Kikkawa, J. & Degnan, S.M. 1998. Effective population size of an island passerine. In: Adams, N.J. & Slotow, R.H. (eds) Proc. 22 Int. Ornithol. Congr., Durban. Ostrich 69: 213.

Effective population size (Ne) is one required to maintain the same amount of genetic variability under the existing demographic constraints and can be estimated from the ecological or genetic data of the population. It is an important parameter for understanding and conserving the evolutionary potential of the population. Estimates of Ne of threatened species are based on population viability analysis of often insufficient demographic information or genetic analysis of allozyme data. These methods usually are not validated or cross-checked. We estimated Ne of the Capricorn Silvereye Zosterops lateralis chlorocephala of Heron Island, Great Barrier Reef, from breeding data over 14 years and also indirectly from changes in allele frequencies for nuclear DNA restriction fragment length polymorphisms over five overlapping generations. The breeding population ranging from 266 to 375 produced 450 to 1380 young per season. The mean survival rate of each cohort to the mean breeding age of three years was 7.74%, giving an estimate of Ne at 182. The standardised variance of allele frequency change was estimated from three endonucleases separately and an Ne of 103 was obtained from the mean. General correspondence of the two independently derived values is remarkable, suggesting the ability of Zosterops to survive with a relatively small Ne.

Key words: Zosterops, white-eye, PVA, conservation, island bird

Matthysen, E., Adriaensen, F. & Dhondt, A.A. 1998. Immigration patterns in relation to patch characteristics and population density of Great and Blue Tits. In: Adams, N.J. & Slotow, R.H. (eds) Proc. 22 Int. Ornithol. Congr., Durban. Ostrich 69: 214.

Natural populations are rarely panmictic, since most birds tend to reproduce close to the site where they were born. In a patchy habitat, the proportion of individuals breeding outside the natal patch, or conversely, the proportion of breeders that are not locally born, is one of the key factors affecting the genetic structure of the population, as well as influencing local and metapopulation dynamics. Immigration rates are the consequence of multiple decisions of individuals socially interacting with one another, and are influenced by the costs and benefits of dispersal. Therefore immigration may reflect the accessibility of different patches within a heterogeneous landscape. Here we report on immigration rates of Great Tit Parus major and Blue Tit Parus caeruleus in a large number of study areas in northern Belgium, including two sets of forest fragments or parks differing in size and degree of isolation, as well as similar-sized study plots in large forests. Since breeding densities have been experimentally manipulated by nestbox availability, this allows us to examine effects of patch size and population size independently. Per capita immigration rates are as expected, higher in smaller study plots, and lower in more isolated populations.

Key words: immigration, habitat fragmentation, Parus

Pettifor, R.A. & Rowcliffe, J.M. 1998. Understanding population behaviour in response to anthropogenic change. In: Adams, N.J. & Slotow, R.H. (eds) Proc. 22 Int. Ornithol. Congr., Durban. Ostrich 69: 214.

Long-term data on productivity, survival and total population size can allow trends in population behaviour to be modelled. Statistical models (we will compare GLMs, GAMs and Index Models) and simple matrix models can examine past and current behaviour but have low predictive power, whilst stage-structured stochastic models incorporating density-dependence and differential emmigration/immigration between sub-populations are potentially more useful. Sensitivity and elasticity analyses of these models, together with assessment of quasi-extinction risks, provide insights into the ecological constraints faced by populations, and thus allows for informed management decisions (e.g. such models of UK wintering populations indicate their greatest sensitivity to mortality rather than productivity, irrespective of population size - this has implications regarding hunting). We apply these modelling techniques across a range of long-term data on wildfowl held by WWT, and conclude that these analyses may provide general management scenarios for a wide range of species (avian and non-avian). However, predicting the response of populations to novel circumstances (e.g. anthropogenic change) requires the use of individual-based, game-theoretic models. We present a year-round model of the Svalbard Barnacle Goose Branta leucopsis population, where we also examine the effects of habitat change at different stages of the annual cycle. The use (and abuse) of these differing approaches in conservation biology will be stressed.

Key words: PVA, population models, statistical models, conservation biology, habitat loss

Clarke, M.F., Jones, D., Painter, J., Moysey, E., Griffiths, R., Crozier, R.H., Robertson, R. & Boag, P. 1998. Contrasting primary sex ratio biases within the co-operatively breeding genus Manorina. In: Adams, N.J. & Slotow, R.H. (eds) Proc. 22 Int. Ornithol. Congr., Durban. Ostrich 69: 215.

In the genus Manorina up to 19 helpers may assist in raising a brood of just two nestlings. We determined the sex of helpers and nestlings using the CHD-W method (a molecular sexing technique based on a coding sequence from the W chromosome called the CHD gene). Helpers were predominantly male in both species we studied; 92% in Bell Miners and 85% in Black-eared Miners. Since helpers in Bell Miners enhance the reproductive success of breeders, the Local Resource Enhancement hypothesis predicts that the primary sex ratio might be biased towards males. Indeed, we found a significant bias towards males (1.63:1) in a sample of 180 nestlings from 93 clutches. By contrast, preliminary analysis of the primary sex ratio in the endangered Black-eared Miner indicates a bias towards females; among 31 nestlings from 14 clutches, 20 were females. Given that females are the dispersing sex in both species, and avoid breeding in their natal colony, and colonies of Black-eared Miners may be occupying isolated, sub-optimal habitats, the female-biased primary sex ratio may be a response to Local Resource Competition, accelerating the decline of this species beyond what might be expected due to habitat fragmentation alone.

Key words: primary sex ratio, co-operative breeding, miners, local resource enhancement, endangered processes

Siriwardena, G.M., Baillie, S.R. & Wilson, J.D. 1998. The determination of farmland bird abundance: Survival or nesting success? In: Adams, N.J. & Slotow, R.H. (eds) Proc. 22 Int. Ornithol. Congr., Durban. Ostrich 69: 215 - 216.

Many species of farmland bird have undergone declines across Europe in recent decades, and although these declines have been linked to agricultural intensification, the exact mechanisms involved are unknown. Mechanisms may be found by first identifying changes in demographic parameters which have caused declines. Annual census data for British farmland bird populations were smoothed to reveal long-term trends, and objectively identified turning points used to define periods with particular trend directions. We present analyses of survival (ring-recovery models incorporating time- and age-specificity) and of nesting success (models of trends in nest failure rates, clutch size, etc. from nest records) for a range of species with contrasting population trends. Estimates of survival and nesting success during periods of increase, stability and decline, together with population models, show which has probably caused long-term trends in each species. Species such as goldfinch had higher survival in periods with healthier trends whereas others (e.g. linnet) had better nesting success in such periods and little variation in survival. These results show that even among phylogenetically and ecologically very similar species the demographic causes of population decline may differ, implying that many different responses to environmental change may underlie the widespread observed declines.

Key words: agriculture, conservation, population dynamics, declines, demography