Common Cuckoos Cuculus canorus are obligate brood parasites, laying eggs in nests of other species. Cuckoo nestlings are often thought to be insatiable, compared to host broods. However, in Reed Warblers Acrocephalus scirpaceus and a few other species, cuckoo nestlings are fed at most only as frequently as a host brood. To add to the small body of knowledge on feeding frequencies and body mass development of Cuckoo nestlings, I studied Cuckoos using another host, the Meadow Pipit Anthus pratensis, in the Dutch dunes (2019–2021). By filming feeding parents (hosts) I determined the provisioning frequency for broods of Pipits and for nestling Cuckoos. I found that nestling Cuckoos receive on average fewer feeds per hour than broods of Meadow Pipits. Furthermore, overall Cuckoos received as many feeds during their 22-day nestling period as a brood of Pipits during their 13 days in the nest. At 13 days of age (day 1 is the day of hatching), the single Cuckoo was as heavy as a brood of four Meadow Pipits. At 22 days, the maximum weights of the nestling Cuckoos were greater than the Meadow Pipit broods and varied between 91 and 105 g. Thus, although Cuckoos are being fed less frequently, young Cuckoos are heavier at fledging than a whole brood of Pipits. This could be because Cuckoos are fed larger prey. Alternatively, Cuckoos may require less food because their thermoregulatory costs could be smaller: they have almost black skin which absorbs solar radiation efficiently, do not have to compete with siblings in the nest and, once older and feathered, have a smaller surface-to-volume ratio than a Pipit brood of four nestlings.

Yellowhammers Emberiza citrinella and Pine Buntings E. leucocephalos hybridize in a c. 2500 km wide sympatric zone in northwest Asia. In the centre of this zone, over the last 50 years males with Yellowhammer and intermediate (i.e. hybrid) phenotypes have been replacing the Pine Bunting phenotype. In the southern part of the zone by contrast, the Pine Bunting phenotype is still common. In this study, we asked if there is a behavioural asymmetry between males depending on their phenotypes and whether this asymmetry can explain phenotypic compositions observed in different parts of the hybrid zone. This study was performed in 2017–2019 in Altai Republic, Russia, in the southern part of the hybrid zone. Songs of Yellowhammer and Pine Bunting are generally similar, although they differ in details. The species also differ in usage of call repertoires: the ‘see’ call is more characteristic for Yellowhammer, while the ‘zieh’ call is used more frequently by Pine Bunting. We performed playback experiments using contrasting stimuli to evaluate responses towards (1) Pine Bunting song vs. Yellowhammer song and (2) songs coupled with ‘zieh’ calls vs. songs coupled with ‘see’ calls. Experiments showed that Yellowhammer and hybrid males were more aggressive towards other males than were birds with the Pine Bunting phenotype. Therefore, the latter might be at a disadvantage in territory acquisition and maintenance. At the same time, males of any phenotype responded more strongly to Yellowhammer playback than to Pine Bunting playback, and to the ‘see’ call than to the ‘zieh’ call. This might counterbalance the competitive ability of different phenotypes in territorial competition, thus allowing Pine Bunting males to avoid being displaced by more aggressive Yellowhammer and hybrid males in the southern part of the hybrid zone.

Prey resources in oligotrophic tropical marine environments are often scattered and unpredictable. Tracking studies of tropical seabirds can provide critical information about ocean habitat affinities, prey choice and the utilisation of surrounding areas, which can be useful for conservation managers. Foraging studies of Red-billed Tropicbird populations in the Caribbean are scarce but increasing. We sought to expand on this by tracking chick-rearing adults using GPS devices and subsequently linking these tracking data to remotely-sensed environmental variables. We related our spatial data to opportunistic sampling of regurgitates in a globally significant nesting colony on Saba, Caribbean Netherlands. Diet samples were dominated by flying fish (Exocoetidae; numerical frequency: 70.73%), but prey items from the squid family (Loliginidae; 9.76%) and the families of flying gurnards (Dactylopteridae; 2.44%) and the ray-finned fish (Carangidae; 2.44%) were also identified, although we were unable to identify 14.63% of samples due to digestion. An additional goal of our study was to compare the foraging ecology of Tropicbirds on Saba with those nesting on St. Eustatius, located circa 25 km south-east. As expected, Tropicbirds nesting on Saba exhibited diurnal foraging patterns, travelling a maximum distance from the colony of 553.7 km, with an average trip length of 117.2 ± 144.6 km (±SD). Adults foraged in shallower, cooler waters with higher chlorophyll a concentrations and higher Exocoetidae species richness compared to travelling points. Despite the proximity of Saba and St. Eustatius, this is contrary to what was found for Tropicbirds nesting on St. Eustatius, where adults foraged in deeper waters with a low Exocoetidae species richness. However, Tropicbirds from Saba and St. Eustatius did exhibit some similarities in their foraging behaviour; specifically, foraging adults traversed multiple exclusive economic zones and marine protected areas, reinforcing our recommendation for nature managers in the Caribbean to create a transboundary network in order to effectively protect and conserve this species.

Flexibility in migratory behaviour may have significant consequences for the adaptability of birds to ongoing environmental changes. Shifts in arrival and departure dates, migration distance and even direction of migration have all been demonstrated over the last few decades in a variety of species, mostly passerines. Less well understood is the spatio-temporal dynamics of migration at so-called migratory divides, i.e. contact zones of populations with different migratory strategies. We used data on 58 White Storks Ciconia ciconia ringed as nestlings in a part of the Central European migratory divide in the Czech Republic during the period 1947–2018 and recovered during their first winter season in October to February, to test the hypotheses of (1) shortening migratory distance over the years and (2) a shift in the proportion of adopted migratory strategies being a proximate factor of changes in migration distance. Using regression analysis, we provided evidence of gradual linear decrease over the years in mean recovery distance of ringed nestlings during their first winter. Results of multinomial regression analysis showed that this decrease has been proximately driven by an increase in the proportion of storks recovered within 500 km from their natal site, a decreasing proportion of storks recovered on the eastern migratory flyway and an increasing proportion of storks recovered on the western migratory flyway. Evidence for Czech Storks migrating via the western migratory flyway first appeared in 2003. The apparent shift of young Storks to migrate via the western flyway from the Czech Republic is most likely a result of immigration by individuals from growing western populations. This leads to diluting of populations migrating via the eastern flyway with migrants using the western flyway. Our study provides evidence that behavioural flexibility may drive dynamics of migration at the migratory divide, with changes of similar magnitudes as those driven by genetic change, as reported in passerines.

Few studies have explored local and global environmental effects on the demographic rates of small seabird species such as storm-petrels. We analysed ringing data (from 2011–2022) to investigate the demographic parameters of a breeding population of Band-rumped Storm-petrels Hydrobates castro, which nest on Farilhão Grande Islet, Berlengas archipelago, Portugal. We used capture-mark-recapture analyses to estimate annual capture probabilities, apparent survival and abundance. The effects of environmental and capture effort-related variables on demographic parameters were then evaluated. The mean annual survival estimate was low (0.68 ± 0.02 SE) in comparison to other storm-petrel species, but this estimate substantially increased to 0.79 ± 0.02 after removing transient individuals. During the study period we determined breeding success in 10 years. In four of these years we also monitored breeding attempts with automatic cameras (2014–2017). Breeding success was low (0.56 ± 0.12 fledglings per active nest) with some observed cases of predation by Yellow-legged Gull Larus michahellis of both adults and chicks. The size of the Band-rumped Storm-petrel population was estimated at 1511.0 ± 266.3 SE individuals in 2017 when the mist netting effort was highest. There is no evidence that this population experienced a large decrease over the last 27 years and the trend from the previous 11 years seems to indicate a fluctuation rather than a clear decrease. The North Atlantic Oscillation Index showed a positive effect on the number of captured birds, while there was a negative effect from moonlight and local winds. We conclude that mist netting is a valuable method for long-term demographic studies on ground-nesting seabirds in which nests are difficult to access, but estimates of demographic parameters are influenced by environmental and capture effort-related variables.

Conservation status assessment of bird species is essential to prioritize conservation and monitoring efforts; however, this is not always possible to achieve due to lack of field data or scientific knowledge. In this context citizen science platforms can act as a data source to prioritize the conservation and research resources within a region or a given taxonomic group. Merging the available information on bird distribution areas from BirdLife International and field observations from eBird, the main citizen science birding app, we create a concern index, using the poorly known buttonquails (Turnicidae) as a case study. This concern index is based on two parameters: scarcity and uncertainty, which ultimately are based on two components, respectively. For every species, we defined scarcity as a combination of its frequency of occurrence (proportion of positive eBird checklists) and its relative range size, while uncertainty is a combination of the eBird effort (density of eBird checklists) and the range accuracy (proportion of positive eBird checklists within the BirdLife distribution area). We found a high correlation (Spearman r = 0.74) between our concern index and the IUCN threat categories for all buttonquail species. Then we apply this concern index to all buttonquail subspecies obtaining a ranked list for these non-assessed taxa, with some island endemic subspecies ranking very high together with the most endangered buttonquail species. Our approach is a very simple method to rank species within a given bird group and prioritize monitoring and conservation efforts. Moreover, it is also suitable for other taxonomic levels as subspecies or even for ecological units as populations, which normally lack a formal conservation status assessment.

Avian growth has been traditionally examined using logistic, Gompertz or von Bertalanffy non-linear equations. Although many studies have analysed nestling mass increase and the factors affecting growth rates, there is little information on nestling growth curves for several avian groups, such as Nuthatches (Sittidae). Moreover, the breeding biology of Nuthatches is still poorly studied because of their reluctance to use nest boxes. Here we examined the growth pattern in Eurasian Nuthatch Sitta europaea nestlings, in addition to studying the breeding biology of this species, in a mixed deciduous forest from Central Portugal during one year. The nestling growth was well fit by the logistic and Gompertz models, whilst the von Bertalanffy equation was less accurate, especially when predicting the asymptotic mass and the mass in the first development stages. Nestlings attained their maximum mass around the age of 18 days, while the largest daily mass increase was achieved when nestlings were 8 days old. Weather conditions did not affect nestling growth. Regarding the breeding biology, the mean date of laying of the first egg was 3 April, the mean clutch size was 5.43 ± 0.78 (±SD) eggs and the mean number of fledglings per clutch was 4.00 ± 2.09. The proportion of eggs that produced fledglings in a nest averaged 72%. Clutch size and number of fledglings were negatively correlated with the laying date. These results contribute to our understanding of Eurasian Nuthatch breeding biology and nestling development, the latter providing useful data for the standardization of ornithological practices, such as determining the appropriate fledgling ringing period.

This study reports on the diet of breeding West African Crested Terns Thalasseus albididorsalis on the basis of otoliths found in excrement (a mixture of pellets and faeces near nests). During the period 1998–2019, towards the end of the incubation period, excrement samples were collected from breeding colonies on islands, along the Atlantic coast from Mauritania to Guinea. In 31 samples with a total of 8956 otoliths we identified 53 fish families, 75 genera and 101 species. Twelve fish families occurred in more than 2% of the samples. The families of Haemulidae, Mugilidae, Pristigasteridae and Sparidae were particularly numerous (found in 10.5–15.7% of samples). The most numerous species were Bigeye Grunt Brachydeuterus auritus (9.8%) and West African Ilisha Ilisha africana (13.3%). There were marked differences in the diets of terns from the Banc d'Arguin (Mauritania), The Langue de Barbarie (northern Senegal) and all more southerly sites together (Senegal, The Gambia, Guinea Bissau and Guinea). On Ile aux Oiseaux, Delta du Saloum, Senegal, we found long-term changes in the diet of the terns with a marked decrease in Brachydeuterus auritus and the family Sparidae and a strong increase in Ilisha africana. Our analyses suggest that breeding West African Crested Terns are food generalists rather than specialists. This is surprising in view of the general trend in terns of the genus Thalasseus. There are indications that chick diets may differ from those of adults.

The conservation of Eurasian Curlews, whose numbers are declining, is aided through an understanding of environmental challenges faced by the birds throughout the annual cycle. In the Moray Firth, northern Scotland, 48 non-breeding Curlews were tagged with geolocators to describe their migrations (routes, destinations and timing), time of breeding and nest success. Based on 19 recaptures, breeding destinations included northern Scotland (21% of tagged birds), Norway (32%), Sweden (21%) and Finland plus Russia (26%). These percentages matched closely with the results from earlier ringing recoveries/sightings. The median last date in the non-breeding area varied for the Fennoscandian birds: 31 March for those migrating to Norway, 11 April for Swedish birds and 15 April for Finnish birds. The amount of migratory fuel accumulated prior to departure in early April was 10–13% of the late winter mass, sufficient for a 1000-km flight, which was similar to the median distance to Norwegian, but not Swedish or Finnish, destinations. Staging en route characterised the migrations to these latter destinations, resulting in longer migration times. Most of the Fennoscandian Curlews migrated across the North Sea with a tailwind component in spring, but all had a headwind component in autumn. A first-year bird remained in northern Scotland for the first summer, but migrated to Russia in the second and did not nest. Arrival on the breeding grounds varied significantly according to destination. Males and females incubated at different parts of the 24-hour cycle, even although there were 24 hours of daylight for most birds. Of birds that nested, 63% hatched clutches, some after a second attempt. Periods of 24 hours of daylight masked the initial departure from the breeding grounds, so only the latter part of the autumn migration and first date back at the non-breeding area were recorded (11 July), with no difference among the Fennoscandian breeding birds.

The White-tailed Eagle Haliaeetus albicilla may aggregate in areas with high fish supply during their autumn movements. Here, we report on data collected between 2008 and 2019 concerning numbers of White-tailed Eagles visiting fishponds located in NE Lithuania, to the east of the Baltic Sea during the autumn months. For comparison, between 2016 and 2018 we also surveyed White-tailed Eagles at another, similarly managed fishpond, located 110 km away in SE Lithuania. The number of White-tailed Eagles at the fishponds in NE Lithuania in October through December increased nearly three-fold between 2008 and 2019. The long-term increase occurred especially in November–December and much less in October, and this, together with sudden increases in numbers in certain years, suggests possible changes in their movement patterns. We found similar numbers of assembling eagles in the two fishponds in October, but in November there were significantly more eagles at the fishponds in NE Lithuania. The size of groups in autumn and the seasonal dynamics therein may differ at different stopover sites that are similar in size, harvesting practice and located in the same region. We suggest this could be due to differences in the surrounding environment.

The duration of parental care varies widely among bird species. The crested terns Thalasseus spp. continue to feed their chicks at least sporadically for several months after fledgling. We recorded provisioning of a juvenile Sandwich Tern T. sandvicensis by its suspected parent at the wintering grounds in southern Namibia, more than 10,000 km away from the nearest breeding site and more than six months after fledging.

Polymelia is a congenital defect characterized by an excessive number of limbs. Leg malformations are unusual and have rarely been reported in birds. The ultimate mechanism that regulates this type of abnormal development is not yet well understood. Here we report polymelia in a nestling of the European Pied Flycatcher Ficedula hypoleuca. We have ringed more than 12,100 Pied Flycatcher nestlings since 1991 and have never observed this malformation before. To our knowledge, there is no study indicating polymelia in any bird of the order Passeriformes. We observed and studied a Pied Flycatcher nestling that had two extra limbs fused at the pelvis. We used X-rays, Computerized Tomography (CT) scan and Magnetic Resonance Imaging (MRI). We describe the observed malformations and discuss potential causes. The malformation could be due to an embryonic cause, such as a parasitic twin (pygopagus parasitic) or a genetic mutation.

Seabirds have long lives, reproduce slowly and usually do not attempt to relay within the same breeding season after nest failure. In Sandwich Terns Thalasseus sandvicensis, renesting has never been documented despite the large number of studies on their breeding ecology and recorded prospecting behaviour after nest failure. In 2020, we observed how three GPS-tagged Sandwich Terns moved 115 km after nest failure for a second breeding attempt. One of these birds hatched a chick from the second clutch successfully. We hypothesize that synchronous breeding usually prevents Sandwich Terns from trying again after nest failure and that the relays observed in 2020 were facilitated by the unusual situation of a successful breeding colony that started three weeks later than all other colonies in the Netherlands.