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Recent sightings in the Netherlands of “pure” and presumably hybrid Pine Buntings (PB) stir discussion on this matter. When do you speak of a hybrid? What do we know of the plumage variation within both species? Were does the hybrid zone, if it exists, occurs? And is it valid to speak of two species at all?
PB in the Netherlands
The PB is a vagrant to the Netherlands with 34 accepted records. Most of the accepted Dutch records are birds caught at coastal ringing stations with Yellowhammer (YH) as lure bird. The number of caught birds went down dramatically in recent years, from one to two birds almost every year, to none in recent years. (source http://www.dutchavifauna.nl/species/witkopgors?#en) - This said, it should be noted that the use of lure birds on the ringing stations is also rare nowadays and some birds don’t react the same on sound recordings as on lure birds. Apart from this, YHs are not as common as they used to be in the west part of the Netherlands, this makes PBs flocking together with YHs also rarer. Because of this, the chance of catching PBs at coastal ringing stations has probably decreased, while numbers of migrating PBs might have stayed the same.
The number of bird watchers and nature photographers increased dramatically, but the number of field sightings of PB hasn’t. The same occurs in our neighbouring countries. This indicates that PB has become extremely rare in western Europe after the milennium.
The recent sighting of the adult male (video's) can be the fourth accepted record since that time. This bird was found at a roost of YHs near Venlo and twitched by many. Within this flock there were one or maybe two female birds showing characters of PB. But were they?
Edit (while writing this post, Marcel Klootwijk found a (pure) first winter female Pine Bunting near Wilhelminadorp, the Netherlands. This (single) bird was a welcome discovery for many who dipped the PB(‘s) before. And while the previous where hard to see, this bird was seen and photographed at close range with all the right features (including underwing coverts)!
Distribution and hybridization
Immature YHs and PBs could look very similar and a good views or photos are needed for certain identification. Even worse, the two species overlap in a zone of central Siberia along the Ob River (near Surgut) and mixed breedings are not uncommon in this zone. These hybrids are fertile and are able to back-cross, which makes it very hard to distinguish a F2 hybrid from a “pure” PB or YH. F2 hybrid PBs could only be identified as hybrids by their yellow lesser underwing-coverts!
The distribution of both species and hybridization is documented in this paper https://www.zoology.ubc.ca/~irwin/PDFs/Irwinetal_BJLS_preprint.pdf (Irwin DE, Rubtsov AS, Panov EN 2009). Although they differ in plumage (e.g. adult males), song, breeding habitat preference and geographical distribution the mtDNA samples show very close resemblance (Irwin DE, Rubtsov AS, Panov EN 2009). This might indicate that hybridisation has occured many times over a large zone. Such a large clinal zone even poses the question if the two species shouldn’t be considered as conspecific?
Birdwatchers are often a bit racist ( ;-) ), they want their birds “pure”. If it's a hybrid, then the bird is not interesting anymore and they move on to the next case. Why don’t these dirty little birds play with the rules and stick to their own species? The species we have put them in! But when do we speak of a hybrid? When do we birdwatchers forgive a cheating ancestor? After how many generations?
In his case the identification of males doesn’t seem to be the problem therefore we focus on the (1st winter) females. The common rule is that PBs can’t show any yellow. If yellow is present this points towards (any influence of)YH.
The question rises if this is true at all and if the bird is a pure or a hybrid YH. In his DB article Danielle Occhiato states that in any plumage no yellow-tones may be present in pure PBs! So maybe it’s even better not to use hybrid at all only the term “pure” considering their close mtDNA..
The crown shows finer and neater dark streaking in PB. The ear coverts and cheeks are brown with 1st winter females and rufous in adult females. In all plumages PB shows a pale spot on the nape which only adult female YH can show but is always very faint. The supercilium of PB females is always wider and paler(grey), often brown/yellowish in YH(hybrids). PBs also seem to have a more erected crest and the eye ring of PB is white. Overall the head is more contrasting and might remember of Lapland Bunting.
For a pure PB no yellow may be present in the wings. Pay extra attention to the outer webs of the primaries. Plate 12 in DB25-2003 on page 6 shows a wing with prominent white flags on the primaries. These white flags are also visible when the wings are closed and this character is very useful for identification.
Breast and flanks
The underparts are heavily streaked with dark brown and blackish spots on a whitish background and become more diffuse towards the flanks. In first winter the spots tend to be brown as they are more rufous in adult females. Also the underground of the spots is greyish in first winter and more whitish in adult females with obviously no traces of yellow.
The dark streaking on the breast of PB show some kind of a necklace before the brown/rufous streaking towards the flanks start, in YH this “necklace” isn’t as clearly present.
No mentionable differences beside the “colder” brown coloration of PB tending more towards a grey mantle making the mantle and grey neck less contrasting in PB. Best seen with overcast conditions, in images one must be aware of the camera settings which can influence the colors.
The rump is the same in both species, there are some features that point towards Pine but these can also be visible in YH with a more eastern origin. In fresh birds the pale fringes on the feathers indicate a first winter.
The tail of PB is about 10% longer, this is not always reliable, but it is visible in the field if birds are in mixed flocks. The tail will also help to age the bird in fresh plumage, if tips are pointed it indicates that it's a first winter bird, the tips are more rounded in adult. The outer tail feathers show more white then in YH. The undertail coverts are less heavy streaked then in YH or can be all absent especially with adult females.
To the human ear the call of PB en YH are the same, the sonogram will show some difference but in the field birds can't be identified by their call to the human ear.
Additional identification features are most welcome to complete this issue, please contact us or add a comment below.