Zosterops Lateralis Chlorocephalus Research Articles

The influence of climatic variation and density on the survival of an insular passerine Zosterops lateralis.

Understanding the influence of environmental factors on population dynamics is fundamental to many areas in biology. Survival is a key factor of population biology, as it is thought to be the predominant driver of growth in long-lived passerines, which can be influenced by both biotic and abiotic environmental conditions. We used mark-recapture methods and generalized linear mixed models to test the influence of density and climatic variation, measured at a regional and local scale (Southern Oscillation Index [SOI] and rainfall, respectively), on seasonal variation in survival rates of an insular population of Silvereyes (Zosterops lateralis chlorocephalus), during a 15-year study period, off the east coast of Australia. We found overall high survival rates for adults and juveniles (81% and 59%, respectively). Local scale climate (i.e. rainfall) and density were the principal environmental factors influencing their survival, both with a negative relationship. A significant interactive effect of density and rainfall influenced survival as they both increased. However, survival remained low when density was at it highest, independent of the amount of rainfall. Nestling survival was negatively influenced by rainfall and density, positively by SOI, and chicks that hatched later in the breeding season had higher survival rates. The regional scale climate variable (i.e. SOI) did not explain survival rates as strongly as rainfall in any age class. Our results contribute to the understanding of insular avian population dynamics and the differential effects of environmental factors across age classes. Climatic predictions expect El Niño events to increase, meaning dryer conditions in eastern Australia, potentially increasing Silvereye survival across age classes. However, the long-term effect of lower rainfall on food availability is unknown; consequently, the outcome of lower rainfall on Silvereye survival rates is uncertain.

Individual heterogeneity and senescence in Silvereyes on Heron Island

Individual heterogeneity and correlations between life history traits play a fundamental role in life history evolution and population dynamics. Unobserved individual heterogeneity in survival can be a nuisance for estimation of age effects at the individual level by causing bias due to mortality selection. We jointly analyze survival and breeding output from successful breeding attempts in an island population of Silvereyes (Zosterops lateralis chlorocephalus) by fitting models that incorporate age effects and individual heterogeneity via random effects. The number of offspring produced increased with age of parents in their first years of life but then eventually declined with age. A similar pattern was found for the probability of successful breeding. Annual survival declined with age even when individual heterogeneity was not accounted for. The rate of senescence in survival, however, depends on the variance of individual heterogeneity and vice versa; hence, both cannot be simultaneously estimated with precision. Model selection supported individual heterogeneity in breeding performance, but we found no correlation between individual heterogeneity in survival and breeding performance. We argue that individual random effects, unless unambiguously identified, should be treated as statistical nuisance or taken as a starting point in a search for mechanisms rather than given direct biological interpretation.

4000 YEARS OF PHENOTYPIC CHANGE IN AN ISLAND BIRD: HETEROGENEITY OF SELECTION OVER THREE MICROEVOLUTIONARY TIMESCALES

Pronounced phenotypic shifts in island populations are typically attributed to natural selection, but reconstructing heterogeneity in long-term selective regimes remains a challenge. We examined a scenario of divergence proposed for species colonizing a new environment, involving directional selection with a rapid shift to a new optimum and subsequent stabilization. We provide some of the first empirical evidence for this model of evolution using morphological data from three timescales in an island bird, Zosterops lateralis chlorocephalus. In less than four millennia since separation from its mainland counterpart, a substantial increase in body size has occurred and was probably achieved in fewer than 500 generations after colonization. Over four recent decades, morphological traits have fluctuated in size but showed no significant directional trends, suggesting maintenance of a relatively stable phenotype. Finally, estimates of contemporary selection gradients indicated generally weak directional selection. These results provide a rare description of heterogeneity in long-term natural regimes, and caution that observations of current selection may be of limited value in inferring mechanisms of past adaptation due to a lack of constancy even over short time-frames.

A wake‐up call for studies of natural selection?

A wake‐up call for studies of natural selection?

Large body size in an island‐dwelling bird: a microevolutionary analysis

Island races of passerine birds display repeated evolution towards larger body size compared with their continental ancestors. The Capricorn silvereye (Zosterops lateralis chlorocephalus) has become up to six phenotypic standard deviations bigger in several morphological measures since colonization of an island approximately 4000 years ago. We estimated the genetic variance-covariance (G) matrix using full-sib and 'animal model' analyses, and selection gradients, for six morphological traits under field conditions in three consecutive cohorts of nestlings. Significant levels of genetic variance were found for all traits. Significant directional selection was detected for wing and tail lengths in one year and quadratic selection on culmen depth in another year. Although selection gradients on many traits were negative, the predicted evolutionary response to selection of these traits for all cohorts was uniformly positive. These results indicate that the G matrix and predicted evolutionary responses are consistent with those of a population evolving in the manner observed in the island passerine trend, that is, towards larger body size.

Large size in an island-dwelling bird: intraspecific competition and the Dominance Hypothesis.

Differences between island- and mainland-dwelling forms provide several classic ecological puzzles. Why, for instance, are island-dwelling passerine birds consistently larger than their mainland counterparts? We examine the 'Dominance hypothesis', based on intraspecific competition, which states that large size in island passerines evolves through selection for success in agonistic encounters. We use the Heron Island population of Capricorn silvereyes (Zosterops lateralis chlorocephalus), a large-bodied island-dwelling race of white-eye (Zosteropidae), to test three assumptions of this hypothesis; that (i) large size is positively associated with high fitness, (ii) large size is associated with dominance, and (iii) the relationship between size and dominance is particularly pronounced under extreme intraspecific competition. Our results supported the first two of these assumptions, but provided mixed evidence on the third. On balance, we suggest that the Dominance Hypothesis is a plausible mechanism for the evolution of large size of island passerines, but urge further empirical tests on the role of intraspecific competition on oceanic islands versus that on mainlands.

Isolation of microsatellite loci in the Capricorn silvereye, Zosterops lateralis chlorocephalus (Aves: Zosteropidae)

AbstractThe Capricorn silvereye (Zosterops lateralis chlorocephalus) is ideally suited to investigating the genetic basis of body size evolution. We have isolated and characterized a set of microsatellite markers for this species. Seven out of 11 loci were polymorphic. The number of alleles detected ranged from two to five and observed heterozygosities between 0.12 and 0.67. One locus, ZL49, was found to be sex‐linked. This moderate level of diversity is consistent with that expected in an isolated, island population.

Genetic monogamy in the absence of paternity guards: the Capricorn silvereye, Zosterops lateralis chlorocephalus, on Heron Island

We investigated the genetic mating system of a socially monogamous passerine bird, the Capricorn silvereye Zosterops lateralis chlorocephalus, on an island of the Great Barrier Reef. There were no cases of extrapair paternity (EPP) among 122 offspring from 53 broods detectable by minisatellite or microsatellite DNA fingerprinting. Behavioral observations of paired birds showed that this was not a consequence of efficacious paternity guards and that females did not engage in extrapair copulation (EPC). Frequency of intrapair copulations was also low, with only 14 cases observed during 199 hours of observations of the 11 focal pairs in the fertile periods of females, and this was consistent with anatomical features of the cloacal protuberance in males. In this population, young birds form life-time pair bonds soon after gaining independence but females are obviously not attempting EPC possibly to redress this early mate choice. This is despite the fact that they breed in high density with a synchronous start and asynchronous spread of laying in a protracted season and males do not positively exhibit mate guarding behavior when females are fertile. Our results support high fidelity of socially monogamous birds on islands and are consistent with the hypothesis that sexual selection is reduced where genetic variation in fitness is limited.

Density dependence in an island population of silvereyes

The population of silvereyes Zosterops lateralis chlorocephalus, on Heron Island, Great Barrier Reef has been monitored accurately since 1965. Between 1979 and 1993, the breeding success of all birds was determined by monitoring nests. The population fluctuated between 225 and 483 individuals. Four cyclones led to substantial mortality. As this data set is long‐term, has little observation error, and is from an effectively closed population, it provides an unusual opportunity to examine density dependence in reproduction or mortality. Using a stochastic logistic model, we found clear evidence of density dependence in adult population size. Logistic regression suggested that fledgling survival decreased with the numbers of birds attempting to breed. There was also some suggestion that adult survival might be density dependent. The fitted stochastic logistic model predicts negligible risks of extinction for this population, in contrast to the predictions of a published population viability analysis. Whilst our statistical model including density dependence may provide better predictions of the “usual” behaviour of a population than a population viability analysis, we suggest that caution should be exercised when statistically fitted models are used to predict the behaviour of the population at extremes, such as near extinction.

Examining threats faced by island birds: a population viability analysis on the Capricorn silvereye using long‐term data

Summary Population viability analysis (PVA) is widely used for assessing the extinction risk faced by endangered species and for evaluating the effects of potential management strategies. However, most PVAs are done in urgency and with limited data. It is therefore important to know how the amount and quality of available ecological data affect the predictions of PVA. How can PVA results be interpreted when we have reliable, long‐term population data? When a species is endangered on islands, as are many endemic bird species, what are the likely causes of the decline, and what is the probable impact of new threats? This paper investigates these issues by modelling 26 years of monitoring data on the population of Capricorn silvereyes Zosterops lateralis chlorocephala Campbell & White of Heron Island on the Great Barrier Reef. Australia. A PVA with parameters estimated from a relatively short (5‐year) period of data collection produced very different predictions of extinction risk compared with a PVA based on more comprehensive ecological data spanning 15–25 years. This indicates that caution should be exercised when using PVA as an indicator of absolute extinction risk. Despite remaining viable over the past 26 years, the Heron Island silvereye population is predicted to be at risk of extinction within the next 100 years, with the magnitude of risk depending on the severity of impact of a range of potentially threatening factors. Increased mortality due to the introduction of new predators or diseases, inbreeding depression or frequent severe storms greatly increased the extinction risk. Habitat reduction, if not very severe, is unlikely to have a large impact. Similar threats and impacts may apply to other island bird populations. Movement or dispersal between neighbouring islands had a mitigating effect on extinction probability and allowed extinct island populations to be re‐established. However, very small islands could only support sink populations that quickly became extinct and required an unrealistically high immigration rate to remain viable in the long term.

The Relationship Between Individual Variation in Song and Ecology in the Capricorn Silvereye

Songs of individual male Capricorn Silvereyes Zosterops lateralis chlorocephala had an average repertoire of 53 syllable types which were organised into over 30 different sequences of three or more syllables. Silvereyes cycled through about half their sequence repertoire in one dawn chorus, repeating each several times. They shared a substantial proportion of their syllable repertoires with other males, especially those in the same age cohort. The relationship of song complexity to the general ecology of the Capricorn Silvereye, and the various influences on that complexity, are discussed.

Post-fledging Parental Investment in the Capricorn Silvereye

SummaryWilson, S.M. & Kikkawa, J. (1988). Post-fledging parental investment in the Capricorn Silvereye. Emu 88, 81–87.We measured relative parental investments of 23 pairs of Capricorn Silvereyes Zosterops lateralis chbrocephala in the post-fledging period to examine the possibility of brood division and to relate parental investment to the survival of parents and offspring. Both parents invested similarly in nestlings and young fledglings. When feeding old fledglings or small broods, one parent tended to invest considerably more, but brood division did not develop. The degree of investment was not related to parental sex, age or dominance status. The strong pair bond, clumping by the fledglings and the small brood size may preclude development of brood division in Silvereyes. Survival of parents and of their young was not related to the relative investment of the members of the pair.

FOOD RESOURCES, TERRITORY DENSITY AND REPRODUCTIVE SUCCESS OF AN ISLAND SILVEREYE POPULATION ZOSTEROPS LATERALIS

SummaryWe examined the relationships between food resources, territory density and some breeding parameters (individual reproductive success and parental age) of a Capricorn Silvereye population during two years' detailed study on Heron Island, Great Barrier Reef. The territorial behaviour of the Silvereyes included dawn song. aggression and distinctive vocalizations. During both years, about 85% of pairs occupied territories although the population sizes differed. The dispersion pattern of territories was not regular and was correlated with the density of figs and human food scraps.Nestlings were fed more figs as they grew, and the parents foraged in fig trees outside their territories. However, only about 60%, of territorial pairs fledged young. Pairs which fledged most young were older, fed more insects to their nestlings, and nested in areas containing more fruiting fig trees per territory.We conclude that food resources were in short supply, and that access to fig trees provided breeding birds with a quick energy source while they searched for insect protein for the young. The data are consistent with the predictions of the ‘sufficient resource‘ hypothesis of the adaptive significance of territoriality but not with its assumption that the principal benefit is the food within the territory.