No abstract

Summary1. In Britain, the song thrush Turdus philomelos is categorized as a species of high national conservation concern because of a large population decline during the last three decades. We calculated a series of annual national population estimates for woodland and farmland habitats combined for the period 1964-2000. We then used turning points analysis to identify seven blocks of years within the period of decline with uniform rates of population change in the smoothed trend. 2. We used recoveries of song thrushes ringed as nestlings, juveniles and adults during April-September to estimate survival rates separately for the post-fledging period, the remainder of the first year and for adults. Daily survival probability was lower during the post-fledging period than during the remainder of the first year or for older birds. 3. There was evidence for variation in survival rates among blocks of years with different rates of population change, particularly for first-year survival. There were significant positive correlations across blocks between mean population multiplication rate (PMR) and both post-fledging and first-year survival. 4. Survival of first-year birds was correlated negatively with the duration of the longest run of frost days and the survival of adults was correlated negatively with the duration of the longest summer drought. Variation among blocks in mean PMR was correlated with block means of the duration of runs of frost days and drought days, but significant correlations between PMR and both post-fledging and first-year survival remained after allowing for the influence of weather on survival. 5. Changes in survival in the first winter, and perhaps also the post-fledging period, are sufficient to have caused the song thrush population decline. The environmental causes of these changes are not known, but changes in farming practices, land drainage, pesticides and predators are all candidates. Adverse weather conditions contributed to the decline, but were not the primary driver.

Latitudinal gradients in population dynamics can arise through regional variation in the deterministic components of the population dynamics and the stochastic factors. Here, we demonstrate an increase with latitude in the contribution of a large-scale climate pattern, the North Atlantic Oscillation (NAO), to the fluctuations in size of populations of two European hole-nesting passerine species. However, this influence of climate induced different latitudinal gradients in the population dynamics of the two species. In the great tit the proportion of the variability in the population fluctuations explained by the NAO increased with latitude, showing a larger impact of climate on the population fluctuations of this species at higher latitudes. In contrast, no latitudinal gradient was found in the relative contribution of climate to the variability of the pied flycatcher populations because the total environmental stochasticity increased with latitude. This shows that the population ecological consequences of an expected climate change will depend on how climate affects the environmental stochasticity in the population process. In both species, the effects will be larger in those parts of Europe where large changes in climate are expected.

During the last 30 years, there have been marked declines in the populations of many British songbirds breeding on farmland, while two of their main predators, sparrowhawk (Accipiter nisus) and magpie (Pica pica), have spread back into areas from which they had disappeared. The causes of the songbird declines remain unclear but given the coincidence in timing, it might appear that increased predation could be responsible. Although many studies have failed to ¢nd links between changes in the populations of breeding songbirds and mortality from avian predators, previous work has, with few exceptions, involved only short-term studies on small spatial scales. Here we use large-scale, long-term data from a national bird census scheme to examine whether magpies and sparrowhawks could have depressed the rates of year-to-year population change in 23 songbird species. Our results indicate that magpies and sparrowhawks are unlikely to have caused the songbird declines because patterns of year-to-year population change did not di¡er between sites with and without these predators.