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Secondary nest usage by a pair of Lesser Whitethroats

By Tom Byars

In Ayrshire, Renfrewshire and Lanarkshire, 9-12 Lesser Whitethroat Sylvia curruca territories have been studied regularly since 1983 (T Byars et al 1991), Scottish Birds 16: 66-76). A Renfrewshire study site contained 2-3 territorial males within a 2 km square south east of Paisley at Brownside Braes (NS 4860). All territories were located in areas of mature Hawthorn Crataegus monogyna scrub with a dense mosaic understorey of Bramble Rubus sp, Gorse Ulex europaeus, Dog Rose Rosa canina and Goat Willow Salix caprea (T Byars et al 1991, Scottish Birds 16: 66-76). Breeding Lesser Whitethroats were individually colour ringed, allowing territorial behaviour, site fidelity and interchange to be studied. Sexing Lesser Whitethroats can be difficult as males develop well vascularised brood patches (S C Norman 1992, Ringing & Migration 13: 167-174; L Svensson 1992, Identification Guide to European Passerines, Stockholm). Research in NE England found that trapped females could be sexed if birds showed distended stomachs and higher body mass through egg carrying, adults can also be sexed on the basis of brood patch size and/or cloacal protuberance (M Boddy 1994, Ringing and Migration 15: 65-78). Trapped Lesser Whitethroats at the study site were sexed using a combination of these features.

















Sexing Lesser Whitethroats

At Brownside Braes on 8 May 1996, A male Lesser Whitethroat was vociferously holding territory (Territory A). It was mist netted and given an orange colour ring that morning and observed singing throughout the day. The following morning, I discovered a new territory within the study site (Territory B) which was located 800 metres away from Territory A. Although the male was singing infrequently, this second territory did contain a breeding pair (Pair B) and efforts were made to mist them. Despite various attempts, only the female from pair B was caught and given a green colour ring. Pair B were observed nest building in a gorse bush that afternoon. The female lined and shaped the nest cup, an indication that pair bond establishment had taken place (S Cramp, {ed}, 1992, Birds of the Western Palearctic, Vol 6: 449-450. Oxford University Press, Oxford). Six days later, a new unringed male had taken over Territory A and the orange ringed male had disappeared. On 16 May, the unringed male on Territory B was singing again, suggesting that mate losshad occurred (S Cramp, {ed}, 1992, Birds of the Western Palearctic, Vol 6: 449-450. Oxford University Press, Oxford. The green ringed female could not be located at Territory B and was never seen again anywhere on the study site. Mist netting at Territory B that morning revealed the orange ringed male, approximately 30 metres away from the now vacant nest site. The following day, the unringed male was still singing on Territory A and the unringed male from Territory B had now moved into a new territory 86 metres away, leaving Territory B vacant.













Unringed male Lesser Whitethroat

Returning to Territory B on 7 June to obtain data on the unused nest site, I heard 2 Lesser Whitethroats alarm calling close by. On inspecting the nest in Territory B, I was surprised to find it contained a clutch of 5 Lesser Whitethroat eggs. Both adults became visibly agitated at my presence and immediately started distraction displays which allowed for close observations of this pair. The male was instantly identified by his orange colour ring, while the female was unringed. This new pair had utilised the old nest and successfully raised 4 young, which left the nest on the 17 June. Although rare, this behaviour has been noted in other birds, eg the Wren Troglodytes troglodytes (J J Sweeney pers comm). This appears to be the first documented account of a pair of Lesser Whitethroats taking on an empty nest.










Observing nesting Lesser Whitethroats

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