Iberian Chiffchaff identification: the state of the art
“And they will run to the highest hill, consult their old books and Ask the dead mystics for wisdom they don't trust” – Don’t Fall In, Kate Tempest
Introduction
Since the beginning of this century, when the species became actually popular, there’s been several attempts to address the identification of the Iberian Chiffchaff. After a long period of only vocal differences described, the early 00s saw the first papers dealing with plumage characteristics, biometrics and moult. Most of these contributions were included in GIA’s monograph on the species, quite a landmark in the species popularization process. All of a sudden, a silent individual could be readily identified in-hand using a formula we all had highlighted in our copies of ‘Svensson’. The species, however, remained officially unidentifiable on field views only.
Almost 20 years later, we now know the habitat and phenology of the species, even in places where it only occurs as a vagrant, like in most European countries. Moreover, a significant part of today’s birders carries a camera with big lens and/or a superzoom camera, what necessarily leads to a big number of high-quality photos of a species that can be considered a skulker. With such nice photos proliferating in our hard drives, it was too tempting to look for that subtle but diagnostic feature. Up to date, we are still missing it, but in our way here we’ve managed to come up with a number of accessory features that all together must be close to make a diagnostic one, if they don’t do yet.
Probably due to the progress being quite slow, research on Iberian Chiffchaff’ identification is spread over several papers. Although I fully recommend to read them all (see References), I’ve tried to compile everything we know about this topic in a single post, benefiting from being able to include a large series of photos. The post is particularly focused on in-the-field identification, what still constitutes the real challenge. In that respect, I’ll go from describing the general appearance of the entire bird to try to cover the entire variation of some key features – including several Common Chiffchaff photos to allow side-by-side comparisons – and a quick look through moult (especially field assessment) and voices. Finally, the main potential pitfalls are discussed, paying special attention to ‘Scandinavian’ Common Chiffchaff Phylloscopus. c. abietinus, a surprisingly similar taxon.
Structure
Overall structure
Most Iberian Chiffchaffs look more elongated and seem to have a longer bill attached to a smaller head in a more powerful body compared to Common. In general, as worn as this statement is, we could say that the impression is somewhat in between a Willow Warbler and a Common Chiffchaff. However, some birds – presumably females – can look surprisingly petite (see for instance that in Figure 4).
Primary projection and spacing
The primary projection is typically described as longer than in Common, but given the variability within collybita, the overlap is quite large. However, the primary spacing seems to be more consistent. Indeed, the space between the third and the fourth primary tip visible in Iberian seems to be of equal length to that between the fourth and the fifth, whereas in Common, the space between the third and the fourth primary tips is generally smaller (Gil-Velasco, 2018).
Figure 1. Comparison between the primary spacing of Iberian Chiffchaff (left) and Common Chiffchaff (right). Note the 4th and 5th spaces (counting inside out) are of equal length in Iberian, whereas the 5th space is longer in Common. Sketch by Martí Franch.
The difference is visible in the field, although you obviously need high-quality photographs, which isn’t always easy in a skulking passerine. However, this feature could be useful to support the identification of a silent bird and it’s something worth paying attention at while waiting for a good candidate to vocalize.
Further research is needed in order to describe the primary spacing of other Common Chiffchaff subspecies, especially abietinus. Given this Scandinavian taxon performs a considerable migration, it would be expectable if they show a more pointed wing, which might also affect the primary spacing. This fact, together with the high variability in collybita, limits the strength of this feature, which should be only treated as supportive.
Plumage
Overall colouration
The most classical Iberian Chiffchaff (≈70%) shows quite bright upperparts, with very little brown, a well-defined eyebrow with highly saturated yellow from the bill to the end of the eye and a paler rear end, pale cheeks with a smooth transition to an almost white throat (just a yellow hue in some), and a golden breast-band, especially large in breast-sides, separating the throat and the pale underparts with just some sparse yellow stripes (Figure 3)
However, there is some gradual variation towards both the dark and the brightest sides. Bright birds aren’t that rare (≈20%; Figure 4). They are similar to the classic Iberian described before, but with lime green upperparts and more yellow in the eyebrow and the face. Dark birds are scarcer, but still a significant part of the population (≈10%). They tend to show dark grey pine or olive tones above (sometimes some scattered green patchy stripes) and very little (sometimes virtually absent) yellow on the head or the underparts (Figure 2).
I have no information on whether this variation is related to the two subspecies described, albeit not accepted as such by all the authors. The percentages mentioned above are rough approximations from the Northern population, the one I’ve been able to study properly. In that context, in my opinion the limit of what it’s identifiable in the field lays somewhere between the birds in Figure 3 and Figure 4, although the closer we get to the former the more extra evidences we should gather.
Figure 2. Iberian Chiffchaff Phylloscopus ibericus. Dark individual. Note the greyer dark olive green above and the limited yellow below. This particular individual shows some yellow in the eyebrow and the flanks, but some can miss even this little amount. Juan Sagardia, Valladolid, April 2017.
Figure 3. Iberian Chiffchaff Phylloscopus ibericus. In-between individual. The most typical overall colouration, but amount of yellow on the face and the underparts very variable, approaching either side of the variation. Juan Sagardia, Valladolid, April 2016.
Figure 4. Iberian Chiffchaff Phylloscopus ibericus. Brightest extreme of the variation. In my opinion, this kind of birds can be safely identified out of range even if completely silent. Miguel Ángel Fuentes, Girona, March 2012.
Head pattern
Eyebrow
In terms of shape, the most classic eyebrow is larger above and in front of the eye and finer towards the end. However, some birds can show still well-defined eyebrows of even width all along or surprisingly subtle, restricted to almost a spot in front of the eye in the most extreme individuals (see Figure 5A). This latter appearance is quite rare though, involving less than 10% of the sample studied.
Colour-wise, the commonest pattern is a brighter yellow first half of the eyebrow, with the rear eye being a quite sudden frontier between this highly coloured region and a generally pale, grey or warmer brown rear end. It’s interesting to note that even in birds with not so prominent eyebrows, this colour pattern still exists. However, it’s worth mentioning that, as usual, light plays an important role, especially regarding the colour of the rear half of the eyebrow: with strong light, it can look quite bright yellow too, diminishing the contrast with the fore part, whereas some fairly typical lights – for instance when foraging under the shade of the canopy – can make this region look darker.
Common Chiffchaffs can also show a similar pattern, but the yellow fore-eyebrow is very rarely as bright, and the rear eyebrow is usually dirtier. Of course, everything is possible when talking of Common Chiffchaffs, but a combination of large bright yellow fore-eyebrow and paler and clean rear eyebrow must draw our attention.
Cheeks
This feature must play a very important role in the ultimate jizz impression we get of an Iberian Chiffchaff. The key thing here is the typical lack of contrast between the cheeks and the throat. Sometimes they are concolorous, both quite pale, but even when the cheeks are more saturated, the transition towards the throat is very gradual, without a well-defined frontier. In some individuals, there is a hint of frontier, but it’s usually restricted to the rear cheeks.
Figure 5. Variation in Iberian Chiffchaff Phylloscopus ibericus face pattern. The eyebrow type depicted in A is quite rare, involving less than 10% of the birds. The birds in pictures B to F show an eyebrow of even width, but light and colour ultimately define its sharpness. G to J show classic large eyebrows in front and above of the eye, quite typical in bright birds. Note that, despite the different light conditions, there is a consistent contrast between the fore and the rear eyebrow, the former being yellower and the latter being generally paler. Moreover, the transition between cheeks and throat is very smooth, leading to a more open-faced impression and a better-defined eye-stripe. The bill in all the photos looks somewhat long, but notice especially the more extensively pale lower mandible. All photos taken by Juan Sagardía in spring in Valladolid.
Figure 6. Variation in nominate type Common Chiffchaff Phylloscopus collybita face pattern. The eyebrow is less conspicuous on average. Usually thinner in front of the eye than in Iberian, and less clean in the rear part. Usually darker cheeks and better-defined contrast between cheeks and throat. Note the shorter and darker bill. A: Lisboa, February 2020, Marcel Gil-Velasco. B: Girona, April 2015, Fran Trabalon. C: Girona, March 2015, Fran Trabalon. D: Girona, March 2015, Fran Trabalon. E: Girona, October 2012, Fran Trabalon. F: Girona, January 2012, Fran Trabalon. G: Girona, January 2012, Fran Trabalon. H, I, J. Lisboa, February 2020, Marcel Gil-Velasco.
General note
The combination of a large eyebrow in front and above the eye and the lack of contrast between the paler cheeks and the throat leads to a more open-faced impression. The pale cheeks below and the large eyebrow above – especially in the individuals that do show such features, which is a very significant part of the population – define a sharp eye-stripe somehow reminiscent of more neatly-plumaged species such as Wood or Bonelli’s Warblers.
Figure 7. Iberian Chiffchaff Phylloscopus ibericus. The pale underparts, opened face and neat impression overall could even remind a Bonelli’s Warbler! Rebollo de Duero, Soria, April 2010. Juan Bécares.
Underparts
For sure one of the most famous features, since birders were already describing Iberian Chiffchaff as “yellower” a long time ago. This statement was certainly based mainly on the colouration of the underparts, and it remains true, to a point. The most classic plumage includes a very pale throat, a hint of an incomplete yellow or buffish-yellow breast band, pale belly, just a yellow wash on the undertail area and the aforementioned sparse yellow stripes. Although some birds are plain yellow underneath, reminiscent of young Willow Warblers in early autumn, most birds should be better defined as pale, with just some yellow stripes across the underparts. When compared to a Common Chiffchaff, rather than being yellower, the main difference resides in the amount and pureness of the white, especially on the throat and the belly. In my opinion, this, together with the less saturated upperparts, highlight the yellow traces of the plumage and justifies the traditional description of the species.
Figure 8. Iberian Chiffchaff Phylloscopus ibericus (left) and Common Chiffchaff Phylloscopus collybita (right). Note that Common can show much yellow on the underparts, sometimes even more than Iberian. However, Iberian tends to show paler throat (extending into the cheeks) and almost pure white belly, leading to a paler overall impression. Barcelona, April 2010, Juan Bécares.
Upperparts
The upperparts of the Iberian Chiffchaff remind me of that of Common but as if we had extracted part (sometimes most) of the brown tones, leaving the grey and the green only. Some individuals are neater than others and they tend to acquire a browner colouration as wear increases and, when Chiffchaffs are done with the pre-nuptial moult, some can have a similarly pale appearance. However, in Common there’s always traces of a more solid brown especially on crown and mantle.
Bare-part colouration
Leg colour isn’t very variable in Iberian: most individuals show dull brown legs, usually slightly paler towards the toes. The toes themselves can also be paler, but there isn’t a big difference with the tarsus as happens, for instance, in some Willow Warblers. The photo below depicts the entire variation: A shows the brightest extreme – which is rare – and F the darkest – which is most often seen in juvenile birds and more rarely in ≥2cy birds (albeit sill commoner than A). E seems to show a richer chestnut brown variant, but it’s probably enhanced by the light.
Regarding the bill, as can be seen in Figure 5, the lower mandible of the bill is more extensively pale and the dark upper mandible is very rarely pure black. In Common, the pale area is usually restricted to a line across the gape, but in Iberian it can occupy the entire base of the mandible. The bird in Figure 5E is an example of how a much Common Chiffchaff alike bill doesn’t rule out Iberian though.
Figure 9. Leg colour variation in Iberian Chiffchaff Phyllosopus ibericus. B to D are all very similar, but included here to depict the changing impression depending on light and angle. A is very rare, with less than 5% of the individuals showing such yellow toes. F is common among 1cy birds, but quite rare even within 2cy birds; very rare in adults. A: Tarifa, September 2016, Andy Butler. B-F: Valladolid, Spring, Juan Sagardía.
Figure 10. Common Chiffchaff Phylloscopus collybita legs are usually described as dark brown or blackish, but they can look surprisingly pale. Birds like A and paler aren’t the commonest, but we might end up seeing one a day if we pay attention enough in Chiffchaff-rich areas. B, C & D are the most classic dark brown, whereas E and F are blacker – not as jet black as in a classic Siberian though. A: Girona, March 2015, Fran Trabalon. B: Girona, January 2012, Fran Trabalon. C: Girona, January 2015, Fran Trabalon. D: Girona, October 2012, Fran Trabalon. E: Girona, April 2015, Fran Trabalon. F: Girona, March 2017, Fran Trabalon.
Side-by-side comparisons
When you ask an Iberian birder if Iberian Chiffchaff is identifiable in the field, some would say yes, but would then have a very hard time describing why. Despite I’ve never particularly sympathised with the ‘jizz school’, I have to say that in this case features that are sometimes too ambiguous in my opinion (such as “expression”) might play a key role. Hence, we thought it was a good idea to include some side-by-side comparisons with Common Chiffchaff to depict the impression you get in different positions and/or light conditions. Although the main purpose of this section is to emphasise how different they can look overall, I fully recommend to check all the features previously described in these photos, so you can sort of guess where does the different jizz come from.
Figure 11. Iberian Chiffchaff Phylloscopus ibericus (left) and Common Chiffchaff Phylloscopus collybita (right). Although this Iberian is an example of a dark bird, note that head differences still stand: better-contrasted eyebrow (yellower from the bill to the rear eye) and diffuse cheeks with a smoother transition to the throat (leading to a more open-faced impression). Left: Valladolid, April 2017, Juan Sagardía. Right: Girona, February 2012, Fran Trabalon.
Figure 12. Iberian Chiffchaff Phylloscopus ibericus (left) and Common Chiffchaff Phylloscopus collybita (right). Very classical Iberian compared to a slightly yellower than average Common. The eyebrows are very similar in this case, but the cheeks are still paler in Iberian. Despite a considerable amount of yellow in this Common Chiffchaff, note the dirtier impression compared to the Iberian, which also shows the classical broken golden breast band. Structure-wise, the Iberian typically offers a smaller head impression. Left: Valladolid, April 2016, Juan Sagardía. Right: Girona, January 2012, Fran Trabalon.
Figure 13. Iberian Chiffchaff Phylloscopus ibericus (left) and Common Chiffchaff Phylloscopus collybita (right). Apart from the head differences already described (which are also present here), note the neater upperparts with quite pure golden tons in scapulars and some back feathers. Left: Valladolid, April 2016, Juan Sagardía. Right: Girona, October 2014, Fran Trabalon.
Figure 14. Iberian Chiffchaff Phylloscopus ibericus (left) and Common Chiffchaff Phylloscopus collybita (right). A comparison between worn individuals. Common Chiffchaff can acquire a paler plumage overall when worn, but they also tend to lose the yellow tones. Iberians usually keep good amount of yellow, especially in the face and the breast band. In this plumage, Iberian is closer to Willow Warbler in my opinion. Left: Valladolid, July 2016, Juan Sagardía. Right: Girona, July 2013, Fran Trabalon.
Moult
One of the greatest news when I grew up as a ringer was the description of Iberian Chiffchaff extensive partial moult. In contrast to both the post-juvenile and pre-breeding moults in Common Chiffchaff (usually restricted to body feathers, wing coverts and at most some inner secondaries and tail feathers), 2cy Iberian Chiffchaffs have moulted a number of outer primaries. Despite P1 is frequently retained, the frequency in which each primary is moulted decreases from there inwards. Indeed, P2 is moulted almost always and P7 (the innermost primary I’ve seen being moulted) much less often. In my opinion, whether this moult corresponds to a post-juvenile or a pre-breeding is still unclear. The moulted primaries are still quite worn, what could point towards them being moulted in late autumn or already in wintering grounds. Anyhow, with patience (and a high-speed camera), this feature can be seen in the field – as shown in Figure 16 – and has already been used to assess vagrants.
Figure 15. Frequency in which each primary is moulted during the post-juvenile moult. Data from Llobregat Delta originally published in Rodriguez, et al. 2013.
Figure 16. Iberian Chiffchaff Phylloscopus ibericus. Classical field views. Even while the bird is foraging in a dense bush, it's possible to capture the moult in a photo. In this case, the moult limit is between p4 and p5. Rebollo de Duero, Soria, April 2010. Juan Bécares.
Voices
Song
As a general rule, the song of an Iberian Chiffchaff is short and well structured (the notes are grouped by type), very rarely above 6.5kHz. The elements found most often are depicted in Figure 17 and the song usually consists in 3-4 distinct phases: first a series of 2-4 elements of a certain type (usually A, B or similar variants of either), then 2-4 elements of another type (A or B again) and ended with 1-2 C elements and between 1 and 5 D types (typically described as ‘the rattle’ that adds the Bonelli’s Warbler impression; Collinson & Melling, 2008).
Figure 17. Classical Iberian Chiffchaff Phylloscopus ibericus song, with the most usual note types labelled. Crémenes, León, August 2019.
The number of isolated elements of a certain type is usually indicative of a mixed singer and indeed the song of those birds sounds more chiff-chaffy. However, not all Iberian Chiffchaffs do the same and short songs – even shorter than usual – including a good variety of elements are fairly typical in pure ibericus (See Song A in Figure 18). Sometimes these short/interrupted songs (and more rarely some full songs too) lack the tidied-up structure of the full song, but are still composed by Iberian-only elements – and this is what we should check. Mixed songs are longer and invariably include several Common Chiffchaff elements, this is, higher pitched “inverted ticks” (as named in Collinson & Melling, 2008) and/or h-shaped notes, both usually well above 7kHz (even reaching 8kHz) (Figure 19). These h-shaped elements are similar to Figure 18C, but are far more common in a Common/mixed singer song.
Figure 18. Songs of an Iberian Chiffchaff Phylloscopus ibericus. Crémenes, León, August 2019.
Figure 19. Mixed singer Phylloscopus cf. ibericus. Stanislas Wroza. Fontainebleau, Seine-et-Marne, Île-de-France. XC: XC557547. Both songs are emitted by the same individual but they are very different nonetheless. Although the duration and frequency are good for Iberian in both cases, the first song includes several h-shaped elements and inverted ticks and doesn’t give the tidied-up impression of the second song, that perfectly fits Iberian.
Whether mixed singing is diagnostic of a hybrid or not isn’t clear yet. While it is true that mixed singers are commoner in areas where both species occur (Bensch, et al. 2002), in these areas young Iberian Chiffchaffs have it easier to incorporate Common Chiffchaff elements in their song during the learning process, due to a higher exposition. This has been proven to be the case in some mixed singers with pure Iberian Chiffchaff genomes, but others have turned out to be hybrids (Bensch, et al. 2002). Therefore, it’s still particularly important to collect genetic material of as many mixed singers as possible in order to clarify the situation. In my opinion, a mixed singer in a vagrant context should be treated with great caution and must remain unidentified on plumage or even biometrics.
Call
The Iberian Chiffchaff call is a very characteristic shy and decreasing Bullfinch-like whistle. The only sources of confusion could be the so-called ‘swee-oo type’ Chiffchaffs or even the Siberian Chiffchaff. In Catalonia, swee-oo calls are very frequent in summer in areas where Common Chiffchaff breeds and, in this context, it’s emitted by young Chiffchaff. Later in the autumn, this call type can become very common in migration stopover sites and usually gets scarcer as winter progresses. Its disyllabic structure is very characteristic and should not lead to confusion, especially when the upwards end is noted. Siberian Chiffchaff in the other hand does have a downward end and, in my experience, they can sound surprisingly similar to Iberian. However, tristis always shows an arched shape, usually with a flat top that makes it sound more homogenous, rather than descending.
Figure 20. Calls of some Chiffchaff taxa. The first two types are emitted by nominate subspecies and presumably by abietinus too. Pure Siberian and Iberian Chiffchaff invariably produce these descending calls, although Siberian shows a flat top of the arch.
Scandinavian Chiffchaff, the real challenge?
The most typical Common Chiffchaff should not represent a real challenge when seen well, but some birds can look quite bright, probably due to a paler background coloration. This appearance can be seen mainly in late winter, when birds are about to undertake the pre-nuptial moult. However, these birds can be told apart by analysing other features such as supercilium shape and colouration, leg colour or overall structure.
The real issue in my opinion resides in abietinus type Chiffchaffs. There’s always been a lot of mystery surrounding this taxon and most references agree it’s indistinguishable from collybita, but the popularization of genetic sampling has recently allowed the identification of some vagrants. It wasn’t until 2014 that the first abietinus for Britain was identified and soon later, in 2015, Ireland had its first one too (Collinson, et al. 2018; O’Mahony, et al. 2015). The photos of these birds were familiar to Iberian birders though, since they all look quite alike Iberian Chiffchaff. The upperparts don’t show any brown tinges and instead are composed by a combination of pale grey and greenish feathers, while the underparts can show some pure white and surprisingly clean yellow. The jizzes are strikingly alike too, both taxa giving a more open-faced impression than collybita due to a similar eyebrow shape and a softer transition from cheeks to throat.
While all this was happening, I was ringing at Falsterbo Bird Observatory, SW Sweden, where we caught a very small number of birds in autumn that fitted this description (Figure 21 & Figure 22). Although it would be too speculative to identify them as abietinus, all I can say is that they look very similar to at least some of those genetically proven to be such. And there’s more: ever since the mid 00s, when I got to see the first photos, I’ve been paying attention to the Common Chiffchaffs ringed in Israel in Spring, to regularly find some birds that also fit this pattern and are quite striking from a W Med perspective. Do all ‘Scandinavian’ Chiffchaffs look like Iberian? Probably not, since they are still Chiffchaffs and hence very variable, but it’s worth to keep their potential resemblance in mind in order to spot either taxa as vagrants.
Figure 21. Top: Common Chiffchaff Phylloscopus collybita, presumably collybita. Middle: Common Chiffchaff Phylloscopus collybita ssp. reminiscent of both abietinus and Iberian Chiffchaff Phylloscopus ibericus. Bottom: Common Chiffchaff Phylloscopus collybita ssp; a very big individual of a Northern population. Falsterbo, Sweden, October 2014.
Figure 22. In front, a Common Chiffchaff Phylloscopus collybita reminiscent of both abietinus and ibericus; in the back, a presumably nominate Common Chiffchaff. Note the neat impression of the bird in front, with a better-defined eyebrow and some purer yellow tones, both in the upperparts and the underparts. Extensively white belly too. Falsterbo, Sweden, October 2014.
Figure 23. Common Chiffchaff Phylloscopus collybita. Fernando Arce. Israel, March 2008. Both the eyebrow and the underparts are very reminiscent of Iberian Chiffchaff: the eyebrow shows a more saturated fore part, the end of the eye being the border between this and a paler rear part. The transition between the cheeks and the throat is smooth and the underparts are clean and pale overall, with just some scattered yellow stripes. A bird like that could be misidentified as a dark-type Iberian (Figure 2), so vocal evidences must be gathered.
Figure 24. Common Chiffchaff Phylloscopus collybita ssp. Fernando Arce. Israel, March 2008. Similar to the bird in Figure 23, but apparently brighter yellow above. The bulky impression doesn’t fit Iberian thought, and could imply an Eastern or Northern influence.
Figure 25. Common Chiffchaff Phylloscopus collybita. Fernando Arce. Israel, March 2008. Again, very Iberian-like eyebrow and face pattern, in this case accompanied by a brighter green back and scapulars. In a vagrant context, this bird should be heard vocalising to clinch the id.
Acknowledgements
First of all, I want to thank Joan Castelló for all the conversations about Iberian Chiffchaff we had. His lessons were the seed of my later obsession. I also want to thank my blog-mates Guillermo Rodríguez for his insights on the Israeli birds, including the photos of several individuals, and Martí Franch, for the sketches included here to depict moult and primary spacing.
As usual, I must thank the photographers who kindly provided the pictures included in the post: Andy Butler, Juan Bécares, Miguel Ángel Fuentes, Fernando Arce and especially Fran Trabalon and Juan Sagardía, whose large photo libraries made this post possible.
Stephen Menzie was there to share the October fun in Falsterbo and I have to thank his inspiration to write this post and the two related articles in British Birds. He also reviewed the English and commented on the contents of the post.
References
Bensch, S., Helbig, A. J., Salomon, M., & Seibold, I. (2002). Amplified fragment length polymorphism analysis identifies hybrids between two subspecies of warblers. Molecular Ecology, 11(3), 473-481.
Castelló, J., Gil-Velasco, M. (2017). Primary moult in Iberian Chiffchaff as a means of ageing and identification. British Birds 110: 476 – 483.
Collinson, J. M., Murcia, A., Ladeira, G., Dewars, K., Roberts, F., Shannon, T. (2018). Siberian and Scandinavian Common Chiffchaffs in Britain and Ireland – a genetic study. British Birds 111: 384–394
Collinson, J.M., Melling, T. (2008). Identification of vagrant Iberian Chiffchaffs – pointers, pitfalls and problem birds. British Birds 101: 174–188.
Gil-Velasco, M. (2017). Primary spacing as a field identification criterion for Iberian Chiffchaff. British Birds 110: 476–483
O’Mahony, B., Farrer, D & Collinson, M. (2015). Genetic identity of wintering Common Chiffchaffs Phylloscopus collybita trapped in County Kerry in 2015. Irish Birds 10: 268-270.
Onrubia, A., Gómez, J., Andrés, T., Zufiaur, F., & Unanue, A. (2013). Identificación en mano y determinación del sexo y la edad del Mosquitero ibérico Phylloscopus ibericus. In: Rodríguez, N., García, J., & Copete, J. L. (eds.), El Mosquitero ibérico. Grupo Ibérico de anillamiento, León.