Resighting Data Research Articles

Seasonal survival and the relative cost of first reproduction in adult female southern elephant seals

The annual cycle of adult female southern elephant seals (Mirounga leonina) can be divided into 2 pelagic phases, separated by relatively short terrestrial phases: breeding and molting. We used resighting data collected from tagged female southern elephant seals at Marion Island during 1986–1999 to investigate seasonal survival during the 2 pelagic phases in relation to reproductive experience. Mean postbreeding (pelagic phase between breeding and molting, about 62 days) survival of primiparous females was 0.830 compared to 0.912 for moreexperienced females. Postmolting (pelagic phase between molting and breeding, about 255 days) survival was 0.847 (0.960 when rescaled to 62 days for comparative purposes) and was not dependent on reproductive experience. Postbreeding survival of experienced females was higher than postmolting survival, but per unit time the opposite applied. A 2-stage survival model, in which survival was constrained to be constant before 1994 (when the population was declining) and from 1994 onward (during the stable phase), had overwhelming support from the data. Postbreeding survival of primiparous females increased from 0.799 before 1994 to 0.880 from 1994 onward. Postmolting survival of all females also increased from 0.817 to 0.872 over the same period. Postbreeding and postmolting mortality risk varied independently over time, demonstrating the importance of an intra-annual approach in population studies of southern elephant seals.

Evidence for density dependent population regulation in southern elephant seals in the southern Indian Ocean

The means by which populations are regulated form a central theme in conservation biology, and much debate has revolved around density dependence as a mechanism driving population change. Marion Island (46°54‘S, 37°45‘E) is host to a relatively small breeding population of southern elephant seals, which like its counterparts in the southern Indian and southern Pacific Oceans, have declined precipitously over the past few decades. An intensive mark–recapture study, which commenced in 1983, has yielded a long time-series of resight data on this population. We used the program MARK to estimate adult female survival in this population from resight data collected over the period 1986–1999. Including concurrent population counts as covariates significantly improved our mark–recapture models and suggests density dependent population regulation to be operational in the population. Although predation may have been involved, it is far more likely that density dependent regulation has been based on a limited food supply. A significant increase in adult female survival was evident which is likely to have given rise to recent changes in population growth.

Mark–resight density estimation for American black bears in Hoopa, California

Accurate and precise population estimates are necessary to answer many management questions, but these estimates are generally unavailable for large carnivores because of their extensive movements, low densities, and secretive natures. These traits also often prevent the bounding of occupied areas necessary to estimate densities. We used a modified Petersen mark–recapture methodology to estimate black bear density in 1998 at 2 study sites on the Hoopa Valley Reservation, California, from mark–resight data. We used culvert traps to capture, radiocollar, and eartag bears, radio telemetry to establish closure, and remote cameras to collect resighting data. We calculated bear densities (90% confidence intervals) of 0.18 (0.09–0.32) and 1.33 (0.54–3.29) bears/km2 on the 2 sites. Knowledge of bear densities can now be incorporated into forest management actions and associated bear control measures on the Hoopa Valley Reservation.

Evidence for density dependent population regulation in southern elephant seals in the southern Indian Ocean

ABSTRACT The means by which populations are regulated form a central theme in conservation biology, and much debate has revolved around density dependence as a mechanism driving population change. Marion Island (46°54′S, 37°45′E) is host to a relatively small breeding population of southern elephant seals, which like its counterparts in the southern Indian and southern Pacific Oceans, have declined precipitously over the past few decades. An intensive mark–recapture study, which commenced in 1983, has yielded a long time-series of resight data on this population. We used the program MARK to estimate adult female survival in this population from resight data collected over the period 1986–1999. Including concurrent population counts as covariates significantly improved our mark–recapture models and suggests density dependent population regulation to be operational in the population. Although predation may have been involved, it is far more likely that density dependent regulation has been based on a limite...

CONSUMER RESOURCE MATCHING IN URBANIZING LANDSCAPES: ARE SYNANTHROPIC SPECIES OVER-MATCHING

Population responses of synanthropic species to urbanization may be explained by the resource-matching rule, which postulates that individuals should distribute themselves according to resource availability. According to the resource-matching rule, urban habitats will contain greater densities if they provide better resources than rural habitats. However, because resource availability is density dependent, individuals in urban areas would ultimately achieve fitness levels comparable to, but no better than, individuals in less urban areas. Some ecologists suggest that synanthropic birds may not conform to the resource-matching rule and may instead overmatch (i.e., overexploit) in urban habitats, ultimately leading to lower fitness despite greater resource levels. Using the Northern Cardinal (Cardinalis cardinalis) as a focal species, we evaluated if Cardinal populations in urban and rural habitats were consistent with predictions of consumer resource matching. During 2003-2006 we documented population density, adult body condition, apparent survival, and annual reproductive productivity of Cardinals in riparian forest stands within urban (n = 8 stands) and rural (n = 6 stands) landscapes in Ohio, USA. Density of Cardinals in urban forests was four times that found in more rural forests. Mark-resight data from 147 males and 125 females over four years indicated that apparent survival rates were similar between urban and rural landscapes (phi = 0.64, SE = 0.039 for males and phi = 0.57, SE = 0.04 for females). Similarly, body condition indices of 168 males, 142 females, and 118 nestlings did not differ significantly between landscapes. Annual reproductive productivity (mean number of fledglings per pair over breeding season) of 294 pairs was comparable for urban (2.4 +/- 0.18 [mean +/- SE] and rural (2.1 +/- 0.18) young birds. Thus, contrary to recent suggestions, we find that high densities of certain synanthropic species in urban landscapes are consistent with expectations of consumer resource matching.

Stopover of migrating birds: simultaneous analysis of different marking methods enhances the power of capture–recapture analyses

Analyses of stopover parameters of migrating birds with Cormack–Jolly–Seber (CJS) capture–recapture models often suffer from low precision due to sparse data sets. Low recapture rates result in low power to detect violations of the underlying assumptions and factors influencing stopover behaviour. We studied stopover behaviour of Palearctic migrant passerines in an oasis in Mauritania, West Africa. Using capture–recapture data and systematic observations of colour-ringed birds, we analysed the effect of increased sample size on probability of stay and recapture probability and the influence of a possible trap response on these parameters. We analysed capture–recapture data with the conventional CJS model and compared the results with those from a multistate model using in addition resighting data. The analyses including resighting data resulted in a higher precision of the estimates of the probabilities of stay compared to analyses using only capture–recapture data of the same individuals. Moreover, the power to detect transients was substantially enhanced. Capture had no effect on the estimates of probability to stay and recapture probability; birds did not leave the stopover site or avoid nets as a reaction to capture. The estimates of probability of stay were up to 15.7% higher when resighting data were included, probably due to the higher power to detect transients and the elimination of the bias induced by non-random temporary emigration when both data types are considered. As a consequence, stopover duration would have been underestimated when only the capture–recapture data were available. We conclude that additional resightings of colour ringed birds can produce more accurate results needed for enhancing our understanding of stopover ecology of migrants.

Monitoring planktivorous seabird populations: validating surface counts of crevice-nesting auklets using mark–resight techniques

Least Auklets ( Aethia pusilla (Pallas, 1811)) are the most abundant species of seabird in the Bering Sea and offer a relatively efficient means of monitoring secondary productivity in the marine environment. Counting auklets on surface plots is the primary method used to track changes in numbers of these crevice-nesters, but counts can be highly variable and may not be representative of the number of nesting individuals. We compared average maximum counts of Least Auklets on surface plots with density estimates based on mark–resight data at a colony on St. Lawrence Island, Alaska, during 2001–2004. Estimates of breeding auklet abundance from mark–resight averaged 8 times greater than those from maximum surface counts. Our results also indicate that average maximum surface counts are poor indicators of breeding auklet abundance and do not vary consistently with auklet nesting density across the breeding colony. Estimates of Least Auklet abundance from mark–resight were sufficiently precise to meet management goals for tracking changes in seabird populations. We recommend establishing multiple permanent banding plots for mark–resight studies on colonies selected for intensive long-term monitoring. Mark–resight is more likely to detect biologically significant changes in size of auklet breeding colonies than traditional surface count techniques.

The effects of nesting success and mate fidelity on breeding dispersal in burrowing owls

Understanding and describing the factors that affect avian breeding dispersal are critical for modeling population dynamics and designing conservation strategies. We investigated the hypothesis that dispersal probability and dispersal distance are affected by nesting success and mate fidelity with band–resight data (1998–2003) from burrowing owls (Athene cunicularia (Molina, 1782)) nesting in southern California. Most owls (167 of 253, 66%) remained near their initial nest (<100 m), and those that moved >100 m dispersed 472 ± 65 m (mean ± 1 SE; n = 86). Both female and male owls whose nests failed were more likely to disperse and dispersed longer distances than owls with successful nests. Failed nesting attempts were also associated with an increased probability of divorce, and divorce was related to increased dispersal probabilities and distances. Moreover, female and male owls tended to be more likely to disperse and to disperse greater distances following the death of a mate than those that remained paired. Although dispersal was related to mate fidelity, nesting success remained an important factor affecting dispersal even after controlling for the effects of mate loss. Our results suggest that nesting failure was the primary factor associated with dispersal probability and dispersal distance in burrowing owls in our population.

Contributions of marginal offspring to reproductive success of Nazca booby (Sula granti) parents: tests of multiple hypotheses

While obligate siblicide is a phylogenetically widespread behavior, known from plants, insects, birds, and other taxa, with important implications for life history evolution, comprehensive evaluations of its costs and benefits to parents are rare. We used 12 years of breeding and band resight data to evaluate the importance of several potential benefits that marginal offspring (the usual victims of obligate siblicide) could provide to parent Nazca boobies (Sula granti), a seabird. We found no evidence for the resource-tracking hypothesis: 99.95% of two-chick broods were reduced to one chick before fledging, and the single exceptional brood probably lost one chick between fledging and independence. Behavioral observations indicated that siblicidal aggression caused most mortality of marginal chicks, and at least contributed to the remainder. We also found no evidence that marginal offspring provide a food resource for other family members. Marginal chicks benefit parents via adoption into other families, and possibly also in the context of progeny choice, but these benefits are minor compared to the insurance that marginal chicks provide against early failure of core (first-hatched) offspring. Further evaluation of the Insurance Egg Hypothesis showed that marginal and core offspring are functionally equivalent in the absence of sibling interactions, and that core offspring incur no detectable costs from behaving siblicidally. Nazca boobies are truly obligate brood reducers, with parents receiving principally insurance benefits from marginal offspring, but many birds and other taxa exhibiting persistent, unconditional sibling aggression do not exhibit universal brood reduction. Insurance is only one of several potential benefits that marginal offspring can confer on parents, and a multi-hypothesis approach to decompose the different types of benefits is required to understand the evolution of clutch size in other obligately siblicidal species.

The effect of climate on adult survival in five species of North Atlantic seabirds

Summary 1 In long-lived species, adult survival is an important life-history trait. Better knowledge of the effects of non-catastrophic climate variation on the adult survival of long-lived seabirds is therefore needed. However, documentation of such effects is still rare. 2 Using capture–mark–resighting data, we modelled the annual survival rates of five species of seabirds, the common guillemot (Uria aalge), Brunnich's guillemot (Uria lomvia), razorbill (Alca torda), Atlantic puffin (Fratercula arctica) and black-legged kittiwake (Rissa tridactyla). The data spanned 14 years of observation at a colony on Hornoya, off Northern Norway in the western Barents Sea. 3 A method of taking into account ring-loss of marked individuals is proposed. This approach made it possible to keep many observations which otherwise would have to be discarded. 4 The estimated survival rates were higher and less variable than most estimates for the same species from other colonies. There was extensive resighting heterogeneity. Ignoring this effect would have underestimated breeding life span by more than 50% in some species. 5 The analysis presented is the first study to document a correlation between the North Atlantic Oscillation (NAO) index and adult survival in North Atlantic seabirds. Other environmental variables considered were sea surface temperatures (SST) and prey stocks. In most of the species, models incorporating climatological variables were considerably better supported than models with constant survival, time-dependent survival or prey effects. SSTs tended to explain more of the variability in seabird survival than did the NAO. 6 The importance of prey availability was evident in some of the species, but climate was generally a better predictor of survival. Nevertheless, the evidence suggests that meteorological parameters affect seabird mortality only indirectly, possibly through the food chain. This conclusion rests on the observations that most NAO effects are lagged, and that survival rates decreased with increasing SSTs.

Increase of a Caribbean leatherback turtle Dermochelys coriacea nesting population linked to long-term nest protection

The leatherback turtle Dermochelys coriacea is considered to be at serious risk of global extinction, despite ongoing conservation efforts. Intensive long-term monitoring of a leatherback nesting population on Sandy Point (St. Croix, US Virgin Islands) offers a unique opportunity to quantify basic population parameters and evaluate effectiveness of nesting beach conservation practices. We report a significant increase in the number of females nesting annually from ca. 18–30 in the 1980s to 186 in 2001, with a corresponding increase in annual hatchling production from ca. 2000 to over 49,000. We then analyzed resighting data from 1991 to 2001 with an open robust-design capture–mark–recapture model to estimate annual nester survival and adult abundance for this population. The expected annual survival probability was estimated at ca. 0.893 (95% CI: 0.87–0.92) and the population was estimated to be increasing ca. 13% pa since the early 1990s. Taken together with DNA fingerprinting that identify mother–daughter relations, our findings suggest that the increase in the size of the nesting population since 1991 was probably due to an aggressive program of beach protection and egg relocation initiated more than 20 years ago. Beach protection and egg relocation provide a simple and effective conservation strategy for this Northern Caribbean nesting population as long as adult survival at sea remains relatively high.

POPULATION ESTIMATION AND BIASES IN PAINTBALL, MARK–RESIGHT SURVEYS OF ELK

To overcome costs and logistical constraints associated with capturing and marking a sufficient number of elk (Cervus elaphus) for population estimation, elk are sometimes marked with paintballs fired from a helicopter flying overhead. Although marking elk with paintballs is a cost-effective marking technique, there are some unique aspects of this method that may bias population estimates. When analyzing the mark–recapture data, the assumption that each animal has an independent and identical probability of being marked and resighted may be violated. Instead, the probabilities of marking and resighting are likely to be a function of herd size. We explored the potential biases of paintball, mark–resight surveys of elk and present alternative methods that could be used to generate population estimates from mark–resight data, including Bailey's binomial MLE, cluster sampling, and bootstrap methods. Our results suggest that population estimates will generally be negatively biased; the magnitude of the bias depends upon the degree of herd aggregation and fidelity. To minimize these biases, we recommend (1) marking animals when they occur in many small herds, (2) marking equal proportions of individuals in each herd, and (3) determining the degree of aggregation and fidelity of elk to herds in each area. Although these recommendations do not facilitate the primary goal of the technique (i.e., the ability to mark numerous animals easily), following them will provide more reliable population estimates.

MODELING SURVIVAL: APPLICATION OF THE ANDERSEN–GILL MODEL TO YELLOWSTONE GRIZZLY BEARS

Wildlife ecologists often use the Kaplan-Meier procedure or Cox proportional hazards model to estimate survival rates, distributions, and magnitude of risk factors. The Andersen–Gill formulation (A–G) of the Cox proportional hazards model has seen limited application to mark–resight data but has a number of advantages, including the ability to accommodate left-censored data, time-varying covariates, multiple events, and discontinuous intervals of risks. We introduce the A–G model including structure of data, interpretation of results, and assessment of assumptions. We then apply the model to 22 years of radiotelemetry data for grizzly bears (Ursus arctos) of the Greater Yellowstone Grizzly Bear Recovery Zone in Montana, Idaho, and Wyoming, USA. We used Akaike's Information Criterion (AICc) and multi-model inference to assess a number of potentially useful predictive models relative to explanatory covariates for demography, human disturbance, and habitat. Using the most parsimonious models, we generated risk ratios, hypothetical survival curves, and a map of the spatial distribution of high-risk areas across the recovery zone. Our results were in agreement with past studies of mortality factors for Yellowstone grizzly bears. Holding other covariates constant, mortality was highest for bears that were subjected to repeated management actions and inhabited areas with high road densities outside Yellowstone National Park. Hazard models developed with covariates descriptive of foraging habitats were not the most parsimonious, but they suggested that high-elevation areas offered lower risks of mortality when compared to agricultural areas.

Spring Dispersal Patterns of Red-winged Blackbirds, <em>Agelaius phoeniceus</em>, Staging in Eastern South Dakota

Red-winged Blackbirds (Agelaius phoeniceus) are very abundant summer residents throughout the Prairie Pothole Region of central North America. In late summer they assemble in post-breeding flocks that cause significant amounts of agricultural damage, particularly in sunflower fields near natal sites. In April 2001, we aerially color-marked ~370,000 Red-winged Blackbirds near Badger, South Dakota (44°48'N, 97°21'W), to determine if migrants staging here were summer residents in sunflower production areas ~350 km to the northwest. We measured patterns of migratory dispersal by collecting birds in 54 randomly selected blocks in the northcentral U.S. and the Prairie Provinces of Canada. The marked specimens (n = 33) were categorized into three polygons based on analyses of banding and re-sighting data and proximity to concentrated sunflower production. We estimated that 82% of the migrants that had staged in eastern South Dakota resided within or on the periphery of the sunflower growing area. These birds probably stay near their breeding territories until at least late August and cause early damage to sunflower, which comprises the majority of damage. Resident birds in Alberta and most of Saskatchewan (18%) might arrive too late in the damage season to impact the sunflower crop significantly.

Estimating Population Size with Correlated Sampling Unit Estimates

Finite population sampling theory is useful in estimating total population size (abundance) from abundance estimates of each sampled unit (quadrat). We develop estimators that allow correlated quadrat abundance estimates, even for quadrats in different sampling strata. Correlated quadrat abundance estimates based on mark–recapture or distance sampling methods occur when data are pooled across quadrats to estimate, for example, capture probability parameters or sighting functions. When only minimal information is available from each quadrat, pooling of data across quadrats may be necessary to efficiently estimate capture probabilities or sighting functions. We further include information from a quadrat-based auxiliary variable to more precisely estimate total population size via a ratio estimator. We also provide variance estimators for the difference between or the ratio of 2 abundance estimates, taken at different times. We present an example based on estimating the number of Mexican spotted owls (Strix occidentalis lucida) in the Upper Gila Mountains Recovery Unit, Arizona and New Mexico, USA. Owl abundance for each quadrat was estimated with a Huggins 4-pass mark–resight population estimator, but with initial capture and resighting probabilities modeled in common across all sample quadrats. Pooling mark–resight data across quadrats was necessary because few owls were marked on individual quadrats to estimate quadrat-specific capture probabilities. Model-based estimates of owl abundance for each quadrat necessitated variance estimation procedures that take into account correlated quadrat estimates. An auxiliary variable relating to topographic roughness of sampled quadrats provided a useful covariate for a ratio estimator.

Optimal allocation of sample sizes between regular banding and radio-tagging for estimating annual survival and emigration rates

Many authors have shown that a combined analysis of data from two or more types of recapture survey brings advantages, such as the ability to provide more information about parameters of interest. For example, a combined analysis of annual resighting and monthly radio-telemetry data allows separate estimates of true survival and emigration rates, whereas only apparent survival can be estimated from the resighting data alone. For studies involving more than one type of survey, biologists should consider how to allocate the total budget to the surveys related to the different types of marks so that they will gain optimal information from the surveys. For example, since radio tags and subsequent monitoring are very costly, while leg bands are cheap, the biologists should try to balance costs with information obtained in deciding how many animals should receive radios. Given a total budget and specific costs, it is possible to determine the allocation of sample sizes to different types of marks in order to minimize the variance of parameters of interest, such as annual survival and emigration rates. In this paper, we propose a cost function for a study where all birds receive leg bands and a subset receives radio tags and all new releases occur at the start of the study. Using this cost function, we obtain the allocation of sample sizes to the two survey types that minimizes the standard error of survival rate estimates or, alternatively, the standard error of emigration rates. Given the proposed costs, we show that for high resighting probability, e.g. 0.6, tagging roughly 10-40% of birds with radios will give survival estimates with standard errors within the minimum range. Lower resighting rates will require a higher percentage of radioed birds. In addition, the proposed costs require tagging the maximum possible percentage of radioed birds to minimize the standard error of emigration estimates.

Estimating survival from joint analysis of resighting and radiotelemetry capture-recapture data for wild animals

Biologists often use more than one marking technique in wildlife studies. For each of the mark types, it is common to conduct a separate analysis of the recapture data to estimate parameters of interest, such as survival rates. Two data types that can be used in estimating survival rates are resighting and radiotelemetry data. The Cormack-Jolly-Seber model is commonly used to analyze the resighting data, while the Kaplan-Meier product limit estimator, modified for staggered entry of animals, is used to analyze the radi otelemetry data. In a study where some animals receive two types of tags and others receive just one tag type, the separate Cormack-Jolly-Seber and Kaplan-Meier analyses do not exploit all of the information in the combined data sets. In this article, we propose a model and likelihood for the combined analysis of resighting and radi otelemetry data. In comparison with the separate analyses, this richer model provides more information about the biology and sampling processes. For example, the richer model permits assessment of assumptions required by the separate analyses and allows estimation of additional parameters. We apply the model to annual resighting and monthly telemetry data from a population of snail kites in Florida. The snail kite is a threatened species of bird in the United States, and our results on survival are very important. In this example, all birds are marked using leg bands and some of them receive radios.

Efficiency Gain from Auxiliary Data Requiring Additional Nuisance Parameters

In a mark-recapture study of an animal population, live-recapture information may be supplemented by resightings from marked animals obtained throughout the period of the study and the two types of data analyzed simultaneously. The resighting data can only contribute to estimates of survival probability if they are jointly modeled with the live-recapture data and require the inclusion of additional nuisance parameters. We show that, under quite general conditions, estimates of the original parameters are estimated with the same, or improved, precision despite the inclusion of the nuisance parameters. Banding data from female goldeneye ducks (Bucephala clangula) are used as an illustration.

Joint analysis of mark—recapture, resighting and ring-recovery data with age-dependence and marking-effect

Using the joint analysis of live-recapture and ring-recovery data described by Burnham in 1993, we generalize it to the case where resightings of live birds are also reported between capture periods. Explicit estimators under 'random' temporary emigration correspond to those suggested by Jolly in 1965. The model is further generalized to admit age-dependence and a temporary marking effect. Explicit maximum-likelihood estimators under random emigration are reported and also contingency table tests for age-dependence and temporary marking effect. The new analysis is illustrated using a five-year subset of data from a 25-year ringing programme for Goldeneye Ducks Bucephala clangula¸. Alternative forms of emigration from the study area are also discussed.

Maternal and birth colony effects on survival of Weddell seal offspring from McMurdo Sound, Antarctica

1. Maternal and birth colony effects on offspring survival to weaning and to reproductive age were examined for Weddell seals using mark–recapture models and over 25 years of mark–resight data from McMurdo Sound, Antarctica. 2. Pre‐weaning mortality (proportion of the total pups found dead on the ice surface) varied significantly with maternal age and was significantly higher for primiparous mothers than for multiparous mothers. 3. Offspring survival to reproductive age also increased significantly with maternal age and experience. Survival of offspring from birth to 6 years of age increased significantly with maternal body length only for male offspring. 4. Increased survivorship of offspring with maternal age was only evident for offspring of multiparous (rather than primiparous) mothers. This suggests that the age when mothers have their first pup may not affect survival of offspring, although lack of effect of age on offspring survival for primiparous mothers may be due to insufficient variation in age of first reproduction in our sample for us to detect a trend. 5. Pre‐weaning survival of offspring also varied significantly among pupping colonies. In 1983, a year of low reproduction and low first‐year survival for the McMurdo population (Testa 1987b; Hastings 1996), first‐year survival was lower for offspring born in Outer Erebus Bay (west of 166·7°E) than for offspring born in Inner Erebus Bay (east of 166·7°E). 6. Maternal effects on offspring survival may be related to quality of breeding site. Maternal age and experience varied significantly among pupping colonies. That pup population size was stable at all colonies over the study period suggests non‐random age‐distribution of mothers may not be an artefact of population growth, but instead result from intraspecific competition for space in good pupping colonies. 7. Offspring survival, pup population size, and mean maternal age at colonies were significantly positively correlated. This suggests that favourable environmental conditions and high maternal or paternal quality may interact to enhance survival of offspring at larger colonies.