Around South Africa in mini-posts: Heuglin's Gull

Please note: The authoritative / observational content of this blog is not my own. Content has been sourced from multiple published papers; the sources of which are broadly acknowledged.

There was some excitement in South Africa's Mpumalanga province recently with the discovery of a 'Lesser Black-backed' - type gull at Mkhombo Dam. Initially identified and locally verified as a vagrant Lesser Black-backed Gull [Larus fuscus], the record, in that context, was good enough to encourage some local interest. Subsequent questions about the validity of the identification were raised informally in the social forum. The idea that this gull was, in fact, a Caspian Gull [Larus cachinnans]; South Africa's first, was proposed for discussion.

Consensus, as it turns out, largely reliant on the knowledge of experts in the candidate-gulls’ respective Northern Hemisphere home-ranges, proved otherwise. By process of elimination, both on form & structure [non-authoritative] and on 'classic' or 'typical' moult-criteria, the vagrant was deemed to be Larus (fuscus) heuglini, treated as a full species elsewhere but only as a subspecies of the lesser black-backed gull complex in South Africa.

The ensuing online discussion highlighted a number of pertinent points:

There is insufficient data to accurately describe, split or lump the lesser black-backed gull complex with any authority.
There is very little understanding of the identification-criteria migrant gulls’ exhibit on their wintering grounds, particularly in the case of long-distance migrants and there is even less authoritative literature on the field-characteristics for the accurate identification of 'stay-over' / seasonally-vagrant gulls, typically out-of-range.

Before we look at the specifics of the Mkhombo gull, a bird in its second year, one or two points on the 'large white-headed gull' complex for context and a synopsis of the basic identification criteria for 2cy gulls in general.[2cy or 2^nd calendar year is the year, commencing January 1, post hatching]

Mitochondrial DNA sequences show that the large white-headed gull complex consists of two clades:

the "Aralo-Caspian' clade including inter alia Larus cachinnans, L.barabensis, L.(fuscus) heuglini, Lesser Black-backed Gull - L.fuscus.ssp. and Kelp Gull L.dominicanus; and
the 'Atlantic' clade which includes most individuals of Herring Gull ie: Larus argentatus.

The taxonomy, however, remains confused.

Until recently it was common practice to subdivide 'Lesser Black-backed Gulls' into five subspecies (ssp.), premised mostly on geographical distribution -

Larus f. fuscus - western Kola Peninsula [North Arctic]
Larus f.graellsii - NW Spain, Netherlands
Larus f.intermedius - Netherlands, Germany
Larus f.heuglini - Kola Peninsula
Larus f.barabensis - Asian Steppes

Recent publications, however, describe and note the classification of 'lesser black-backed gulls' [ie: NW dark-mantled gulls] into three species and they are: -

Lesser Black-backed Gull [Larus graellsii incl. intermedius]
Baltic Gull [Larus fuscus]
Tundra / Siberian or Heuglin's Gull [Larus heuglini]

Notwithstanding, on the basis of gene-flow scrutiny no differentiation has been found between any of the graellsii and intermedius populations. There is no evidence purporting a significant reproductive barrier between fuscus, graellsii and intermedius. Fuscus and heuglini, however, indicate some reproductive isolation even though there is evidence of continuing low levels of gene flow in spite of the apparent ecological separation of Larus fuscus fuscus and heuglini within the narrow contact zone. Extensive sharing of haplotypes, between these two taxa, may simply reflect the recent split from a shared ancestor [ie: ancestral polymorphism] rather than gene flow.

Northern Hemisphere authorities, however, adopted [subsequently reversed by the CSNA given the gene flow betw. fuscus, graellsii and intermedius] the proposed split of Lesser Black-backed Gull as follows:

Lesser Black-backed Gull [Larus graellsii and intermedius]
Baltic Gull [Larus fuscus]
Siberian Gull [Larus heuglini]

In South Africa the earliest classification of Lesser Black-backed Gull remains the status quo [ie: L.f.ssp incl. L.f.heuglini]. Why that is remains a mystery.

The distribution of wintering Larus fuscus.ssp. is little understood. Early literature split the migratory distribution of Larus fuscus as follows:

Larus f.fuscus - East Africa [ie: east of the Rift Valley and the eastern seaboard]
Larus f.graellsii and intermedius - predominantly West Africa and the western seaboard, southwards to Namibia.

In a recent study ring-recoveries of some 300 individuals of ssp. Fuscus, in Africa, concluded differently:

Whilst the Rift-Valley lakes are important for wintering Larus.f.fuscus most recoveries for this ssp. were from west of the Rift Valley and into the Congo Basin; and even further westwards to the west coast of Africa.
L.f.graellsii and intermedius recoveries were concentrated on the west coast of Africa.

There is evidence, therefore, to support the idea that the majority of Larus (f).fuscus [Baltic Gull] use the Eastern Flyway, on their winter-migration, whilst both Larus (f).graellsii and L.f.intermedius [ie: Lesser Black-backed Gull] use the Western Flyway.

Neither graellsii nor intermedius were ever recovered south of the equator. Given the prevalence of Larus f.fuscus [ie: 'Leapfrog migration'] south of the equator, 'Lesser Black-backed Gull' [or more accurately, Larus fuscus ie: Baltic Gull] may be more common in South Africa than is generally accepted.

The situation on Africa’s eastern seaboard remains unclear, however. The presence of a fourth taxon - Larus (fuscus) heuglini ie: Heuglin's / Siberian / Tundra Gull may be prevalent.

L.(fuscus) heuglini breeds in Arctic Russia and was thought to comprise of two forms: heuglini in the west; and taimyrensis in the east. It has subsequently been 'confirmed' that taimyrensis has no taxonomic validity. In addition Heuglini is, genetically at least, more worthy of species status than fuscus but the taxonomy remains unclear.

Given the complexity of the large white-headed gull complex, MOULT is relied on to support identification. There are, however, four important caveats:

Birds ringed as pulli, on the breeding grounds, are often misidentified given the geographical overlap of the ssp.
The identification criteria, in most published literature, is very time-specific - MANY of the features described for birds in the boreal [N. Hemisphere] summer do NOT hold true during the austral [S. Hemisphere] winter; esp. for vagrants.
Gulls are notoriously variable and rarely conform to ‘typical’ moult timing. Stress, injuries or poor condition can delay moult.
It is not uncommon for out-of-range birds to moult in a manner consistent with the moult-cycle of taxon / taxa present in that location. Vagrant Lesser Black-backed Gulls, in North America, regularly show moult-events consistent with the moult-cycles of local taxa.

The current view* on the identification of out-of-range [excl. the S. Hemisphere] 2^nd CY fuscus, heuglini and cachinnans is as follows: [*based on many field-observations of 2^nd CY birds; on the breeding grounds, in the boreal summer ie: June - Aug]

Any 2cy LBBG-type gull with fresh primaries / tail and a combination of worn brown and blackish upperparts is reliably fuscus.
Some heuglini individuals are indistinguishable from graellsii and intermedius.
There is no [critical] comparative analysis of 1^st cy heuglini and fuscus or for 3^rd- 4^th cy heuglini.
Fuscus and heuglini show significant variations in moult-timing and moult extent.
2^nd cy heuglini are often indistinguishable from same aged cachinnans [Caspian Gull] both in colouration and structure.
The following criteria, however, have been mooted to reliably separate 2cy heuglini from 2cy cachinnans:

heuglini show dark inner primaries, dark under-wing coverts, a broader tail-band and fresh rectrices. Cachinnans does not.
heuglini is comparatively smaller than cachinanns.
cachinanns shows more juvenile wing-coverts; with extensive wear.
adult scapulars appear in heuglini at the start of the boreal summer - in cachinnans this is thought to occur later in the year.

Despite assertions to the contrary, there is very little empirical evidence, premised on comparative analysis, to reliably exclude one ssp. in favour of another particularly from photographic evidence only. That applies to gulls on their wintering grounds generally and perhaps, more implicitly, in the case of vagrant gulls specifically.

2cy L.(fuscus)heuglini