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In many taxa, reproductive performance increases throughout the lifespan and this may occur in part because older adults invest more in reproduction. The mechanisms that facilitate an increase in reproductive performance with age, however, are poorly understood. In response to stressors, vertebrates release glucocorticoids, which enhance survival but concurrently shift investment away from reproduction. Consequently, when the value of current reproduction is high relative to the value of future reproduction and survival, as it is in older adults, life history theory predicts that the stress response should be suppressed. In this study, we tested the hypothesis that older parents would respond less strongly to a stressor in a natural, breeding population of common terns (Sterna hirundo). Common terns are long-lived seabirds and reproductive performance is known to increase throughout the lifespan of this species. As predicted, the maximum level of glucocorticoids released in response to handling stress decreased significantly with age. We suggest that suppression of the stress response may be an important physiological mechanism that facilitates an increase in reproductive performance with age.

Between 2002 and 2019, monitoring of Common Tern and Little Tern along the Sava River near Zagreb, Croatia was conducted. Natura 2000 site “Sava kod Hrušćice” was designated to protect colony at river islands, with estimated population sizes of 100–150 pairs of Common and 20 pairs of the Little Tern. Flooding of the colony caused breeding failure in several years. Common Terns moved to breed on islands in gravel pits with a total population around 150 pairs, while Little Tern did not breed after 2010. In the last few years, terns have not bred at Hrušćica and the only colony inside the Natura 2000 site is situated on a breeding platform at Siromaja gravel pit. Channelling of rivers and hydropower plants are the main threats, changing natural dynamics of water level and causing reduction of gravel sediment in rivers.

An overview of the long-term (1980–2019) population development of colonial Laridae species in continental part of Slovenia, their nest sites at anthropogenic water bodies, and various conservation measures with special focus on Common Tern along the Slovenian/border part of the Drava River is given. Breeding of these species occurs only on the westernmost fringes of the Pannonian plain, on lowland floodplains of the main rivers of the Danube Basin. Altogether, breeding of Common Tern and Black-headed Gull was recorded at 11 and 10 sites, respectively. Lake Ptuj is the single site with mixed-species colony residing there in all years of the study period, while at Ormož (two sites) it was established during the early 1990s in the Slovenian territory but moved completely to the Croatian side by the second half of the 2010s. At all other sites, a suitable breeding habitat became available or was provided by management in just a few years, or created only recently. The percentage of Common Tern national population breeding in continental Slovenia was usually well above 50% throughout the 1980s and 1990s (52–136 pairs), while in the last 16 years (77–258 pairs) it ranged between 40.8% and 69.0%. Breeding of Black-headed Gull remains largely limited to continental Slovenia. Overall, continental populations of both species in the last few years have been substantially higher compared to the most of the study period. Long-term trends were estimated as a moderate increase for Common Tern and a strong increase for Black-headed Gull. Since 2006, the Mediterranean Gull has been a regular breeder at Lake Ptuj (up to 28 pairs), the only such site in Slovenia. Three general types of conservation measures implemented at different nesting locations are described in detail: (1) measures to create/increase the total surface of breeding habitat – the construction of new breeding structures, such as artificial islands and nesting rafts, (2) measures to maintain and enhance breeding habitat through recurring management activities, and (3) measures aimed to increase chick/nest survival and improve breeding success.

Light‐level geolocators were used to record the annual migration cycles of black terns Chlidonias niger (9 individuals, 11 journeys) and common terns Sterna hirundo (7 individuals, 11 journeys) breeding in southernmost Sweden. The black terns used two different non‐breeding (Oct–Mar) regions along the Atlantic coast of Africa, either north of the equator between Senegal and Liberia (3 individuals) or south of the equator between Gabon and northern Namibia (5 individuals). All the common terns travelled to non‐breeding quarters south of the equator, mainly along the coasts of Namibia and South Africa. One juvenile common tern was tracked during the first twenty months of its life. This bird spent its first northern winter in South Africa, after which it migrated north of the equator to spend the northern summer as a one‐year‐old non‐breeder in tropical waters off Ghana, after which it returned to South Africa for its second northern winter. This record demonstrates that one‐year‐old terns may undertake extensive intra‐African migration to distant over‐summering areas. Comparing geolocator results from Swedish and Dutch black tern populations indicate that they have similar migration habits, with a possible tendency of relatively more individuals migrating south of the equator in the more northerly Swedish population (leap‐frog migration). Comparing geolocator and ringing results among common tern populations indicates a fascinating and complex pattern of scale‐dependent geographic segregation and intermixing along the coasts of Africa.

Little is known about the intestinal microflora of Common Terns (Sterna hirundo) and their capacity to disseminate human gastrointestinal pathogens along migratory flyways. Common Tern chicks on Pettit Island in Barnegat Bay, New Jersey, USA, were evaluated for carriage of bacterial and protozoan pathogens. Oropharyngeal and cloacal swabs were taken for culture-based detection of bacterial gastrointestinal pathogens during the 2009 and 2010 breeding seasons. Bulk fecal samples were also taken during the 2010 nesting season to determine Cryptosporidium oocyst loads. Of 125 birds sampled in 2009, none carried Salmonella and only one carried Campylobacter. Moreover, the majority of birds sampled in the 2009 breeding season carried Escherichia coli and Klebsiella ozaenae, members of bacterial family Enterobacteriaceae. In 2010, one of the 54 birds sampled carried Salmonella and none had Campylobacter in their intestines. Similar to the birds sampled in 2009, the 2010 cohort showed relatively high carriage of E. coli and K. ozaenae. Microscopic examination of fecal smears obtained in 2010 revealed that 39 of 54 Common Tern chicks had microscopic structures consistent with Cryptosporidium oocysts in their intestines. These data indicate that Common Tern chicks in Barnegat Bay present low to no threat to public health through the spread of human gastrointestinal pathogens.

Telomere length is associated with cellular senescence, lifestyle and ageing. Short telomeres indicate poor health in humans and reduced life expectancy in several bird species, but little is known about telomeres in relation to phenotypic quality in wild animals. We investigated telomere lengths in erythrocytes of known-age common terns (Sterna hirundo), a migratory seabird, in relation to arrival date and reproductive performance. Cross-sectional data revealed that, independent of age, individuals with short telomeres performed better: they arrived and reproduced earlier in the season and had more chicks in the nest. The latter effect was stronger the older the brood and stronger in males, which do most of the chick provisioning. Longitudinal data confirmed this pattern: compared with birds that lost their brood, birds that raised chicks beyond the 10th nestling day experienced higher telomere attrition from one year to the next. However, more detailed analysis revealed that the least and most successful individuals lost the fewest base pairs compared with birds with intermediate success. Our results suggest that reproductive success is achieved at the expense of telomeres, but that individual heterogeneity in susceptibility to such detrimental effects is important, as indicated by low telomere loss in the most successful birds.

The ongoing worldwide outbreak of high pathogenicity avian influenza (HPAI) H5 of the Goose/Guangdong (Gs/GD) lineage has caused unprecedented mortality amongst an ever-expanding number of avian and mammalian species. This includes black-headed gulls, which suffered high mortality from this virus in 2022 and 2023. We performed pathological and virological analyses on black-headed gulls and a common tern found dead in a breeding site in The Netherlands in spring 2023 in order to characterise the pathogenesis of HPAI Gs/GD virus infection in these species. We found a strong tropism of HPAI H5 Gs/GD virus for the central nervous system, pancreas, lungs, and the preen gland, with acute necrosis as the main associated lesion. Affected organs revealed high virus RNA loads and intralesional expression of virus antigen, confirming HPAI H5 Gs/GD virus infection as the cause of disease and mortality. Genetic sequencing showed that this HPAI H5 Gs/GD clade 2.3.4.4b virus belonged to the gull-adapted BB genotype. Comprehensive pathogenesis studies in a cohort of wild animals naturally infected with HPAI H5 Gs/GD virus are scarce, yet the knowledge gained may fill knowledge gaps to help mitigate future HPAI outbreaks.

In the context of rapid climate change, phenological advance is a key adaptation for which evidence is accumulating across taxa. Among vertebrates, phenotypic plasticity is known to underlie most of this phenological change, while evidence for micro-evolution is very limited and challenging to obtain. In this study, we quantified phenotypic and genetic trends in timing of spring migration using 8,032 dates of arrival at the breeding grounds obtained from observations on 1,715 individual common terns (Sterna hirundo) monitored across 27 years, and tested whether these trends were consistent with predictions of a micro-evolutionary response to selection. We observed a strong phenotypic advance of 9.3 days in arrival date, of which c. 5% was accounted for by an advance in breeding values. The Breeder’s equation and Robertson’s Secondary Theorem of Selection predicted qualitatively similar evolutionary responses to selection, and these theoretical predictions were largely consistent with our estimated genetic pattern. Overall, our study provides rare evidence for micro-evolution underlying (part of) an adaptive response to climate change in the wild, and illustrates how a combination of adaptive micro-evolution and phenotypic plasticity facilitated a shift towards earlier spring migration in this free-living population of common terns. Lay Summary Empirical evidence for evolutionary change underlying vertebrate adaptation to current global change is very rare. This may be due to phenotypic plasticity being the main mechanism underlying adaptation, or to challenges associated with the empirical testing of genetic changes in the wild, in particular data limitations. In this study, we tested whether an observed phenotypic advance in the timing of arrival from spring migration in a wild seabird population was due to an evolutionary response (i.e., genetic change) and/or to phenotypic plasticity or other non-genetic effects. To do so, we applied a bivariate “animal model” to a 27-year data set from a pedigreed population of common terns located at the North Sea coast of Germany. We found an evolutionary response to selection favoring earlier arriving individuals. Additionally, we could show that two different theoretical models predict a qualitatively similar evolutionary response as the one we estimated, both in terms of direction and magnitude. As such, our study provides a rare empirical case where estimated and predicted evolutionary responses agree, and suggests an evolutionary response in the timing of avian spring migration, although it played a smaller role than phenotypic plasticity in the common tern response to rapid climate change. Overall, these findings show the use of disentangling the relative, and often complementary, contributions of plastic and evolutionary changes to better understand adaptive processes and predict responses to future changes.

Seabirds introduce aquatically-derived nutrients into their terrestrial nesting environments, often leading to vegetative overgrowth that degrades nesting habitat suitability over time. In this study we capitalized upon the process of salt suppression that naturally occurs in salt-spray plant communities in order to reintroduce habitat heterogeneity (the mix of open substrate and plant cover) on a common tern (Sterna hirundo) breeding colony in the Gulf of Maine. In 2019, we randomly assigned hypersaline spray and control treatments within 10 experimental blocks across the site, and recorded plant community and seabird reproductive responses. We compared the habitat heterogeneity resulting from salt treatment to estimates of optimal habitat heterogeneity measured within productivity monitoring areas at this site during 2016–2020. We observed an average reduction of approximately 24% plant cover in hypersaline plots relative to control plots (t18 = −5.56, p < 0.001), constituting a substantial increase in nesting habitat heterogeneity. Common tern nesting density was 80% higher in treatment plots relative to controls (t18 = 2.6, P = 0.020), yielding 88% more chicks hatched per square meter (t18 = 3.3, P = 0.004). We suggest that the application of hypersaline spray may serve as an effective and practical habitat management technique in management-dependent systems such as tern nesting colonies.

While nest usurpation and subsequent incubation of eggs and even brooding of chicks from other species has been reported for Common Terns (Sterna hirundo), such behavior is considered rare. We report an observation of a Common Tern pair usurping the Least Tern (Sternula antillarum) nest and brooding the Least Tern chicks. While the Least Tern pair attempted to provide care for the chicks, the Common Terns displayed aggressive behavior and defended the nest. Though both species attempted to feed the chicks, no feeding events were observed due to harassment from the other species. Neither pair was observed nesting prior to or following this event, and all chicks are believed to have been lost to predation. We discuss the possible scenarios leading to the observed usurpation event, the possibility that usurpations are more common than previously believed, and the need for different monitoring methods to elucidate the causes of usurpations.