animal to return to its previously used location in consecutive years [59][60][61]. To analyze the annual fidelity of individual caribou to wintering areas, we calculated transition probabilities using the R package TraMineR [62]. ...
Many animals migrate to take advantage of temporal and spatial variability in resources. These benefits are offset with costs like increased energetic expenditure and travel through unfamiliar areas. Differences in the cost-benefit ratio for individuals may lead to partial migration with one portion of a population migrating while another does not. We investigated migration dynamics and winter site fidelity for a long-distance partial migrant, barren ground caribou ( Rangifer tarandus granti ) of the Teshekpuk Caribou Herd in northern Alaska. We used GPS telemetry for 76 female caribou over 164 annual movement trajectories to identify timing and location of migration and winter use, proportion of migrants, and fidelity to different herd wintering areas. We found within-individual variation in movement behavior and wintering area use by the Teshekpuk Caribou Herd, adding caribou to the growing list of ungulates that can exhibit migratory plasticity. Using a first passage time–net squared displacement approach, we classified 78.7% of annual movement paths as migration, 11.6% as residency, and 9.8% as another strategy. Timing and distance of migration varied by season and wintering area. Duration of migration was longer for fall migration than for spring, which may relate to the latter featuring more directed movement. Caribou utilized four wintering areas, with multiple areas used each year. This variation occurred not just among different individuals, but state sequence analyses indicated low fidelity of individuals to wintering areas among years. Variability in movement behavior can have fitness consequences. As caribou face the pressures of a rapidly warming Arctic and ongoing human development and activities, further research is needed to investigate what factors influence this diversity of behaviors in Alaska and across the circumpolar Arctic.
... Chapter 3 TERRESTRIAL AND FRESHWATER SYSTEMS frequently, but changes are also becoming evident in what were considered to be very constant calving grounds. During 42 years of monitoring the Bathurst herd, the average annual overlap in calving grounds was 43%, forming two geograph- ically consistent clusters (1966-1984 and 1996-2011), broken by a brief period at peak caribou densities when the calving ground shifted ( Gunn et al. 2012). In early springs plants tend to grow and flower earlier, and calving grounds tend to have high concentrations of plants that provide a burst of nutrient-rich early growth that is crucial to caribou milk production ( Griffith et al. 2002). ...
... Based on Bergerud's (1996Bergerud's ( , 1988 observations and definitions of ecotypes, migratory caribou move north of treeline at calving to areas of lower wolf density. In Ontario, however, the migratory southern Hudson Bay caribou population, constrained by the Hudson Bay coastline, has only 5% of its annual range north of treeline (Gunn et al., 2012;Newton et al., 2015). The Hudson Bay Lowland, in contrast to the boreal forest to the south, has lower wolf densities (Berglund et al., 2014;Patterson, 2009;Poley et al., 2014). ...
... The herd was estimated at 32,000 animals (±5300 SE) in 2009 representing a 70 % decline over 3 years (Adamczewski et al. 2009). The distribution of barren-ground caribou is density dependent such that range expansion and contraction are a function of the size of the herd (Simmons et al. 1979;Bergerud et al. 1984;Heard and Calef 1986;Messier et al. 1988;Couturier et al. 1990;Gunn et al. 2012). Gunn et al. (2013) reported that the winter range of the Bathurst herd has contracted since 2002, corresponding to the decline in the herd. ...
Wolves (Canis lupus) that den on the tundra of the central Arctic prey primarily on migratory barren-ground caribou (Rangifer tarandus groenlandicus). Prey densities in the vicinity of den sites are low, however, for a period each summer when caribou migrate to their calving and post-calving ranges. Eskers provide substrate where wolves can excavate den sites, but these landforms make up only a small proportion of the tundra landscape. We investigated the factors that influenced den site selection for wolves on the summer range of the Bathurst caribou herd, Northwest Territories, Canada. We used a long-term data set (1996–2012) of wolf den locations to develop a series of resource selection function (RSF) models representative of broad land-cover types, esker density, and annual variation in seasonal prey availability. We compared a temporal sequence of RSF models to investigate whether wolves altered selection patterns in response to a 90 % decline in caribou abundance (1996–2012). Eskers were selected denning habitat; the distribution of eskers may be limiting when wolf density is high. Covariates representing the seasonal distribution of caribou from early (5–18 July) and late (19 July–22 August) summer were the best predictors of den occurrence; these areas represented reliable availability of caribou over the greatest portion of the denning period. As the caribou herd declined, the seasonal summer ranges contracted northward towards the calving ground. Wolves did not exhibit a similar response. As such, the period of spatial separation between breeding wolves at den sites and the main distribution of caribou increased when herd abundance was low. The lack of a behavioural response is consistent with wolf–prey dynamics observed in other studies that suggest wolves strive to maintain consistent territories even following large decreases in resource availability. Such behaviours reduce fitness and have implications for pup survival and population growth.
... The Buchans herd appeared to be the most effective at avoiding predation, which is interesting given that it is the herd that undergoes the longest annual migration to calving grounds (Mahoney and Schaefer 2002a). This suggests that migratory caribou may face a trade-off between migration distance and its expected benefit in terms of reduced predation risk and increased foraging opportunities (Gunn et al. 2012); such a trade-off is likely to exist in terrestrial species given the high costs associated with migratory behaviour (Alerstam et al. 2003). Considering the observed variability in Newfoundland caribou migratory movements (Rayl et al. 2014), it appears that this trade-off may lead to variable migratory behaviour across herds. ...
Habitat selection is a multi-level, hierarchical process that should be a key component in the balance between food acquisition and predation risk avoidance (food – predation trade-off ). However, to date, studies have not fully elucidated how fine- and broad-scale habitat decisions by individual prey can help balance food versus risk. We studied broad-scale habitat selection by Newfoundland caribou Rangifer tarandus , focusing on trade-off s between predation risk versus access to forage during the calving and post-calving period. We improved traditional measures of habitat availability by incorporating fine-scale movement patterns of caribou into the availability kernel, thus enabling separation of broad and fine scales of selection. Remote sensing and fi eld surveys served to create a spatio-temporal model of forage availability, whereas GPS telemetry locations from 66 black bears Ursus americanus and 59 coyotes Canis latrans provided models of predation risk. We then used GPS telemetry locations from 114 female caribou to assess food – predation trade-offs through the prism of our refi ned model of caribou habitat availability. We noted that migratory movements of caribou were oriented mainly towards habitats with abundant forage and lower risk of bear and (to a lesser extent) coyote encounter. These findings were generally consistent across caribou herds and would not have been evident had we used traditional methods instead of our refined model when estimating habitat availability. We interpret these fi ndings in the context of stereotypical migratory behaviour observed in Newfoundland caribou, which occurs despite the extirpation of wolves Canis lupus nearly a century ago. We submit that caribou are able to balance food acquisition against predation risk using a complex set of factors involving both finer and broader scale selection. Accordingly, our study provides a strong argument for using refined habitat availability estimates when assessing food – predation trade-offs.
... Notwithstanding this, we contend that our approach offers transparent and conservative results regarding selection because the analyses summarized individual selection and then pooled the individual responses into herds rather than a more uniform (and less appropriate) multiherd pooling.The Buchans herd appeared to be the most effective at avoiding predation, which is interesting given that it is the herd that undergoes the longest annual migration to calving grounds(Mahoney & Schaefer 2002b). This suggests that migratory caribou may face a trade-off between migration distance and its expected benefit in terms of reduced predation risk and increased foraging opportunities(Gunn, Poole & Nishi 2012); such a trade-off is likely to exist in terrestrial species given the high costs ...
The objective of this thesis is to better understand the demography and habitat
selection of Newfoundland caribou. Chapter 1 provides a general introduction of
elements of population ecology and behavioural ecology discussed in the thesis. In
Chapter 2, I examine the causes of long-term fluctuations among caribou herds. My
findings indicate that winter severity and density-dependent degradation of summer
range quality offer partial explanations for the observed patterns of population change.
In Chapter 3, I investigate the influence of climate, predation and density-dependence
on cause-specific neonate survival. I found that when caribou populations are in a
period of increase, predation from coyotes and bears is most strongly influenced by the
abiotic conditions that precede calving. However, when populations begin to decline,
weather conditions during calving also influenced survival. I build on this analysis in
Chapter 4 by determining the influence of climate change on the interplay between
predation risk and neonate survival. I found that the relative equilibrium between bears
and coyotes may not persist in the future as risk from coyotes could increase due to
climate change. In Chapter 5, I investigate the relationships in niche overlap between
caribou and their predators and how this may influence differential predation risk by
affecting encounter rates. For coyotes, seasonal changes in niche overlap mirrored iii
variation in caribou calf risk, but had less association with the rate of encounter with
calves. In contrast, changes in niche overlap during the calving season for black bears
had little association with these parameters. In Chapter 6, I examine broad-level habitat
selection of caribou to study trade-offs between predator avoidance and foraging
during the calving season. The results suggest that caribou movements are oriented
towards increased access to foraging and the reduction of encounter risk with bears,
and to a lesser extent, coyotes. Finally, I synthesize the major findings from this thesis
and their relevance to caribou conservation in Chapter 7, to infer that Newfoundland
caribou decline is ultimately driven by extrinsic and intrinsic elements related to
density-dependence. Reduction in neonate survival emerged from nutritionally-stressed
caribou females producing calves with lower survival.
... This fundamental relationship is exemplified by species like caribou (Rangifer tarandus (L., 1758)) where changes in population size and space use often vary in concert. Numbers of migratory caribou may rise or fall by 100-fold in a few decades (Bergerud 1996; Vors and Boyce 2009; Couturier et al. 2010; Mahoney et al. 2011), swings in demography that may move in tandem with size of the home range (Schaefer and Wilson 2002; Couturier et al. 2010), fidelity to calving grounds (Gunn et al. 2012), and timing of migration (Mahoney and Schaefer 2002). Population structure, too, may be altered (Hinkes et al. 2005). ...
Understanding the relationship between abundance and distribution is central to ecology but may require broad-scale observations, especially for long-lived, mobile animals like caribou (Rangifer tarandus (L., 1758)). We tested the link between demography and spatial ecology of Newfoundland caribou, coincident with their numerical growth (1980s, 1990s) and decline (2000s). We analysed site fidelity, rate of movements, timing of migration, and population organization from telemetry observations of more than 600 adult females. Site fidelity was consistent across herds, intensifying near the onset of calving and peaking in late summer, a cycle that may reflect selection for postpartum security. Late-summer fidelity was stronger in the 1980s than 1990s, a trend that reversed itself during the 2000s. Weekly movements were lower in the 2000s, but with no clear differences between the 1980s and 1990s. Timing of migration changed: in the 2000s, spring migration of the Buchans herd occurred 3 weeks earlier and autumn migration 3 weeks later compared with the previous decade, the reversal of a 40-year trend. Herd affinities, revealed by fuzzy membership coefficients, diminished by the 2000s. These changes are consistent with the hypothesis of limitation by summer forage competition. Space use represents a useful gauge of numerical changes in caribou.
Barren ground caribou (Rangifer tarandus granti) are distributed in herds that seasonally use specific geographic regions within an annual range, with varying levels of fidelity during different periods (e.g., calving, insect relief, wintering). As a result, caribou management is generally tailored to individual herds that often range across administrative boundaries. Herd ranges can shift over time, seasonal ranges of adjacent herds often overlap, herds merge, and there is often little genetic differentiation among adjacent herds. If substantial herd interchange occurs, it would have important management implications by influencing estimates of herd size, herd composition, and harvest rates. We compiled satellite telemetry data from 2003–2015 for 4 large arctic caribou herds to quantify herd interchange rates. We calculated a metric of herd interchange based on the relationship of caribou locations to typical weekly herd ranges (all yrs combined) and the distance to other radio‐collared caribou from each of the 4 herds (yr specific). Although herd membership cannot always be clearly defined based on location, this metric provides an objective measure of the strength of evidence of herd membership that can be used to make comparisons among herds and time periods. We also calculated herd overlap and quantified how it varied throughout the year. Herd interchange was rare in the 2 larger herds, generally occurring when caribou overwintered with an adjacent herd, whereas herd interchange from the 2 smaller herds was more frequent and could last longer than a year. Although sample sizes were limited, there were no clear patterns in herd interchange with year or annual herd size. The 2 smaller herds had large seasonal overlap with adjacent herds, suggesting that herd interchange may be related to spatiotemporal herd overlap and relative herd size. Our results can help managers understand herd interchange and overlap to make management decisions, interpret research results, and develop more accurate population models. © 2020 The Authors. The Journal of Wildlife Management published by Wiley Periodicals LLC on behalf of The Wildlife Society. The amount of herd overlap and interchange varied among 4 arctic caribou herds in northern Alaska, USA, and Yukon Territories, Canada. Caribou from the 2 larger herds rarely switched herds and herd interchange occurred more frequently for caribou of the 2 smaller herds.
The severity of recent declines of barren-ground caribou (Rangifer tarandus groenlandicus) across the central Canadian Arctic has led to harvest restrictions and concerns about population recovery. Wolves (Canis lupus) are the main predator of barren-ground caribou; however, the extent that wolves influence the decline and recovery of caribou herds is unknown. Such uncertainty confounds management responses (e.g., reducing harvest, predator control). We investigated wolf–caribou dynamics on the summer range of barren-ground caribou in the Northwest Territories and Nunavut, Canada (i.e., Bathurst caribou herd). Our primary objective was to test for a numerical response of wolves to changes in the abundance and spatial distribution of caribou. Caribou experienced a >90% decline in abundance over the study period (1996–2014). Using long-term data sets (1996–2012), we developed regression models to investigate relationships between abundance indices of wolves and range-use patterns of caribou. We monitored the movements of adult female wolves fitted with global positioning system (GPS) collars representing individual packs throughout the 2013 and 2014 denning periods. We also investigated pup recruitment, an index of population decline, at a time of low caribou abundance. Finally, we developed a series of stochastic population models to understand how pup recruitment influenced wolf densities on the Bathurst range over the period of caribou decline. As caribou abundance decreased, the late-summer distribution of the Bathurst herd contracted toward the calving ground. These movements correlated with low rates of wolf pup recruitment and high den abandonment, suggesting a regulatory mechanism whereby wolf reproductive success was limited by the low availability of caribou within the denning areas. Furthermore, these data suggested a numerical response, where wolf densities decreased as caribou numbers declined. In 2014, wolf density was estimated at <4 wolves/1,000 km2. Our results suggest that these wolves exhibited a relatively strong numerical response to a single, declining prey base. Given the continued decline in the Bathurst caribou herd, the tundra wolf population on the Bathurst range has likely declined below our 2014 estimate.
Intensities of warble fly larvae, Hypoderma tarandi (L.), were examined in slaughtered reindeer (Rangifer tarandus tarandus L.) from different summer grazing areas of Finnmark County, northern Norway. To test the hypothesis-that larval abundance decreases with increase in post-calving migration distance (i.e., distance from calving grounds), herds with differing migration distances were sampled. The prevalence of infection in the total sample of 1168 animals was 99.9%. The study revealed significant differences in larval abundance among herds from different summer grazing areas. Herds with post-calving migrations have significantly lower larval abundances than herds remaining on or near the calving grounds for the whole summer. Between-herds variation in abundance of H. tarandi larvae is assumed to reflect differing densities of the infective stage (adult flies) on the herds' summer ranges. Larval abundance in a herd is in turn negatively correlated with the distance between the main larval shedding areas (i.e., calving grounds) and the areas of greatest transmission (i.e., summer pastures). These results are discussed in relation to transmission of other parasites common to Rangifer and suggest that this host's post-calving migration may be a behavioural adaptation that reduces levels of parasitic infections.
The five naturally occurring and one transplanted caribou (Rangifer tarandus granti) herd in southwestern Alaska composed about 20% of Alaska's caribou population in 2001. All five of the naturally occurring herds fluctuated considerably in size between the late 1800s and 2001 and for some herds the data provide an indication of long-term periodic (40-50 year) fluctuations. At the present time, the Unimak (UCH) and Southern Alaska Peninsula (SAP) are recovering from population declines, the Northern Alaska Peninsula Herd (NAP) appears to be nearing the end of a protracted decline, and the Mulchatna Herd (MCH) appears to now be declining after 20 years of rapid growth. The remaining naturally occurring herd (Kilbuck) has virtually disappeared. Nutrition had a significant effect on the size of 4-month-old and 10-month-old calves in the NAP and the Nushagak Peninsula Herd (NPCH) and probably also on population growth in at least 4 (SAP, NAP, NPCH, and MCH) of the six caribou herds in southwestern Alaska. Predation does not appear to be sufficient to keep caribou herds in southwestern Alaska from expanding, probably because rabies is endemic in red foxes (Vulpes vulpes) and is periodically transferred to wolves (Canis lupus) and other canids. However, we found evidence that pneumonia and hoof rot may result in significant mortality of caribou in southwestern Alaska, whereas there is no evidence that disease is important in the dynamics of Interior herds. Cooperative conservation programs, such as the Kilbuck Caribou Management Plan, can be successful in restraining traditional harvest and promoting growth in caribou herds. In southwestern Alaska we also found evidence that small caribou herds can be swamped and assimilated by large herds, and fidelity to traditional calving areas can be lost.
Parturient female caribou from the Teshekpuk caribou (Rangifer tarandus granti) herd (TCH) have been observed across the western North Slope, but most cows that were seen with calves during the calving period were in the area surrounding Teshekpuk Lake. During surveys conducted between 1994 and 2003, 155 (91%) of 171 collared cows seen with calves were within an area given protected status in the 1998 Bureau of Land Management Final Integrated Activity Plan/Environmental Impact Statement (IAP/EIS). The percentage of adult collared cows seen with a calf between 1994 and 2003 has ranged from 44% to 86%, with a mean of 66%. The years with the lowest percentage of collared cows seen with calves were 1997 (50%) and 2001 (44%). In 1997 most of the herd migrated much farther south than usual, and in 2001 unusually deep, persistent snow restricted spring migration, resulting in fewer cows returning to the traditional calving area during the calving period. When snowmelt dates were earlier, calving locations were farther north. Average standardized travel rates for parturient cows were significantly greater before they had calves (7.25 km/day) than after 3.89 (km/day). Geographically, protections granted by the 1998 BLM IAP/EIS appear to adequately cover the concentrated calving grounds, allowing for variance in the annual distribution of calving cows.
We synthesize findings from cooperative research on effects of petroleum development on caribou (Rangifer tarandus granti) of the Central Arctic Herd (CAH). The CAH increased from about 6000 animals in 1978 to 23 000 in 1992, declined to 18 000 by 1995, and again increased to 27 000 by 2000. Net calf production was consistent with changes in herd size. In the Kuparuk Development Area (KDA), west of Prudhoe Bay, abundance of calving caribou was less than expected within 4 km of roads and declined exponentially with road density. With increasing infrastructure, high-density calving shifted from the KDA to inland areas with lower forage biomass. During July and early August, caribou were relatively unsuccessful in crossing road/pipeline corridors in the KDA, particularly when in large, insect-harassed aggregations; and both abundance and movements of females were lower in the oil field complex at Prudhoe Bay than in other areas along the Arctic coast. Female caribou exposed to petroleum development west of the Sagavanirktok River may have consumed less forage during the calving period and experienced lower energy balance during the midsummer insect season than those under disturbance-free conditions east of the river. The probable consequences were poorer body condition at breeding and lower parturition rates for western females than for eastern females (e.g., 1988 - 94: 64% vs. 83% parturient, respectively; p = 0.003), which depressed the productivity of the herd. Assessments of cumulative effects of petroleum development on caribou must incorporate the complex interactions with a variable natural environment.
Predator swamping was an adequate explanation for the occurrence of within-season reproductive synchrony when the predator exhibits a Holling type-II functional response (specialist predator), but, in the case of a generalist predator switching from alternative prey (Holling type-III functional response), which is expected to be a common functional-response type when particular prey are unavailable at certain times of the year, highly asynchronous reproduction may be the best reproductive strategy. In particular, when prey switching occurs at high offspring densities and/or the satiation density of the predator is high relative to the total reproduction of the prey population, the peak predation rate is expected to occur when reproduction is completely synchronous. Spatially clumped prey populations are expected to experience a dramatically increased predation rate if reproduction is synchronized within clumps but not between them. Hence, it is predicted that mechanisms promoting within-group reproductive synchrony are even less likely to have evolved as a predator-swamping strategy in patchily distributed populations, in which reproduction cannot be synchronized between groups. -from Author
Habitat selection is a crucial process in the life cycle of animals because it can affect most components of fitness. It has been proposed that some animals cue on the reproductive success of conspecifics to select breeding habitats. We tested this hypothesis with demographic and behavioral data from a 17-yr study of the Black-legged Kittiwake (Rissa tridactyla), a cliff-nesting seabird. As the hypothesis assumes, the Black-legged Kittiwake nesting environment was patchy, and the relative quality of the different patches (i.e., breeding cliffs) varied in time. The average reproductive success of the breeders of a given cliff was predictable from one year to the next, but this predictability faded after several years. The dynamic nature of cliff quality in the long term is partly explained by the autocorrelation of the prevalence of an ectoparasite that influences reproductive success. As predicted by the performance-based conspecific attraction hypothesis, the reproductive success of current breeders on a given cliff was predictive of the reproductive success of new recruits on the cliff in the following year. Breeders tended to recruit to the previous year's most productive cliffs and to emigrate from the least productive ones. Consequently, the dynamics of breeder numbers on the cliffs were explained by local reproductive success on a year-to-year basis. Because, on average, young Black-legged Kittiwakes first breed when 4 yr old, such a relationship probably results from individual choices based on the assessment of previous-year local quality. When breeders changed breeding cliffs between years, they selected cliffs of per capita higher reproductive success. Furthermore, after accounting for the potential effects of age and sex as well as between-year variations, the effect of individual breeding performance on breeding dispersal was strongly influenced by the average reproductive success of other breeders on the same cliff. Individual breeding performance did not appear to influence the probability of dispersing for birds breeding on cliffs with high local reproductive success, whereas individual breeding performance did have a strong effect on dispersal for birds that bred on cliffs with lower local reproductive success. This suggests that the reproductive success of locally breeding conspecifics may be sufficient to override an individual's own breeding experience when deciding whether to emigrate. These results, which are supported by behavioral observations of the role of prospecting in recruitment, suggest that both first breeders and adults rely on the reproductive success of conspecifics as 'public information' to assess their own chances of breeding successfully in a given patch and to make settling decisions. A corollary prediction is that individuals should attempt to breed near successful conspecifics (a form of social attraction) in order to benefit from the same favorable local environmental conditions. Such a performance-based conspecific attraction mechanism can thus lead to an aggregative distribution of nests and may have played a role in the evolution of coloniality.
The George River caribou herd increased from 15,000 animals in 1958 to 700,000 in 1988 - the largest herd in the world at the time. The authors trace the fluctuations in this caribou population back to the 1700s, detail how the herd escaped extinction in the 1950s, and consider current environmental threats to its survival. In an examination of the life history and population biology of the herd, The Return of Caribou to Ungava offers a synthesis of the basic biological traits of the caribou, a new hypothesis about why they migrate, and a comparison to herd populations in North America, Scandinavia, and Russia. The authors conclude that the old maxim, "Nobody knows the way of the caribou," is no longer valid. Based on a study in which the caribou were tracked by satellite across Ungava, they find that caribou are able to navigate, even in unfamiliar habitats, and to return to their calving ground, movement that is central to the caribou's cyclical migration. The Return of Caribou to Ungava also examines whether the herd can adapt to global warming and other changing environmental realities.