Comparing population trend estimates of migratory birds from breeding censuses and capture data at a spring migration bottleneck

Constitutive immune function is not associated with fuel stores in spring migrating passerine birds

Migratory birds may either upregulate their immune system during migration as they might encounter novel pathogens or downregulate their immune system as a consequence of trade-offs with the resource costs of migration. Support for the latter comes not least from a study that reports a positive correlation in autumn migrating birds between fuel stores and parameters of innate and acquired immune function, that is, energy-exhausted migrants appear to have lowered immune function. However, to our knowledge, no study has tested whether this pattern exists in spring migrating birds, which may face other trade-offs than autumn migrants. Here, we investigate if in spring there is a relationship between fuel stores and microbial-killing ability, a measure of innate immune function, and total immunoglobulin (IgY), a measure of acquired immune function, in four migrating bird species: chaffinches (Fringilla coelebs), dunnocks (Prunella modularis), song thrushes (Turdus philomelos) and northern wheatears (Oenanthe oenanthe). Our findings indicate no significant correlation between fuel stores and either microbial killing ability or IgY levels when considering all species collectively. When analysing species separately, we found a significant negative correlation between fuel stores and microbial-killing ability in chaffinches and a positive correlation between fuel stores and IgY levels in wheatears. In song thrushes, but not in any of the other species, there was a significant negative correlation between relative arrival date and microbial-killing ability and between arrival date and IgY levels. Sex did not affect immune function in any of the species. Our study suggests that the relationship between immune function and fuel stores may be different during spring migration compared to autumn migration. Differences in the speed of migration or pathogen pressure may result in different outcomes of the resource trade-off between investment in immune function and migration among the seasons.

Co-migration fidelity at a stopover site increases over time in African-European migratory landbirds

Migratory species are changing their timing of departure from wintering areas and arrival to breeding sites (i.e. migration phenology) in response to climate change to exploit maximum food availability at higher latitudes and improve their fitness. Despite the impact of changing migration phenology at population and community level, the extent to which individual and species-specific response affects associations among co-migrating species has been seldom explored. By applying temporal co-occurrence network models on 15 years of standardized bird ringing data at a spring stopover site, we show that African-European migratory landbirds tend to migrate in well-defined groups of species with high temporal overlap. Such 'co-migration fidelity' significantly increased over the years and was higher in long-distance (trans-Saharan) than in short-distance (North African) migrants. Our findings suggest non-random patterns of associations in co-migrating species, possibly related to the existence of regulatory mechanisms associated with changing climate conditions and different uses of stopover sites, ultimately influencing the global economy of migration of landbirds in the Palearctic-African migration system.

Long-Term Ringing Data on Migrating Passerines Reveal Overall Avian Decline in Europe

The loss of biodiversity is shaping today’s environment. Bird ringing is a citizen science research tool that can determine species population dynamics and trends over a large geographic area. We used a 17-year time series to assess population trends of 74 passerine species based on ringing data from autumn migration in Slovenia (south-central Europe). We defined seven guilds of species according to geographic location, ecological, migratory, breeding, and life-history traits. Almost all guilds showed declining trends, except for the group of species of northeastern European origin, which showed a stable trend. The greatest decline was in low-productivity wetland specialists. Forest birds, seed-eaters, and high-productivity species experienced the smallest declines. The general declines in avifauna across a range of life-history and behavioural traits, and across a range of spatial and ecological scales, suggest widespread environmental change in Europe. Our data indicates that recent trends are toward ecosystem homogeneity, with an impoverished avifauna, including a few species that are increasing in abundance. These are the species with higher productivity and flexible behaviour, such as short-distance migrants, that have the greatest chance of prevailing in the recently rapidly changing environment because of their ability to adapt to changes in a timely manner.

Diet of breeding Eleonora's falcon Falco eleonorae in Algeria: Insights for the autumn trans-Mediterranean avian migration

How environmental changes are affecting bird population dynamics is one of the most challenging conservation issues. Dietary studies of top avian predators could offer scope to monitor anthropogenic drivers of ecosystem changes. We investigated the diet of breeding Eleonora's falcon in an area of Northeastern Algeria in the years 2010-2012. Feathers and insect remains originating from prey plucking behavior were analyzed, providing insights into the seasonally changing diet of this raptor, as well as the trans-Mediterranean avian migration. A total of 77 species of birds (16 Sylviidae, 11 Turdidae, and 4 Emberizidae), 3 species of insects, and 1 lizard were identified among prey remains, reflecting a diverse diet. Diet composition and prey abundance varied seasonally, faithfully correlating with the passage of migrant birds as recorded from bird ring recoveries. Our findings suggest that dietary studies of predators might be deployed to investigate changes in bird migration. We discuss our results in the context of trans-Mediterranean migration, with early-season prey mainly comprising trans-Saharan migrants (Apus apus and Merops apiaster) and late-season prey being dominated by Mediterranean winter migrants (Erithacus rubecula, Turdus philomelos, Sylvia atricapilla, and Sturnus vulgaris). Notably, we observed a significant reduction in species richness of passerine remains in 2012, potentially highlighting a decline in the diversity of avian migrants.