| Rodents abundance evaluation: |
low
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Breeding conditions:
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Numbers of Arctic Foxes were above expected in 2006 in the study plot. Five territories were occupied, and the density was 0.22 fox/sq.km, which was almost twice the value in 2005. While numbers were low overall, they did not decrease after the lemming low in 2005. The study plot was inhabited only by territorial animals, while wandering foxes were not recorded during the whole breeding season. Survival was high during winter 2005/2006, based on rare records of winter carcasses and stable numbers in 2005 and 2006. Arctic Foxes were observed at an average density 0.15 animal/km across the island, and landscapes did not differ notably in the frequency of records. High fraction of Arctic Foxes, participating in reproduction in the study plot (80%, n=10), indicated successful wintering, given low lemming numbers for the second year in a row. Mean brood size was 5.1 cub (n=16). Adult foxes actively destroyed nests and captured adult birds and chicks. Approximately 10 Wolverines and 15 Wolves have currently inhabited the island. In spite of the marked decrease of ungulate numbers Wolves do not apparently suffer from shortage of food, as indicated by observations of partly consumed prey bodies and intact bodies of reindeers and muskoxen, failing to survive the winter. While Wolves were also taking adult Snow Geese and destroyed bird nests, their impact on Snow Goose population was not considerable. In contrast 16% of Snowy Owl clutches on the island were lost in 2006 due to predation by Wolves and Wolverines.
Lemming abundance was not sufficient for successful reproduction of Snowy Owls in 2006, and their diet could entirely consist from other prey in July, during hatching of tundra birds. A body of Arctic Fox cub was found in the owl nest for the first time in 13 seasons of observations. Lemmings prevailed in owl diet only in May and June, while other prey comprised 79-100% (n=105) of it in July and August. The diet of owl chicks consisted of chicks of waders, Snow Buntings, Long-tailed Skuas, adult females of Common Eider and Snow Goose, depending of abundance and availability of prey species.
Numbers of Snowy Owls were low to average in 2006 in different landscapes, and they occurred at an average density of 0.31 bird/km (range 0.05-0.77, total transect length=484 km). Given low lemming numbers all regions of the island were inhabited by owls, but they were most abundant in Northern Mountains and the Gusinaya River valley, while owl numbers in the eastern part of the island were lower.
Proportion of juvenile owls was high in 2006 among breeding birds, particularly males. Major part of the population participated in the reproduction in 2006 in spite of low lemming abundance, and proportion of breeding birds among owl females was 80% in the study plot (n=10) and 74% (n=89) elsewhere on the island.
Early spring allowed early and synchronous start of egg-laying by Snowy Owls, and 95% of pairs started incubation from 14-31 May. Early breeding resulted in large clutch size: 28% of females laid 8 eggs, while mean clutch size was 6.8 eggs (n=18). Large clutches are typical for the owl population of the island in early seasons even at low lemming numbers, however, 19.4% of nests (n=31) were deserted during incubation, partly due to a high proportion of juvenile females. Unfavourable weather conditions during egg-laying resulted in perishing of 13% eggs (n=94) prior to hatching, particularly in large clutches. Chicks fledged in 58% of pairs, but mortality of chicks in the first 10 days after hatching reached 72% (n=82) in 2006, and was apparently due to shortage of primary food. Low lemming abundance was the main reason for perishing of 6% of chicks (n=23) after leaving nests. In total, 18% of chicks fledged, while mean brood size was 1.4 chick (n=23). All chicks in 4 broods were eliminated by Wolves and Wolverines.
Short-eared Owls have not bred on the island since 1994, and only 3 solitary birds were observed in 2006. Pomarine Skuas were not recorded in the inner parts of the island during summer period, and their breeding was not recorded.
Numbers of Long-tailed Skuas were average in the study plot, where they occurred at a density 0.67 bird/km, and 93% (n=30) of birds bred. Nest density was 0.31 nest/sq.km in the study plot, and breeding success was extremely low due to heavy predation on broods by rodent-specialists failing to find lemmings. Chicks of Long-tailed Skuas constituted 10.5% (n=105) of diet of Snowy Owls. Not a single brood of skua was found in the study plot in the first half of August, and adult birds left their territories after perishing of chicks. Long-tailed Skuas occurred in usual numbers elsewhere on the island, and started breeding by 10 June. Rare fledgelings were recorded in the valleys of the Tundrovaya and Gusinaya rivers and on the Southern Plain.
Snow Geese and/or Common Eiders nested in the vicinity of 81% of nests of Snowy Owls (n=31). Mass arrival of Snow Geese to the study plot was observed on 25-26 May, while formation of colonies occurred from 25 May to 8 June. Good weather after 20 May favoured start of nesting by geese immediately after arrival and somewhat earlier than normal. Mixed colonies of Snow Geese and Common Eiders prevailed (64%, n=14), while single-species colonies constituted 21% and 14% for Snow Goose and Common Eider, respectively. Breeding success of Snow Geese, nesting in the vicinity of owl nests, was on average 38% in 2006 (0-95%, n=13). All nests failed in 4 colonies, which size did not exceed 18 nests, while breeding success in other colonies varied from 12-95% (n=9). Snowy Owls and Arctic Foxes captured 0.01% and 1.4% of incubating geese, respectively (n=567). Mean clutch size was 4.22 egg (n=108) in Snow Geese, while mean brood size in small colonies was 3.44 chick (n=82) by the time of migration of families to moulting sites. Remains of geese chicks and adult birds were recorded during brood-rearing period at all dens of Arctic Foxes with cubs on the northern plain of the island in the Tundrovaya River valley, but a fraction of taken birds was not high overall due to low fox numbers.
Breeding of Brent Geese was not recorded in the vicinity of owl nests in 2006. A majority of Common Eiders (92%, n=106) bred in the vicinity of owl nests, and only 8% of nests were solitary. Eider nests were found in the vicinity of 63% of nests of Snowy Owls. Mean clutch size was 4.76 egg (n=21) and mean brood size was 4.33 chick (n=3) in eiders. The last eider was observed on nesting territory on 17 July. Nest success of the Common Eider was 26% (n=97) and 22% (n=9) for nests associated with Snowy Owls and solitary ones, respectively. Snowy Owls from 12 different pairs captured 14% (n=97) of eider females incubating in the vicinity of owl nests. Thus, nesting near owls was not beneficial to Common Eiders in a season with low lemming abundance.
Low reproductive success of Snow Geese associated with Snowy Owls was mostly due to predation by Arctic Foxes. The latter became possible in a low lemming season, as some owls deserted their clutches, while others protected nests less actively, having been made to survey wide areas in search for food. Also, at least 5 nests of Snowy Owls were destroyed by Wolverines and Wolves. In contrast to large colonies of geese with high reproductive success, all nests in all large colonies of Common Eiders failed. Thus could be due to earlier breeding by geese compared with eiders, which allowed them to hatch before failure of the majority of nests of owls.
Numbers of common waders were below average in 2006 in the inner parts of the island, which could be due to cold weather at their arrival in early June. Waders could have moved from mountain regions to the southern plain with more favourable spring conditions. Wader chicks constituted 13.25% (n=105) of Snowy Owl diet in July, and breeding success of waders was very low due to predation pressure. Broods of waders were rarely recorded in the study plot and other regions of the island.
Thus, poor weather in June and strong predation pressure were responsible for low reproductive success in most species of birds on Wrangel Island in 2006.
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