Thorsten Johannes Skovbjerg Balsby

In Denmark, the Danish Environmental Protection Agency conducts nationwide systematic monitoring (NOVANA) of breeding birds listed under Annex I of the Birds Directive, but monitoring is restricted to designated Special Protection Areas (SPAs). Because several Annex I species also breed outside SPAs, national population estimates reported to the EU Commission (in accordance with Article 12 of the Birds Directive) have therefore largely relied on expert assessments. Here, we test a reproducible approach for national population estimation for nine Annex I breeding bird species by integrating NOVANA counts inside SPAs with opportunistic citizen science records from Birdlife Denmark’s database DOFbasen outside SPAs. For DOFbasen, we used records from the most recent five-year period (2019 – 2023) and, for each site, retained the maximum number of breeding pairs after filtering by breeding evidence and phenology. Finally, national totals were calculated as NOVANA + DOFbasen. Comparison with the Danish Breeding Bird Atlas (2014 – 2017) and expert assessments indicates that the approach provides reliable national estimates for four species (Bittern, Black-tailed godwit, Dunlin, Ruff), minimum estimates for two (Common crane, Marsh harrier), maximum estimates for two highly fluctuating species (Spotted crake, Corn crake), and an underestimate for one (Eagle owl). Reliable performance was associated with small and/or declining populations concentrated in SPAs and/or high detectability, whereas large, widely distributed populations, strong interannual fluctuations, and low detectability outside SPAs reduced robustness. To conclude, the method shows promise for several species, but also highlights the challenges associated with fluctuating and large populations and the inherent uncertainty in opportunistic citizen science data.

Night-brooding of nestlings in cavity-nesting avian species carries predation risks to parents. Anecdotally, several species are known to shift from constant adult night-brooding behaviour to leaving nestlings unattended at night during offspring development but the timing, speed of change and sex-specific differences between parents, and the factors shaping this behaviour have rarely been described. Moreover, the location and nature of night roosts used by adults whilst provisioning nestlings has received little research attention. We studied breeding Common Starlings Sturnus vulgaris and hypothesised that, in such a cavity-nesting species, 1) nestlings would only be night-brooded until they achieved thermal independence, 2) since the species is frequently polygynous, female parents would most likely exclusively night-brood offspring despite provisioning by both sexes and 3) night-brooding would be more likely during nights with lowest temperatures. Nightly video recordings throughout the nestling phase at eight Starling nests together with data from 18 Starlings fitted with GPS-loggers during 26 nights provided support for hypotheses 1) and 2), while we found no support for hypothesis 3). All tagged male Starlings always roosted far from the nesting site (up to 8 km) independent of nestling age; all females brooded nestlings, usually up to the first seven-nine days after hatching (when the nestlings achieve thermoregulation), but roosted with males after day 10, when all tagged Starlings from the same nesting ‘colony’ roosted together. These results confirm differential sex-related parental effort in provisioning Starlings, suggesting females only night-brood until young achieve homeothermy.

Great Cormorants Phalacrocorax carbo sinensis use vocal communication during the breeding season, with males being particularly vocal earlier in the breeding cycle and showing individual variation ...

We analysed intraclutch egg-size variation over the laying sequence in relation to clutch size, and the relation between clutch size and female body condition, in the Common Eider Somateria mollissima during an 8-year period. The aim was to assess if eiders adjusted egg size within the laying sequence depending on clutch sizes in response to body condition, as such an adjustment could have adaptive implications on reproductive success through a size advantage for the hatchlings. The analyses were performed on a population level; and then at the individual level using data from recaptured females that changed clutch size between years. Based on 1,099 clutches from 812 individual females, population clutch size averaged 4.13 eggs (range: 1–6), with 4- and 5-egg clutchesconstituting c.70% of all clutches, taking turns in being the most represented clutch size. Clutch size was positively related to female pre-laying body condition at both the population and individual levels. Egg size varied significantly within and between clutch sizes and changes were significantly related to the laying sequence. First eggs were significantly larger in 4-egg clutches and second eggs marginally smaller than in 5-egg clutches, a pattern also found among individual females changing clutch size between years. The relationship between female pre-laying body condition and clutch size, and the intraclutch egg-size pattern indicate that both clutch size and egg size are actively adapted to the pre-breeding body condition of the female. As egg size potentially optimise reproductive success through a size advantage in hatchlings, the observed pattern of intraclutch egg-size variation suggests that female eiders possesses a finely tuned conditional dependent mechanism that may optimize reproductive output in years were females are in suboptimal body condition for breeding.

Increasing goose population sizes gives rise to conflicts with human socioeconomic interests and in some circumstances conservation interests. Grazing by high abundances of geese in grasslands is postulated to lead to a very short and homogeneous sward height negatively affecting cover for breeding meadow birds and impacting survival of nests and chicks. We studied the effects of spring grazing barnacle geese Branta leucopsis and brent geese Branta bernicla on occupancy of extensively farmed freshwater grasslands by nesting and brood‐rearing waders on the island Mandø in the Danish Wadden Sea. We hypothesized that goose grazing would lead to a shorter grass sward, negatively affecting the field occupancy by territorial/nesting and chick‐rearing waders, particularly species preferring taller vegetation. Goose grazing led to a short grass sward (Tringa totanus, black‐tailed godwit Limosa limosa, oystercatcher Haematopus ostralegus and lapwing Vanellus vanellus as well as by chick‐rearing black‐tailed godwit and lapwing was short vegetation height. Distance to shrub cover and elevation were less important. Hence, despite very intensive goose grazing, we could not detect any negative effect on the field occupancy by nesting nor chick‐rearing waders, including redshank and black‐tailed godwit, which are known to favor longer vegetation to conceal their nests and hide their chicks. Possible negative effects may be buffered by mosaic structures in fields and proximity to taller vegetation along fences and ditches.

Cooperation is a complex behaviour found in many kinds of organisms and occurs between individuals of the same and different species. Several studies have examined the intentionality of this behaviour by testing the animals’ understanding of the need for a partner when working in pairs. The mammalian species tested express such understanding, whereas most tested birds fail, especially when the test involves a delayed access to the setup by one of the co-operators. In the present study, the cooperative problem-solving capability of four peach-fronted conures (Eupsittula aurea) was investigated with the loose string test. All four parrots solved the paradigm by simultaneously pulling the ends of the same string to bring a platform with a food reward within reach. They were also capable of solving the task when one of the co-operators was delayed, even when visually isolated from each other. To further test their comprehension and to exclude the birds relying on task-associated cues, we video-recorded the trials and quantified possible cues and strategies for timing the pulling behaviour (e.g., sound of the partner’s door when opening, sound of steps of partner approaching). The preferred cue to start pulling was to wait for their partner’s arrival to the string. The number of vocalisations was significantly higher during visually isolated conditions and for successful trials compared to failed trials, suggesting possible information exchange. Our findings show that peach-fronted conures can solve a cooperative task, and that cooperation success is not determined by external cues or by partner identity or affinity.

Lead is a widely used and toxic heavy metal that poses a serious hazard to wildlife species and their ecosystems. Lead is used for production of hunting ammunition. Via gunshot or rifle projectiles, it spreads in ecosystems and may end up in predators and scavengers feeding on wounded or dead animals shot with lead-based ammunition. To assess to what degree Danish raptors are subject to lead contamination, we measured the content of lead in liver tissue from Danish birds of prey (n = 137). Additionally, the study included values for 54 other trace elements. In our analysis, emphasis was put on interpretation of lead levels. Levels of cadmium, mercury and selenium were also discussed, while data for the remaining elements were provided for reference purposes. Bismuth was included to assess if lead originated from bismuth gunshot used as an alternative to lead shot. Concentrations of lead, cadmium, mercury and selenium were generally below the levels in similar studies of birds of prey in other northern European countries and none exceeded known and generally accepted threshold values for adverse health effects. As for lead, this is possibly related to the phase out of lead shot for hunting since 1986. The study confirms results from other studies showing that bismuth shot contains traces of lead that is deposited with bismuth in the target animal.