Ecology of a Two-barred Crossbill (Loxia leucoptera) irruption to Norway in 2019–20: altitudinal migration and interspecific habitat differences

Cover photo: A male Two-barred Crossbill (Loxia leucoptera). Photo: Frode Falkenberg. Two-barred Crossbills (Loxia leucoptera) have cyclic irruptions to Norway, but are generally uncommon and breeding is rare. Here I analyse data on a large irruption occurring in 2019–20 to assess the magnitude of the irruption and the ecological niche of the species. The irruption lasted one year, starting in July 2019 and ending in June 2020. Total numbers reported by birdwatchers to the website of the National Biodiversity Information Centre in Norway were ca. 7,000 individuals. Breeding indications were reported from nearly 100 sites. Analyses of elevation of records indicated that birds were often seen at low elevations before the breeding season in February–June, but moved to higher elevations during the breeding season. In a focal study area in SE Norway, breeding season surveys along elevational gradients indicated that Two-barred Crossbills occurred at higher elevations, and often close to summits of hills, perhaps representing preferences for more open forest habitats. Two-barred Crossbills often co-occurred with other seed-eating bird species, but presence was more closely related to numbers of Common Redpoll (Acanthis flammea), than to Eurasian Siskin (Spinus spinus) or cogeneric Common Crossbill (L. curvirostra). Similarly, the Common Redpoll also increased strongly in abundance with elevation, whereas the other two species did so to a lesser degree. These data suggest that the Two-barred Crossbill favors montane forests during the breeding season, and thereby has a different niche than the Common Crossbill which is distributed more widely across all elevations.

Use of two distant nesting areas as a breeding strategy of Golden Eagles (Aquila chrysaetos) in Valdres, southeast Norway

Sixteen occupied nesting territories of Golden Eagle Aquila chrysaetos were studied between 2000 and 2020, in the southern part of the valley of Valdres, southeast Norway. Most of the study area consists of rolling hills dominated by spruce Picea abies with numerous clear-cuts. It was concluded that at least six (about 40%) of the eagle pairs in these territories move back and forth between two alternate, distant nesting areas 5.3 km apart (average). In two additional pairs, a second nesting area was considered possible. In the remaining eight, only one nesting area was found. The periods of one nesting area in use varied from 2–19 years, before moving to the other nesting area. This result was supported by panoramic mid-day surveillance of the air space between the two nesting areas during 2014–2020. The maintenance of a second core nesting area is most likely a strategy for moving to a more favourable hunting area and might be initiated by a new mate in the pair. The move might also be influenced by avoidance of a close neighbouring pair. A switch of nesting areas, as indicated by this study, could significantly affect results, when the number of eagle pairs in a certain area is counted. Cover photo: Golden eagle. Photo: Frode Falkenberg.

Population size and trend, and habitat selection of Common Moorhens (Gallinula chloropus) in Oslo and Akershus, southeastern Norway

The Common Moorhen Gallinula chloropus is listed as vulnerable (VU) in Norway due to a small population size (estimated at 110–215 pairs in 2015). The population size is considered stable. More than one quarter of the population (30–70 pairs) is thought to occur in the region of Oslo and Akershus, SE Norway. In 2018, I conducted a comprehensive survey of known and potential breeding sites to assess current population size in Oslo and Akershus. I recorded 74 pairs in 58 sites. To analyse the population trend, I collected all known records of Common Moorhen during the breeding season for the period 1995–2018. Analyses indicated that population size was stable overall. However, compared to data from 1982, sites with the largest number of pairs in 1982 have had declining population size, and these sites also had high nutrient levels. On the other hand, several new breeding sites in recently created ponds in parks, on golf courses and wastewater treatment plants have been established. Dammed ponds were occupied more often than natural waterbodies, and occupied sites were in general at nutrient-rich sites at low elevation close to the coast. Occupancy rate (proportion of years surveyed with Common Moorhen presence) during 1995–2018 was higher for dammed ponds than for natural waterbodies, and higher for smaller wetlands. Thus, analyses suggested that the most suitable sites for Common Moorhen were nutrient-rich small ponds at low elevation close to the coast, and in such sites the Common Moorhen appears to have a stable, but small population size. Cover photo: Common Moorhen. Photo: Arild Breistøl.

Osprey observed when drowned by its prey

An Osprey Pandion haliaetus was observed by eight people as it was dragged under water by a fish and drowned. Several unsuccessful attempts to leave the water surface caused the Osprey gradually to sink in the water until it disappeared. The area was searched by boat, but the fish and Osprey were gone, and it was confirmed that there were no fishing nets in the area. This is a rare event, and sometimes rejected by scientists. The good view by eight persons, and detailed descriptions I received from two independent observers make the observation credible. The Osprey was an adult bird. In addition to traditional explanations that claws can become fastened to scales, bones or flesh of the fish, I propose that muscle spasm may be a reason why the osprey was not able to release its grip. Cover illustration: Fishing Osprey. Photo: Jørn Bøhmer Olsen.

Breeding biology and long-term population dynamics of the Pied Flycatcher Ficedula hypoleuca in Skibotn, Northern Norway

The breeding biology and population dynamics of the Pied Flycatcher Ficedula hypoleuca in pine, mixed and deciduous forests in Skibotn, Northern Norway (69°20’N, 20°20’E), was studied during 33 years (1987–2019; in total 1775 completed first clutches). It was a period during which summer temperatures remained relatively stable, but late spring (May) temperatures tended to become warmer. The number of breeding pairs declined over the study period but varied less than in populations living in more harsh subalpine environments. In spite of the northern location, females started to lay eggs in a relatively late phenological phase of the environment and in relatively warm weather. The date of birch leafing and the date of egg-laying advanced during the study period, and they correlated with each other. Mean clutch size was 6.4 eggs, and mean number of fledglings/nest 4.6. Date of egg-laying was earlier and clutch size larger in deciduous forests than in pine and mixed forests, but this did not translate into better fledgling productivity. The main causes of nesting failure were predation by small mustelids and starvation in warm and dry summers. However, in the long run the population seemed to be able to maintain itself without immigration, i.e., act as a source population. The possibility that during warm climate periods of the past, breeding biology and population dynamics of Pied Flycatchers over large geographical areas might have resembled those in Skibotn is discussed.

Breeding population increase and range expansion of the Great Crested Grebe Podiceps cristatus in southeastern Norway

The Great Crested Grebe Podiceps cristatus is listed as near-threatened on the Norwegian red list due to small population size, estimated in 2015 at 220–380 pairs. Population size is considered to be stable. Approximately one quarter of the population (50–100 pairs) is thought to occur in Oslo and Akershus, but this estimate is not based on detailed data, and the only previous systematic estimate was 90–100 pairs in 13 sites in 1982. In 2018, I conducted a comprehensive survey of all known and potential breeding sites to assess current population size in Oslo and Akershus. I recorded 233 pairs in 34 sites, suggesting a large increase in population size. To analyse the population increase in more detail, I collected all known records of Great Crested Grebes during the breeding season for the period 1995–2018. Analyses confirmed that there has been a strong increase, at a yearly rate of 4.2%. The increase was both due to increases in already established populations (69% of total increase), and establishment of new sites (31%). New sites were colonised in particular the last 10–15 years, and new sites were located gradually further away from the sites that were already used in 1982, indicating continuous range expansion. New sites were also located in smaller, less nutrient-rich lakes at higher elevations, perhaps indicating occupation of lower quality sites that could limit further population increase. Similar population increases have also occurred in other parts of the distribution range of Great Crested Grebes in Norway, and I present a new population size estimate for Norway at 531–634 pairs. Hence, the species no longer qualifies for red listing, and one may consider to downlist the species to least concern.

Using δ2H stable isotope analysis to identify the origin of a first winter Red-breasted Goose Branta ruficollis caught in Norway

We analysed the stable isotope of hydrogen (δ2H) in first generation median wing covert feathers sampled from a free-flying first winter Red-breasted Goose Branta ruficollis caught in Storøya, Norway in late April 2019. The highly depleted δ2H values corresponded to those predicted if the bird had grown feather in the Russian Arctic breeding areas of this species and were much more highly depleted compared to predicted values of a bird raised in captivity in Norway or elsewhere in western Europe. Although these data do not provide irrefutable proof of the individual’s wild origin, these results strongly suggest that even records outside the normal range for this species can potentially constitute a genuinely wild-reared individual.

Reindeer carcasses provide foraging habitat for several bird species of the alpine tundra.

On August of 2016, 323 wild tundra reindeer (Rangifer tarandus) were struck by lightning on southern Norway, killing virtually the entire herd. During fieldwork for parallel studies in 2017, we opportunistically registered the occurrence and behavior of avian species on carcasses from this mass die-off. Several passerine species other than corvids were observed actively foraging on arthropods that are typically associated with carcass decomposition, such as blowfly (Calliphoridae sp.) adults and larvae. In decreasing order of abundance, five passerine species (excluding corvids) were observed taking arthropods at the site: the Meadow Pipit (Anthus pratensis, least concern), Northern Wheatear (Oenanthe oenanthe, least concern), Common ReedBunting(Emberizaschoeniclus, near threatened), Bluethroat (Lusciniasvecica, near threatened), and Lapland Bunting (Calcarius lapponicus, vulnerable) (status according to IUCN). Previous studies on these passerines have documented a very sharp decline in their abundance in Norway. The insect abundance associated with ungulate carcasses may provide an important food resource for such species, and may impact their life history, fitness, and eventually population dynamics. This mechanism, however, has received almost no attention from the scientific community, and warrants further investigation.

Woodpeckers in Western Norway: the White-backed Woodpecker is still the most common species

The pine-dominated forests of Western Norway have been found to harbour viable populations of woodpeckers, including the highly specialized White-backed Woodpecker Dendrocopos leucotos. The aim of this study was to investigate to what extent there were any changes in frequencies of woodpeckers, in particular the White-backed Woodpecker and the Grey-headed Woodpecker Picus canus, by resurveying 60 plots (each 1 km2) originally surveyed during 1994/1995. The resurvey was performed in 2013/2014. The White-backed Woodpecker was found to be the most common woodpecker species in both time periods. The Grey-headed Woodpecker was found to have a statistically significant decline from 27% of the 60 plots in 1994/95 to only 12% in 2013/14. The other four species all increased in frequency; although none of those increased frequencies were found to be statistically significant. We discuss possible explanations to why pine forests in Western Norway constitute a valuable habitat for the White-backed Woodpecker at the same time as it has drastically declined in other parts of Norway and Western Europe. In general, the reduced frequency of Grey-headed Woodpecker is not fully understood, although we suggest that cold winters during the years prior to the surveys in 2013/14 may be an important factor.

Population decline of the Eurasian Curlew in Akershus (southeastern Norway)

The Eurasian Curlew (Numenius arquata) is a species in decline, classified as near threatened (NT) worldwide, and vulnerable (VU) in Norway. In Akershus county, southeastern Norway, the population of breeding Eurasian Curlews was estimated at 50-60 pairs in 1982. No recent update exists of this population size estimate. In this study, we assessed the population size in 2017 in Akershus, and examined how the population size changed between 1971 and 2017 by using historical observation records of Eurasian Curlews. We estimated that there were 30 territories in Akershus in 2017 and found that the population declined by 47% since 1995 and 77% since 1971. In the period 1995-2017, the yearly rate of decline was 2.8%. We discuss possible reasons for the decline, such as intensive agricultural practices, high nest predation rates, and large-scale threats on the wintering grounds.