Ageing and sexing of European Bee-eater (Merops apiaster)

8^th March 2021 | Ringing

UPDATED on 21^st July 2021 – morphometric sexing and sexing in general was updated.

» Česká verze zde (Czech version): Určování stáří a pohlaví vlhy pestré (Merops apiaster).

Abbreviations:

greater coverts: GC
median coverts: MC
lesser coverts: LC
primary coverts: PC
carpal covert: CC
primaries: P
secondaries: S
tertials: T
alula: Al

European Bee-eater is probably not the commonly trapped species and most of the bird ringers probably trap them only by mere chance. It may then seem odd to start a new series of articles about ageing and sexing European birds with this particular species. When we consider the fact that my wife and I study a population of bee-eaters quite intensively, it suddenly becomes quite a logical choice. I believe that information and photos in this article might be useful to at least some of the bird ringers around. In the end, most of the sexing and ageing criteria are quite apparent also on photographed animals and this article could be useful to birders, too.

Quick index: Moult | Ageing | Sexing | Other cases | References

You can find more photographs of this species on this website in the Gallery section: Merops apiaster.

Each photograph in this article is accompanied by a number in brackets – a link to the gallery (opened in a new tab) where you can see other photos of that individual.

Moult

Adult: SC, wp
Juvenile: SC, wp

The moult of the European Bee-eater could be simplified as a complete post-breeding and post-juvenile moult and partial pre-breeding moult in winter. Reality is a bit more complex, and one moult usually blends into other without clear temporal separation. On top of that, some individuals skip part of the moult altogether.

Moult of adults

Adult birds start their “complete” post-breeding moult when they are still breeding. First to moult are the body feathers, mostly on back and throat. Moult of remiges starts in August with P1 and continues until about P3. Then the moult is suspended and is resumed at the wintering-grounds. Before the departure from breeding grounds, bee-eaters acquire their eclipse (non-breeding, winter) plumage, with mostly green upperparts (fig. 1.3) – and as such possibly quite unfamiliar to many people. Extent of moult of body feathers during post-breeding moult is highly variable and most often than not only part of the body feathers is moulted (fig. 1).

On the wintering grounds, post-breeding moult transiently changes into partial pre-breeding moult which involves replacement of body feathers including the ones moulted during post-breeding moult. This moult basically overlaps with the previous one because suspended moult of remiges is resumed on the wintering grounds as well as the moult of rectrices. This moult leads to the typical breeding (summer) plumage (fig. 1.2).

Part of the adult birds does not start moulting the body feathers before migration and start to moult them on the wintering grounds. In that case, no eclipse plumage is acquired, and the birds moults right into new breeding plumage. Some of the adult birds do not moult remiges before the migration either.

Moult of juveniles

Post-juvenile moult of juvenile birds moulting from juvenile plumage (fig. 1.4) is like the post-breeding moult of adults. Unlike adults, juveniles do not start to moult remiges before migration. Otherwise, it is also a complete moult. Similarly to adults, moult of body feathers starts when still on breeding grounds and juveniles moult into similar (green) eclipse plumage. This moult also blends into the partial pre-breeding moult like in adults. The overall timing is just a bit shifted (fig. 1.1).

Another similarity with adults is the (quite) common skipping of body moult prior migration. Then, the green eclipse plumage is not acquired at all but juveniles moult right into the breeding plumage.

Fig. 1.1: Progress of the moult of body feathers and remiges during the year in adults and juvenile birds. Hatched squares represent time when only some birds moult, solid colour represent time when most birds moult. Line in adult remiges represents suspended moult.

Fig. 1.2: Breeding (summer) plumage of European Bee-eater. [CZEP20-275]

Fig. 1.3: Non-breeding (eclipse or winter) plumage of European Bee-eater. [CZEP19-264]

Fig. 1.4: Juvenile plumage of fledged offspring. [CZEP20-299]

Ageing

Even though European Bee-eaters undergo a complete post-juvenile moult, it is still possible to distinguish between last year’s birds (2CY, euring 5) and older individuals (2CY+, euring 6) in the spring. We focus on pattern and colouration of PC as there are practically no other useful criteria in plumage. Ageing the European Bee-eater is in this aspect similar to ageing the Blue Tit (Cyanistes caeruleus).

Bee-eaters arrive at the breeding grounds with relatively fresh plumage in good condition. Their plumage remains surprisingly intact and not even birds in August show excessive abrasion of remiges. Only tail streamers seem very prone to wearing (fig. 2.3.4), bleaching is evident only on feathers of back and tips of P not covered by S. This, and the fact that plumage of adults and juveniles grows at almost same time, makes using general wear of feathers unusable for ageing.

A particular exception is the inner P which adult birds replace at the breeding grounds. In that case, such feathers are two to three months older than the rest of the wing and are more bleached and worn compared to adjacent S. This difference becomes progressively less evident quite fast during the breeding season if we consider bleaching of the tips of P. Not all individuals start their P moult at the breeding grounds, too (see Moult)

2CY (euring 5) – last year’s birds, birds in their first year of life

Last year’s birds can be recognized by the pattern and colouration of PC and they contrast with surrounding feathers. PC markedly contrast with Al, GC and P, their colouration is less saturated, and they look lighter in colour than surrounding feathers. They have two-colour pattern – outer vane is blue-green, inner one is grey or brown (depends on bleaching; fig. 2.1.1).

Colouration of iris is same as in older birds – deep rich red (fig. 2.2.2). There is no moult limit between inner P and surrounding remiges (fig. 2.2.3).

Fig. 2.1.1: PC of two 2CY (euring 5) individuals, examples of variability. Left female, right male. [CZEP19-230, CZEP20-276]

Fig. 2.1.2: Head and iris coloration of a 2CY (euring 5) bird. Iris is rich red. [CZEP19-255]

Fig. 2.1.3: Wing of a 2CY (euring 5) female. No moult limit in remiges present. [CZEP20-167]

2CY+ (euring 6) – older birds, birds in their second year of life or older

PC of before last year’s and older birds are of same colour as surrounding feathers (vivid blue-green), they do not contrast with Al, GC or P. They lack the colour pattern as 2CY birds, both outer and inner vane are of same colour (or with a small grey wedge-shaped marks) and they’re not worn (fig. 2.2.1).

Colouration of iris is, as in 2CY birds, deep rich red (fig. 2.2.2). Some birds may exhibit a moult limit in P, then inner 1—3 P are a bit more bleached than the rest of feathers (fig. 2.2.3).

Fig. 2.2.1: PC of two 2CY+ (euring 6) individuals, examples of variability. Left female, right male. [CZEP20-231, CZEP19-262]

Fig. 2.2.2: Head and iris coloration of a 2CY+ (euring 6) bird. Iris is rich red. [CZEP20-227]

Fig. 2.2.3: Wing of a 2CY+ (euring 6) male. Note the moult limit in P. [CZEP20-226]

1CY juv (euring 3J) – birds in juvenile plumage, this year’s birds

Colouration of the juvenile (this year’s) individuals of the European Bee-eater may recall the adult birds in eclipse plumage (especially the back) but can be separated by following criteria. Their overall coloration is duller, upperparts and wing colouration is mossy green (fig. 1.4) rather than blue-green like in adults and they lack brown wing patch in coverts (fig. 2.3.1).

Colour of iris is not deep rich red but dark brownish grey (fig. 2.3.2); central pair of tail feathers lack streamers and do not protrude other rectrix, this makes tail round (fig. 2.3.3) – beware of adult birds with worn streamers (fig. 2.3.4)!

Fig. 2.3.1: Detail of PC of juvenile (1CY juv, euring 3J) birds, examples of variability. [CZEP20-299, CZEP20-315]

Fig. 2.3.2: Colouration of juvenile (1CY juv, euring 3J) individual. Note the back, wing and iris colouration. Iris is not red like in adults but rather dark brownish grey. [CZEP20-299]

Fig. 2.3.3: Tail of a juvenile (1CY juv, euring 3J) individual with its typical shape lacking the central pair’s streamers. [CZEP20-299]

Fig. 2.3.4: Very worn tail of an adult (2CY+, euring 6) male – central pair of rectrix is worn to such an extent that the tail recalls that of a juvenile individual. This can be confusing when seen in flight. [CZEP19-227]

Sexing

When sexing, we focus primarily on differences in coloration of LC, MC, GC and S. Females ten to be less vividly coloured then males, but this difference is (at best) perceivable only when both of the pair can be compared, otherwise practically useless.

Adult bird

For sexing adult birds – that means last year’s (2CY, euring 5) and also older (2CY+, euring 6) – we inspect pattern and colouration of LC, MC, GC and possibly also S.

♂ Coverts of males are basically wholly rufous and of single colour, and they form a large rufous wing patch. Inner GC and T are always green as well as LC in the carpal area (forming a small green patch. Inner S are rufous with dark tip, outer ones have green tinge.

♀ GC of females are rufous with a green edge variable in thickness, sometimes very thin but always present. MC and LC are always rufous with green edge or are completely green, not completely brown like in males and they do not form uniform brown wing patch. S are brown with green edge, sometimes thin.

Fig. 3.1: Typical appearance of female (left) and male (right) coverts and S in 2CY (euring 5) birds. [CZEP20-213, CZEP20-212]

Fig. 3.2: Typical appearance of female (left) and male (right) coverts and S in 2CY+ (euring 6) birds. [CZEP20-168, CZEP20-227]

European Bee-eater shows a certain size dimorphism – males tend to be bigger than females. According to a study [link] on a dataset of bee-eaters across its breeding distribution it is possible to sex this species using a formula containing wing length and tail streamer length. This formula should be able to correctly sex ca. 90 % of birds, on our dataset^[1], however, we achieved “only” 82 % of correctly sexed birds. Tail streamers can wear of quite quickly and by the end of breeding, some birds can have none (fig. 2.3.4). Alternatively, only wing length can be used, which is not that precise (our dataset 80 %): male ≥ 147.5 mm, female < 147.5 mm (tab. 3.1, fig. 3.3). Tail streamer length and body mass can be also used to sex but they somehow overlap more than winglength. Both these values change during the breeding season, wear was mentioned but birds also tend to get lighter during the breeding season.

Tab. 3.1: Distribution of wing length of bee-eaters from Czechia, n is the sample size.

Fig. 3.3: Boxplot showing the wing length, streamer length and body mass of males and females of the European Bee-eater. Box shows 50 % of all observations, line inside box shows median, and cross marks average value, whiskers show minimum and maximum values. For sample size see tab. 3.1.

Sexing birds using broodpatch is not possible as both sexes incubate eggs and brood youngs and develop broodpatch. That is also never as vascularized as in passerines.

Fig. 3.4: Well developed brood patch of a male European Bee-eater, early stage 5 (brood patch is starting to regrow feathers). Both sexes develop same looking brood patch and this criterion is therefore not usefull for sexing.

Juvenile birds

Sexing of juvenile (1CY juv, euring 3J) birds is theoretically possible, practically we can sex just the extreme cases. Variability in colouration seems to be quite extensive and some criteria needs to be verified. Juvenile P are shorter than those of adult birds and cannot be used to sex birds. Tail streamers are not present at all.

Differences in colouration of coverts and remiges are similar to adult birds but much less pronounced. Males have S mostly rufous brown with green tinge, GC are brownish green. S of females are rather green with brown tinge, GC are green. LC and MC are green in both sexes. It is necessary to remember that whole wing of juvenile birds is mossy green and all remiges have slight brown tinge (fig. 3.5).

Fig. 3.5: Differences in colouration of coverts and remiges of juvenile individuals (1CY juv, euring 3J); left probably male, right probably female. [CZEP20-299, CZEP20-315]

Other cases

More complicated cases

Ageing European Bee-eater is not exactly complicated, most of the individuals can be sexed with certain amount of experience. Only few individuals can be a little ambiguous (fig. 4.1). If we are not certain by the age of a certain bird, then do not assess it.

Fig. 4.1: PC of last year’s female (2CY, euring 5) that are rather richly coloured and does not contrast much with surrounding feathers. Age can be assessed based on the pattern of PC (inner vane) and their shape – very worn. [CZEP19-253]

On the other hand, sexing can be sometimes difficult, especially in older females or last year’s males, in such cases the rufous wing patch can be similar in size. Males always have GC, MC and LC as well as S without green tinge.

Fig. 4.2: Pattern and colouration of coverts and S of a 2CY+ (euring 6) female. Extent of brown is quite great for a female, but all rufous feathers have also a greenish edge or tinge, most noticeably on LC. [CZEP20-231]

Fig. 4.3: Pattern and colouration of coverts and S of a last year’s male (2CY, euring 5). Extent of rufous on LC is a bit smaller than usual, but nonetheless it is far more extensive than most females. No brown feathers have noticeable green edge or tinge like in females. [CZEP20-279]

Fig. 4.4: Pattern and colouration of coverts and S of a last year’s male (2CY, euring 5). LC and MC are not pure rufous but have an edge of different colour. That edge is not green but golden. Only males can sometimes show edges of coverts of such colours. Also, the extent and colouration of GC and S is clearly indicative of a male bird. [CZEP20-276]

Recaptures

Following pictures show repeat trapping of same individuals. You can clearly see how the pattern and colouration change between last year’s and older birds, and how such colouration does not change with age later in life.

» 2CY (euring 5) → 3CY (euring 7)^[2]

Fig. 4.5: Example of a change of pattern and colour of PC between one year old and two years old bird, female. [CZEP19-260, CZEP20-229]

Fig. 4.6: Example of a change of pattern and colour of PC between one year old and two years old bird, male. [CZEP19-223, CZEP20-190]

» 2CY+ (euring 6) → 3CY+ (euring 8)^[2]

Fig. 4.7: Example of PC of recapture of an older bird (at least two years old in first case), female. [CZEP19-257, CZEP20-283]

Fig. 4.8: Example of PC of recapture of an older bird (at least two years old in first case), male. [CZEP19-256, CZEP20-282]

References

Baker, J. 2016. Identification of European Non-Passerines. British Trust for Ornithology.

Blasco-Zumeta, J. & Heinze, G.-M. 2020. European Bee-eater (Merops apiaster). URL , accessed: 24^th February 2021

Costa J. S., Hahn S., Rocha A. D., Araújo P. M., Olano-Marín J., Emmenegger T., Alves J. A. 2020. The discriminant power of biometrics for sex determination in European Bee-eaters Merops apiaster. Bird Study 67(1), 1–10. doi:10.1080/00063657.2020.1728229

Cramp S. & K. E. L. Simmons (eds) 2020. BWP: Birds of the Western Palearctic app. NatureGuides Ltd.

Demongin, L., Lelièvre, H. & Candelin, G. 2016. Identification Guide to Birds in the Hand. Laurent Demongin, Peronnas.

Fry, C. H. 1984. The Bee-eaters. T & AD Poyser, London.

[1] Which is not verified by e.g., molecular sexing, though, only by trapping whole pair (helpers seem to be quite rare at our site).

[2] The 3CY (euring 7) category and the 3CY+ (euring 8) cannot be distinguished from the 2CY+ (euring 6) category in the field and they would be aged as 2CY+. They are called like that here for the only purpose of stressing out that the age of the bird is either known exactly or in greater detail.