Dutch Birding volume 37 (2015) no 3

Presumed hybrid Common x Blue Rock Thrush at Den Helder in April 2013

On 11-15 April 2013, a first-summer male rock thrush Monticola (with many traces of juvenile plumage) stayed at Den Helder, Noord-Holland, the Netherlands. It mostly stayed on roof tops, coming down to forage on sandy patches or to perch on stone heaps. On at least the first two nights, the bird was observed roosting in a niche on a floodlit church. The bird was first reported as Common Rock Thrush M saxatilis, based on its small size, relatively short tail, blue upperparts, mostly orange underparts and rufous tail (except for the brown central tail-feathers). However, some characters were at odds with this species, such as the mix of orange and blue feathers on the underparts, wing-to-tail ratio, wing formula, bluish lesser coverts, bluish uppertail-coverts with orange tips, brownish outer web to the tail-feathers and absence of white patch on the back. The possibility of Asian Blue Rock Thrush M s philippensis was considered but dismissed because of the tail length, rufous tail and orange instead of dark red belly coloration. This reduced the possibilities to either an aberrant Common or a hybrid Common x Blue Rock Thrush M solitarius. This hybrid type has been described before (most recently from 1929, collected) and could well explain the mix of characters, including structural ones, of the Den Helder bird. The Dutch rarities committee (CDNA) concluded that the hybrid option was a good explanation but that an aberrant Common could not be ruled out with certainty. This means that, at least up to now, the bird was not accepted as any taxon and has not been admitted to the Dutch avifaunal list.

André J Boven, Hoofdweg 320a, 9681 AP Midwolda, Nederland
(aj.bovenhetnet.nl)
Hans Schekkerman, Pernéstraat 37, 1901 AV Castricum, Nederland
(Hans.Schekkermansovon.nl)
Enno B Ebels, Joseph Haydnlaan 4, 3533 AE Utrecht, Nederland
(ebelswxs.nl)