Henk P. van der Jeugd

Barn Swallows are known to use alternative reproductive strategies but resulting levels of extra-pair parentage differ widely between studies. In this one-year pilot study (2021) we examined extra-pair parentage in 50 broods across four Barn Swallow populations in The Netherlands, with a focus on whether these levels complicate the estimation of individual (lifetime) reproductive success based on field observations of breeding and provisioning behaviour. We found that 31.3% of nestlings from 56.0% of broods were of extra-pair origin. Of these nestlings, 94.9% were related to the social female but sired by an extra-pair male, while in 0.5% of cases the opposite occurred, involving an extra-pair female. Another 4.6% of nestlings were unrelated to either social parent, indicating ‘egg dumping’. Based on these results we found that visual observations overestimated the average reproductive output of cuckolded males by as much as 48%. Additionally, we observed that extra-pair offspring were not randomly distributed within broods, and there were no consistent differences between breeding locations. The complexity of alternative mating systems is further illustrated by the observation of a polygynous male with offspring in broods of four different females, and broods of two females that each contained offspring from three different males; while their social males may also engage in extra-pair copulations themselves. Overall, we conclude that an accurate assessment of individual reproductive success requires full-scale genetic sampling of local breeding populations.

Aim: To understand species population ecology, we need to study how trends of demographic drivers change over time and space, especially so under current rapid climate change. However, knowledge on long‐term trends of survival and productivity, especially using multiple species over large spatial scales, is scarce. Here, we examined the long‐term trends of adult survival and productivity of European songbirds, their relation to temperature gradients within species' ranges, and different traits across multiple species over large spatial scales. Location: Ten countries in Europe. Time Period: 2001–2021. Major Taxa Studied: 28 songbird species. Methods: We used bird ringing data from the European Constant Effort Ringing scheme (CES), with 1.2 million captures of birds. We investigated the long‐term trends of adult survival and productivity in relation to temperature gradients within species' ranges. We also tested differences in the long‐term trends of demographic measures in relation to species' migratory strategies (long‐distance migrants vs. short‐distance migrants and residents) and long‐term population trends. Results: There was no apparent major change in the long‐term trends of either adult survival or productivity, and they did not show differences along the range gradient. Long‐term trends of productivity differed between migratory strategies: long‐distance migrants showed more negative trends in productivity than short‐distance migrants and residents, while survival trends were similar between the two groups. Trends in both adult survival and productivity had equal positive connections with the population trends of songbirds. Main Conclusions: The difference in long‐term trends of productivity between migratory strategies highlights the importance of different conservation efforts for long‐distance migrants compared to short‐distance or resident birds.

Increasing urbanization poses significant risks to organisms confronted with new and rapidly changing environments. Long-term exposure to urban environments may involve changes in the immune system and its interrelated processes. A properly functioning immune system is essential for health maintenance, including the prevention of infection, so identifying and understanding the links between urbanization and immunological changes can offer insights into wildlife health and disease emergence. Here, we investigated the relationship between three measures of innate immune defenses in nestling and adult Turdus merula (Eurasian Blackbird) and the level of urbanization. Two of these immune defenses were positively correlated with urbanization in adults, but no relationships were found between urbanization and immune defenses in nestlings. Additionally, no apparent trade-offs between body condition and immune defenses measures were observed. Our results suggest that urbanization does not negatively impact immune defenses in T. merula, but immune defenses may be upregulated in adult birds to cope with the environmental stressors related to urban habitats. Our study highlights the complexity of immune regulation in heterogeneous environments, such as urban environments, and emphasizes the need for more empirical research to disentangle how specific ecological pressures shape immune function across life stages.

Background: Crimean-Congo haemorrhagic fever (CCHF) is a tick-borne disease endemic to Africa, Southern Europe, and Western Asia. Its main vectors, Hyalomma ticks, can spread to and possibly establish populations in non-endemic regions via migratory birds. Methods: We summarized the association between migratory birds and Hyalomma ticks by analysing spatial and temporal patterns in tick prevalence and infestation intensity on migratory birds through a systematic review of studies conducted in Europe, Western Asia, and Northern Africa between 1954 and 2022. Results: We reviewed 37 studies and retrieved data from one additional unpublished datasets. Overall, we collected data on the occurrence of 3876 ticks, most of which were in immature life stages, from 1553 individuals of 75 migratory bird species. The prevalence of ticks from both the Hyalomma genus and Hyalomma marginatum complex ticks (H. marginatum and H. rufipes ) on migrating birds declined significantly with increasing latitude of the sampling sites in spring. Additionally, we found that the infestation intensity of both tick groups on migratory birds was significantly higher among intra-Palearctic migrants than in Afro-Palearctic migrants. Conclusions: This review underscores the role of migratory bird species in spreading Hyalomma ticks across the Northern Hemisphere and highlights their potential as an early warning system for CCHF outbreaks. This study demonstrates the need to integrate migratory birds into CCHF surveillance systems as sentinels of Hyalomma tick dispersal, providing a reference framework for monitoring tick-borne risks along avian migration routes. Graphical

Long distance migrants like Barn Swallows have to operate within narrow time-frames to fit post-breeding migration, wintering (including wing moult) and pre-breeding migration into their annual cycle. As a short-lived passerine, Barn Swallows have evolved towards maximising annual reproduction in order to increase fitness. To achieve this, a timely arrival at the breeding location is an important precondition to realise the necessary two breeding attempts. Ringing recoveries indicate that Dutch Barn Swallows winter across large parts of Central and Southern Africa. But ringing recovery data have limitations to analyse spatial-temporal occurrence throughout the annual cycle. We used geolocators to describe spatial and temporal aspects during migration and wintering of 20 Barn Swallows, breeding in The Netherlands (2011–2013). The results of our geolocator research and the ringing research showed broadly similar northern and southern limits of the wintering areas: from Ghana to South Africa. However, the centres of gravity of both methods differed completely. Geolocator-based wintering areas were mainly in the Congo Basin, from which no ringing recoveries are known. We evaluated correlations between timing, duration and distance travelled for the different stages of the annual cycle. The result that a longer post-breeding migration distance did not lead to a later arrival at the breeding location in the subsequent year, was contrary to our expectations. Despite methodological improvements made between research years, the results of this study came with a ‘cost’ for the birds involved. In both research years, the return rates of both geolocator groups were 17 percent points lower than their respective control groups.

In the current warming climate, many organisms in seasonal environments advance their timing of reproduction to benefit from resource peaks earlier in spring. For migrants, the potential to advance reproduction may be constrained by their migration strategies, notably their ability to advance arrival at the breeding grounds. Recent studies show various changes in migration strategies, including wintering closer to the breeding grounds, earlier departure from the wintering grounds or faster travels by spending less time at stopover sites. However, whether such changes lead to earlier arrival or earlier breeding remains an open question. We studied changes in migration and reproduction timing in 12 populations of nine migratory birds, including seabirds, shorebirds, birds of prey and waterfowl breeding at Arctic sites bordering the Greenland and Barents Sea, a region undergoing rapid climate warming. The timing of migration and reproduction was derived from tracking and field data and analysed to study (1) how timing has changed in response to the changing moment of snowmelt at the breeding grounds and (2) what adjustments in migration strategies this involved. We found that in years with early snowmelt, egg‐laying in multiple populations advanced, but only two waterfowl populations also advanced arrival in the Arctic. In contrast, arrival in the Arctic generally advanced with time, even when snowmelt or egg‐laying dates did not advance. Earlier arrival with time was mostly explained by populations traveling to the Arctic faster, likely spending less time at stopover sites. Inability to forecast conditions in the Arctic may limit birds to adjust migration timing to annually varying snowmelt, but we show that several species, particularly waterfowl, are able to travel faster and advance the timing of migration over the years. The question remains whether this reflects adaptations to Arctic climate change or other factors, for example, environmental changes along the migratory route.

Emerging vector‐borne pathogens (VBPs) pose significant global health threats, occasionally causing widespread outbreaks with high wildlife mortality. Understanding climatic and anthropogenic drivers behind transmission dynamics in wildlife populations is key to mitigate health risks. Citizen science data may allow us to investigate the spread of pathogen outbreaks as it can be collected over a wide range within short time periods. In this study, we explored the use of wildlife mortality data to model the annual occurrence of VBPs. Making use of retrospective Eurasian blackbird Turdus merula mortality data collected within a citizen science program, we employed an ecological niche modelling framework to model the annual distribution of Usutu virus (USUV) in the Netherlands. Based on these modelled USUV occurrence probabilities, we assessed the impact of the USUV occurrence on blackbird populations. Our results reveal a rapid south–north spread of USUV across the Netherlands within three years with an average expansion rate of 91 km year ^‐1 . This spread led to significant population declines in blackbirds, reducing population numbers by 30% compared to pre‐USUV levels. Our study demonstrated that citizen science bird mortality surveillance can generate useful data to model the spatial transmission intensity of VBP over time that follows the declines in wildlife populations. Higher winter and spring temperatures, which may have facilitated early development of mosquito populations and resulted in greater mosquito abundance later in the year, had a positive impact on the USUV occurrence estimates. While our approach proved successful in modelling pathogen distribution using a well‐monitored species like the blackbird, it may be less effective for pathogens that cause minimal wildlife mortality or affect species that are harder to detect. This study provides a first example of a spatio‐temporal estimation of USUV transmission intensity in Europe, offering a framework for investigating the annual impact of USUV outbreaks on wildlife populations.

Disentangling contributions of different hosts to disease emergence and spread is highly complex, but critical for improving predictions, surveillance, and response. This is particularly challenging in wildlife, with pathogens often infecting multiple species and data collection being difficult. Using the emergence of Usutu virus (USUV) in the Netherlands as a case study, we demonstrate the use of an Approximate Bayesian Computation framework on diverse data sources to uncover hidden drivers of spatio-temporal wildlife disease emergence. We calibrated single- and multi-host mechanistic transmission models to five types of wildlife surveillance and research data, describing molecular and serological evidence of USUV in wild birds. Although Eurasian blackbirds, the primary sentinel species, were the most severely affected species, our models indicated that USUV could not persist in blackbirds alone. Our data-model framework provided statistical support for additional bird species to have contributed to transmission. This population of bird species is characterised by limited infection mortality, a longer lifespan, and likely further dispersal than blackbirds. Immunity in this reservoir population appears to have protected blackbirds from further USUV-related population decline. Continued surveillance could help identify these reservoir species. Our results underscore the importance of considering multiple host populations to understand outbreak dynamics. Neglecting the multi-host context of disease transmission can impact the reliability of future predictions and the projected impact of intervention strategies. Significance statement Understanding which species contribute to disease transmission is critical for improving predictions, surveillance, and response. However, disentangling these contributions is difficult, especially for wildlife diseases where pathogens often infect multiple species and data collection is notoriously difficult. Using Usutu virus (USUV) in the Netherlands as an example, we show how a computational framework can integrate diverse wildlife data sources to uncover hidden contributors to transmission. Our findings show that blackbirds, the most severely affected species, cannot sustain USUV transmission alone. Additional bird species, characterised by lower infection mortality and longer lifespans, are important for transmission and may protect more vulnerable species. This work emphasizes that studying transmission in a multi-host context is crucial for accurate outbreak predictions and effective interventions.