sábado, 23 de febrero de 2013

Leucistic Sturnus as a pitfall for Rosy Starling

Rosy Starling (Pastor roseus) is a rarity in Spain despite there are annual records of the species (28 records/birds in mainland and Balearics up to 2010 plus another in the Canary Islands, Source: @CR_SEO).

Besides well marked adults, some moulting individuals from juvenile/1w plumages to adult also occur with rather frequency. However, some distant pale and black starlings are not always true Rosy Starlings but can be leucistic starlings.

In recent times there have been at least three cases of Spotless Starlings (Sturnus unicolor) showing extensive white areas in underparts and mantle:

1. Sturnus unicolor, Valencia, 9 February 2013 (Javier Sánchez at SVO forum)

© Javier Sánchez

2. Sturnus sp. (c.f. unicolor), Aldea del Rey, c.10 February 2013 (Gustavo Barba Alcaide blog)

© Gustavo Barba

3. Sturnus unicolor, Odiel marshes, Huelva, 17 February 2013 (Marie Montalva and  José Luís Anguita,  reported to this blog)

© Marie Montalva, José Luís Anguita

© Marie Montalva , José Luís Anguita

© Marie Montalva, José Luís Anguita

© Marie Montalva, José Luís Anguita

Other sightings are quoted elsewhere, showing more variation (c.f. Manolo García Tarrasón to SVO forum):

(Source: Flickr)
(Source: www.avesibericas.es)

These birds show at first sight some similarities to Rosy Starlings (Pastor roseus) in plumages  reported in Spain (source: Rare Birds in Spain), but can be easily told from the 'true' Rosies.

Juvenile Rosy Starlings can look dull in autumn and start acquiring greenish coverts and yellow bill by mid winter. By february they can show dark heads but still being pale overall. Depending on light, some spring adults can even seem 'whitish' instead of pink below, thus recalling the above 'leucistic' starlings. See a number of photos published previously in the website featuring different plumage stages of Pastor roseus.

© Marcel Gil. 22.10.2006

© Pablo Fernández García 1.12.2011
© Fernando Arce. 20.1.2006

© Álvaro Rodríguez Pomares 23.1.2012

© Álvaro Rodríguez Pomares 23.1.2012

© Daniel Roca. 5.2.2006
© Ferran López. 12.2.2007

© Ferran López. 23.3.2007

© Albert Cama 3.4.2011

© José Luis Anguita. 11.4.2010
© David Cuenca. 29.4.2010

© Ricard Gutiérrez, 1.5.2010

© Daniel López Velasco. 26.5.2007

Leucism, produced by a lack of melanin in feathers, is variable but can be partial (unlike albinism). Partially leucistic birds can have normally coloured feet, bill and eyes, as it happens with the above birds. van Grouw (2013) (British Birds 106: 17-29) quotes leucism as being bilaterally symmetrical, again as it happens in the above birds.

Interestingly, all three birds reported above show a similar pattern of leucism! Distances between them and close observations favour the option of being three different birds indeed.

How to tell a leucistic Sturnus from a Rosy Starling (Pastor roseus)?

First of all, in any roseus, some greyish, brownish or rosy pale parts should be expected. Not pure white should be present (beware of light conditions of photos such as that of 3.4.2011 above though!).

Second, no roseus shows any dark strikes in the underparts as the leucistic birds above. Bird of 20.1.2006 shows some kind of breast band related to the future 'black head' being moult in that moment. But no strikes in the body and belly.

Third, and as another clue, bill structure in roseus is very different from that of Sturnus unicolor or S.vulgaris, with a more curved culmen and lower mandible as well as wider base.

Sturnus unicolor, Juncosa, Lleida, 26.8.2007 ♂ ♀ 'winter' plumage © Ricard  Gutiérrez / Pastor roseus, Llobregat delta, 12.2.2007 1w © Ferran López / Sturnus vulgaris, Juncosa, Lleida, 1w, 19.8.2007 © Ricard Gutiérrez

Note differences in shape of bills that should have to prevent any possible confusion.

Leucism, color aberrations can 'create' some tricky birds. In those cases of plumage variation one should always remember that despite odd colours, 'structure' remains and can be very helpful when facing these odd, strange but also nice and challenging birds.

Acknowledgements. José Luís Anguita provided detailed information on the 3rd 'leucistic' and Manolo García Tarrasón plus the 'birdzarre birders crew' gave details on the other two birds. Thanks all.

2 comentarios:

Juan Manuel Pérez de Ana dijo...

Muy interesante.

Paco Chiclana dijo...

el 10 de julio de 2014 observo en vuelo un estornino en Calatilla, Odiel, Huelva, que puede ser el mismo registrado en febrero de 2013