Theoretical studies have shown that variation in density regulation strongly influences population dynamics, yet our understanding of factors influencing the strength of density dependence in natural populations still is limited. Consequently, few general hypotheses have been advanced to explain the large differences between species in the magnitude of population fluctuations. One reason for this is that the detection of density regulation in population time series is complicated by time lags induced by the life history of species that make it difficult to separate the relative contributions of intrinsic and extrinsic factors to the population dynamics. Here we use population time series for 23 bird species to estimate parameters of a stochastic density-dependent age-structured model. We show that both the strength of total density dependence in the life history and the magnitude of environmental stochasticity, including transient fluctuations in age structure, increase with generation time. These results indicate that the relationships between demographic and life-history traits in birds translate into distinct population dynamical patterns that are apparent only on a scale of generations.

Summary1. The increasing population of cormorants (Phalacrocorax carbo sinensis) in Europe since 1970 has led to con¯icts with ®shery interests. Control of cormorant populations is a management issue in many countries and a predictive population model is needed. However, reliable estimates of survival are lacking as input for such a model 2. Capture±recapture estimates of survival of dispersive species like cormorants suer from an unknown bias due to permanent emigration from the study area. However, a combined analysis of resightings and recovery of dead birds allows unbiased estimates of survival and emigration. 3. We use data on 11 000 cormorants colour-ringed as chicks in the Danish colony Vorsù 1977±97 to estimate adult survival and colony ®delity. Recent statistical models allowing simultaneous use of recovery and resighting data are employed. We compensate for variation in colour-ring quality, and study the eect of population size and winter severity on survival, as well as of breeding success on ®delity by including these factors as covariates in statistical models. 4. Annual adult survival¯uctuated from year to year (0Á74±0Á95), with a mean of 0Á88. A combination of population size in Europe and winter temperatures explained 52±64% of the year-to-year variation in survival. Dierences in survival between sexes was less than 1%. Cormorants older than I12 years experienced lower survival, whereas second-year birds had survival similar to adults. Colony ®delity declined after 1990 from nearly 1 to I 0Á90, implying 10% permanent emigration per year. This change coincided with a decline in food availability. 5. Apparently, survival was more severely aected by winter severity when population size was high. This could be caused by saturation of high-quality wintering habitat, forcing some birds to winter in less good habitat where they would be more vulnerable to cold winters. There was thus evidence for density dependence in adult survival, at least in cold winters. 6. The high population growth rate sustained by European Ph. c. sinensis in the 1970s and 1980s can partly be accounted for by unusually high survival of immature and adult birds, probably caused by absence of hunting, low population density and high food availability.

Summary1. The population of great cormorants Phalacrocorax carbo sinensis breeding in northern Europe has increased from 5000 pairs around 1970 to c. 100 000 pairs in the late 1990s, leading to serious conflicts with fishery and aquaculture interests. Management action, including widespread culling, has been taken in several countries. 2. Since 1990, presumed density-dependent declines in demographic performance have appeared in cormorant populations. We employed an extended Leslie matrix model to study the interaction between culls and density-dependence in regulating breeding and autumn population sizes, with emphasis on evaluating the effects of culling. 3. During 1979-92, the breeding population of great cormorants in northern Europe increased by 18% year -1 , in accordance with observed life-cycle parameters before the appearance of density-dependent declines. 4. We modelled six scenarios with varying assumptions about the strength of densitydependence in adult survival and the proportions of breeding cormorants. A series of cull estimates was also included. Scenarios with moderate or strong levels of density-dependence provided predictions that fit the observed numbers of breeding pairs, whereas scenarios without density-dependence in survival overestimated real population growth. 5. The most well-supported scenarios indicated that the effect of culls at the present level (1998-99: 17 000 cormorants shot) was limited (< 10% reduction at equilibrium). Increasing the annual cull to 30 000 still had a limited effect, whereas shooting 50 000 birds year -1 led to population extinction within 20-40 years. Shooting a fixed proportion of the population exceeding a threshold, through density-dependent culling, could eliminate differences among scenarios and stabilize the population. 6. We conclude that culls probably have had a limited effect on cormorant populations, but if carried out in a density-dependent way they could stabilize numbers near a desired level. However, a reduction in the number of cormorants may not lead to a similar reduction in conflicts, and actions to control damage rather than cormorant populations are likely to be more cost-effective. If culling is to be continued, we recommend the adoption of an adaptive and co-ordinated management strategy across Europe. We also advocate the need to account for density-dependent mechanisms in general culling strategies.

Summary 1.A central question in population ecology is how to estimate the effects of common environmental noise, e.g. due to large-scale climate patterns, on the synchrony in population fluctuations over large distances. We show how the environmental variance can be split into components generated by several environmental variables and how these can be estimated from time-series observations. 2. With a set of time-series observations from different locations not necessarily covering the same time span, it is shown how the spatial autocorrelation of the residual variance component, not explained by the covariates and corrected for demographic stochasticity, can be estimated using classical multinormal theory. 3. Some previous results on spatial scaling in continuous linearized models on log scale are extended to also provide the scaling for the residuals. This is shown to be close to the spatial scaling of the autocorrelation in the environmental noise and only weakly affected by migration. 4. The logistic model of local population dynamics with the NAO index as the only covariate is fitted to 22 populations of the Continental great cormorant Phalacrorax carbo sinensis . The spatial scale of the environmental noise is estimated to be about 155 km. The NAO index alone accounts for about 10% of the total environmental variance and nearly all of the regional environmental variance (long-distance environmental autocorrelation).

While the factors influencing reproduction and survival in colonial populations are relatively well studied, factors involved in dispersal and settlement decisions are not well understood. The present study investigated exchanges of great cormorants Phalacrocorax carbo sinensis among six breeding colonies over a 13‐year period when the breeding population in Denmark increased from 2800 to 36 400 nests. We used a multistate capture‐recapture model that combined multisite resightings and recoveries to examine simultaneously recruitment, natal dispersal, breeding dispersal and annual survival of first‐year, immature and breeding great cormorants. Mean survival of first‐year birds (0.50±0.09, range=0.42–0.66 among colonies) was lower than survival of breeders (0.90±0.06, range=0.81–0.97). Mean survival of immature birds over the study period was 0.87±0.08. Dispersal from a colony increased with decreasing mean brood size in the colony in both first‐time and experienced breeders. The choice of the settlement colony in first‐time breeders was affected by conditions in the natal colony and in the colonies prospected during the pre‐breeding years. In particular, first‐time breeders recruited to colonies where they could expect better breeding success. Experienced breeders relied mainly on cues present early in the season and on their own breeding experience to choose a new breeding colony. Newly established colonies resulted mainly from the immigration of first‐time breeders originating from denser colonies. Dispersal was distance‐dependent and first‐time breeders dispersed longer distances than breeders. We suggest that the prospecting behaviour allows first‐time breeders to recruit in nearby as well as more distant potential breeding colonies. Dispersing breeders preferred to settle in neighbouring colonies likely to benefit from their experience with foraging areas. We discuss the importance of these movements for growth and expansion of the breeding population.