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

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

Cover photo: Common Moorhen.  Photo: Arild Breistøl. 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. 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

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.   Cover photo: A pair of Great Crested Grebes with chicks. Photo: Frode Falkenberg

Relative magnitude of two Northern Hawk-Owl (Surnia ulula) irruptions to southern Norway: comparison of citizen data and survey data

Cover photo: Northern Hawk-Owl Surnia ulula. Photo: Terje Lislevand.  The Northern Hawk-Owl Surnia ulula occurs as an irruptive species to southern parts of Fennoscandia. Large numbers of individuals were recorded in the autumns of 2016 and 2020 but assessing the relative magnitude of irruptions is challenging. Systematic surveys allow direct comparisons but are time-consuming and will therefore be limited in time and space. Citizen data may provide large amounts of information for wide areas but may in particular suffer from spatial biases in the observation effort of birdwatchers. I compared the 2016 and 2020 irruptions of Northern Hawk-Owls to southern Norway, and found that citizen data indicated that numbers in 2016 were ca. 2–3 times larger than in 2020. However, the relative magnitude differed geographically, and the 2016 irruption was larger in western and southern counties, whereas the difference was smaller in eastern counties (in particular in Innlandet county). Systematic surveys in eastern regions (Oslo and Akershus) indicated that Northern Hawk-Owl densities were similar in the two irruption years. Overall, Northern Hawk-Owls were recorded at the same rate (approximately one owl per 16 km), and density was estimated to be 0.09–0.18 individuals/km2 in 2016 and 0.08–0.13 individuals/km2 in 2020. Thus, citizen data and survey data from the same geographical region concurred. However, due to the geographical variation in relative irruption size and spatial variation in observer density and biases in observation effort, the overall difference between the two years is difficult to assess. The 2016 irruption was likely larger than the 2020 irruption, but the difference was probably smaller than suggested by citizen data

Breeding population increase and range expansion of the Whooper Swan Cygnus cygnus in Oslo and Akershus

The Whooper Swan Cygnus cygnus has shown a remarkable breeding population increase and range expansion in Northern Europe during recent decades. Here, I summarize the temporal and spatial pattern of the expansion in Oslo and Akershus counties in southeastern Norway, and assess current and potential future competition with Mute Swans Cygnus olor. The first breeding was reported in 1999, and in 2003 breeding was reported at a second site. From 2006 the number of breeding sites increased rapidly, and the species has now been reported breeding from a total of 20 sites. However, at least seven sites have been abandoned after one or a few years of breeding, leading to temporary decreases in population size in some years. Abandoned sites had lower breeding success than sites that are still occupied. Current population size is 11–14 pairs. TRIM-analyses indicated a rate of increase of 7% per year. In about two thirds of the occupied sites, breeding was preceded by one or more years with presence of pairs that did not breed. Non-breeding Whooper Swans have been observed during summer (16 May–July) at a further 24 sites, suggesting that population size is likely to continue to increase. Oslo and Akershus also has an increasing population of Mute Swans, currently estimated at ca. 50 pairs, but Mute Swans have been recorded breeding at only three of the sites (15%) where Whooper Swans have bred, and at one of these sites there has been no temporal overlap. Thus, the two swan species have had limited interaction at breeding sites so far. However, among the 24 sites that have had non-breeding Whooper Swans during summer, Mute Swans have bred at 10 sites (42%, still present at most sites). This suggests that further expansion of Whooper Swans may soon lead to increased competition with Mute Swans, but there are also numerous other potential breeding sites without Mute Swans present

Lone male loser? Effects of spatial isolation on male pairing success in the ortolan bunting Emberiza hortulana.

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Changes in the breeding bird communities on mires and in surrounding forests in southeastern Norway during a 40-year period (1976–2015)

The breeding bird communities of 18 mires and surrounding forests in southeastern Norway were censused in 1976–77 and in 2015. We found that 53% of the mire species with sufficient data for analyses showed significant changes. Red-throated Diver Gavia stellata, Common Crane Grus grus, Wood Sandpiper Tringa glareola and Tree Pipit Anthus trivialis increased, whereas Northern Lapwing Vanellus vanellus, Black-headed Gull Chroicocephalus ridibundus, Meadow Pipit Anthus pratensis and Yellow Wagtail Motacilla flava decreased. There were also near significant decreases of Eurasian Curlew Numenius arquata and Eurasian Skylark Alauda arvensis. Population changes did not differ between short- and long-distance migrants. Species with marginal populations on mires declined more than mire specialist species, suggesting an indirect negative influence of problems in other habitats. Overall, there was a significant 19% decline in number of mire species, but a non-significant 7% increase in number of individuals. The bird community in the forests surrounding the mires showed significant population changes for 42% of the species with sufficient data for analyses, with increases in many resident forest species, but less so for migrants. Overall, there was a near significant 12% increase in number of forest species, and a significant 28% increase in number of individuals. Mire- and forest-associated species did not differ in population trends. Among short-distance migrants (mire and forest species combined), species wintering in agricultural habitats had more negative population changes than species wintering in other habitats. Thus, the breeding bird community on and around mires in this part of Norway has undergone large changes during the last 40 years. We discuss our results in relation to general trends of bird communities in northern Europe

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

A male Reed Warbler and Marsh Warbler hybrid (Acrocephalus scirpaceus × A. palustris) in Norway documented with molecular methods

We present molecular evidence for a hybrid between Reed and Marsh Warbler (Acrocephalus scirpaceus × A. palustris) from south-eastern Norway. The hybrid was a male and was shown to have a Reed Warbler mother and a Marsh Warbler father. Biometric measurements were intermediate between parent species, and the hybrid sang a mixed song, which in our sample was dominated by Marsh Warbler like sections. The hybrid was only seen sitting in reed, and hence, to our knowledge the habitat was most similar to that of Reed Warblers. We propose that documentation of song and habitat of hybrids for which maternal and paternal species are known is valuable for testing hypotheses concerning development of song and habitat selection

Ortolan Buntings Emberiza hortulana mimicking other species and other dialects of own species

We present new acoustically well-documented observations of Ortolan Bunting males from Norway, singing atypical songs with syllables copied from other species and from non-neighbouring, distant populations of own species. The first case concerns a male singing strophes containing syllables of Yellowhammer and Ortolan Bunting, both most probably copied in central Europe. These songs often had broken syntax. The second case concerns a male that sang both typical strophes and strophes that had the initial part copied from a local dialect of Redwing, but with a typical syntax for the species. That male mated successfully. Causes and consequences of vocal mimicry in the species are discussed