Advancement of spring arrival in a long-term study of a passerine bird: sex, age and environmental effects.

In migratory birds, mistimed arrival might have negative consequences for individual fitness, causing population declines. This may happen if arrival time is not synchronized with breeding time, especially when earlier springs favour earlier reproduction. We studied spring arrival time to the breeding areas in a pied flycatcher Ficedula hypoleuca population in southern Norway during a 30-year period (1985-2014). We investigated trends in arrival both for the entire population and for different population fractions (e.g. early vs. late arrivals). We also studied sex and age class differences, along with repeatability of arrival. Finally, we explored how arrival is influenced by environmental conditions at the areas birds use throughout the year, using mixed-effects models and quantile regressions with individual-based data. Spring arrival advanced over five days, at a similar rate through the entire population. Males and adult birds arrived earlier than females and yearlings. Arrival was significantly repeatable for males and females. Birds arrived earlier in years with high temperature and rainfall at the breeding grounds, and low NDVI both on the Iberian Peninsula and in central Europe. Later fractions of the population showed a steeper response to these environmental variables. This intra-population heterogeneity in the responses to the environment probably stems from a combination between the different selection pressures individuals are subject to and their age-related experience. Our results highlight the importance of studying how migration phenology is affected by the environment not only on the breeding grounds but also on the other areas birds use throughout the year.

To make the most of what we have: extracting phenological data from nestling measurements.

To study the ecological and evolutionary effects of climate change on timing of annual events, scientists need access to data that have been collected over long time periods. High-quality long-term phenology data are rare and costly to obtain and there is therefore a need to extract this information from other available data sets. Many long-term studies on breeding birds include detailed information on individually marked parents and offspring, but do not include information on timing of breeding. Here, we demonstrate how a study of repeated standard measurements of white-throated dipper Cinclus cinclus nestlings in our study system in southernmost Norway can be used for modeling nestling growth, and how this statistical model can be used to estimate timing of breeding for birds with sparser data. We also evaluate how the accuracies of nestling growth models based on different morphological traits (mass and feather length) differ depending on the nestling age, present user guidelines and demonstrate how they can be applied to an independent data set. In conclusion, the approach presented is likely to be useful for a wide variety of species, even if the preferred measurement may differ between species.

Challenging claims in the study of migratory birds and climate change.

Recent shifts in phenology in response to climate change are well established but often poorly understood. Many animals integrate climate change across a spatially and temporally dispersed annual life cycle, and effects are modulated by ecological interactions, evolutionary change and endogenous control mechanisms. Here we assess and discuss key statements emerging from the rapidly developing study of changing spring phenology in migratory birds. These well-studied organisms have been instrumental for understanding climate-change effects, but research is developing rapidly and there is a need to attack the big issues rather than risking affirmative science. Although we agree poorly on the support for most claims, agreement regarding the knowledge basis enables consensus regarding broad patterns and likely causes. Empirical data needed for disentangling mechanisms are still scarce, and consequences at a population level and on community composition remain unclear. With increasing knowledge, the overall support ('consensus view') for a claim increased and between-researcher variability in support ('expert opinions') decreased, indicating the importance of assessing and communicating the knowledge basis. A proper integration across biological disciplines seems essential for the field's transition from affirming patterns to understanding mechanisms and making robust predictions regarding future consequences of shifting phenologies.

Climate effects on population fluctuations of the white-throated dipper Cinclus cinclus.

1. Climate change may have profound consequences for many organisms. We have studied fluctuations in a population of the white-throated dipper Cinclus cinclus during 31 years (1978-2008) in a river system in southern Norway in relation to both large-scale and local weather conditions occurring during the non-breeding season. 2. Multiple regression and partial least squares regression were used to model the growth rate of the population, accounting for population size in the previous year. 3. Population growth was influenced by North Atlantic Oscillation (NAO), mean winter temperature, precipitation and timing of ice formation on the main lake in the river system in autumn. These variables explained 84% of the variation in population growth over the 31-year study period. 4. Local winter conditions played a prominent role in explaining the population fluctuations, which is plausible because the dipper depends on open water for foraging. In the study area, winters can be harsh and rivers and lakes may freeze and severely affect the subsequent population size of the dipper in spring. 5. The breeding population of the dipper does not seem yet to have reached a level where all possible territories in the area have been occupied, even after mild winters, and the estimated carrying capacity is also decidedly lower (66 breeding pairs) than the number of available territories. If the trend of milder winters continues, the population might increase in the future. However, strong climate variation is expected to continue in the future, and hence periods of rapid growth of the dipper population will probably be followed by severe declines.

Rapid advance of spring arrival dates in long-distance migratory birds.

Several bird species have advanced the timing of their spring migration in response to recent climate change. European short-distance migrants, wintering in temperate areas, have been assumed to be more affected by change in the European climate than long-distance migrants wintering in the tropics. However, we show that long-distance migrants have advanced their spring arrival in Scandinavia more than short-distance migrants. By analyzing a long-term data set from southern Italy, we show that long-distance migrants also pass through the Mediterranean region earlier. We argue that this may reflect a climate-driven evolutionary change in the timing of spring migration.