Following up on my recent post about roosting behaviour of the harriers in Chiang Saen, in this post, I’d like to focus more on the identification of the 4 species that can be regularly seen in Thailand. These 4 species include Eastern Marsh Harrier (Circus spilonotus), Western Marsh Harrier (C. aeruginosus), Pied Harrier (C. melanoleucos), and Hen Harrier (C. cyaneus). I have to say that I’m no expert on the identification of raptors, but I’d like to share some identification tips based on my observation.
Abundance
One of the first clues for identification is the abundance of each species. In Thailand, Eastern Marsh and Pied Harrier are the 2 widespread and relatively common species. Both of them can be found throughout the country, although Pied Harrier tends to become less abundant southward. The other 2 species, Western Marsh and Hen Harrier are both rare, so when finding a harrier in Thailand, there’s a much better chance that it would be either of the former 2 species rather than the latter two.
Structure
When looking at a harrier, structure is one of the first things that I pay attention to. Both Eastern and Western Marsh Harrier share the same body structure which typically appears bulky, with broad and blunt-tipped wings. The head and face also appear more pointed than in other species, particularly when seen from the side. On the other hand, Pied Harrier typically appears to have a flatter face, with narrower and more pointed wings. Hen Harrier also has a rather flat face, but the wings are much broader than in Pied Harrier. Both Pied and Hen Harrier are also smaller and appear slimmer than the 2 marsh harriers. By structure alone, it is usually enough to tell Pied and Hen Harrier from marsh harriers. However, identification of the 2 marsh harriers has to be done by looking at the plumage.
Note wing and head shape
The Confusing Pairs
Identification of the adult male of each species is usually quite straightforward. However, other plumage types of every species can be extremely variable and lead to a lot of confusion. Here are some of the species pairs that often confuse birders.
Eastern vs. Western Marsh Harriers
The most confusing plumage type of both species is probably the juvenile plumage. Both species share a similarly patterned juvenile plumage consisted of dark chocolate-brown plumage with pale area on the head. Eastern Marsh Harrier particularly has an extremely variable juvenile plumage with variable amount of paleness on the head and chest. The best way to identify the juvenile of both species is by looking at the facial and underwing pattern. Juvenile Western Marsh Harrier typically has more restricted pale patch on the head and throat with consistently dark lores unlike in juvenile Eastern Marsh Harrier, which typically has pale lores. The underwing of Western Marsh Harrier is also typically darker with little or no pale wash at the base of primaries. However, it seems that not all individuals can be safely identified due to the extreme plumage variation, as well as hybridisation which is not unusual among the 2 species. No wonder why they were once widely treated as the same species.
Another pair of plumage types that often confuses birder is between adult/subadult female Eastern Marsh Harrier and adult/subadult male Western Marsh Harrier. Both share similarly streaked head and breast with rufous colouration on the belly and undertail coverts. The best way to separate the two is by checking the wing and tail pattern. In adult/subadult male Western Marsh Harrier, the upperwing is largely grey with black wing tips. The underwing often appears plain whitish with prominent black wing tips and trailing edge. The adult-type tail feathers are also pale grey and unbarred when seen from above. On the other hand, adult/subadult female Eastern Marsh Harrier has variably brownish-grey upperwing with dark barring. The underwing also lacks the prominent black wing tips and the uppertail is brownish with prominent barring.
Subadult male Eastern Marsh Harrier vs. adult female Pied Harrier
Plumage-wise, this seems to be the most commonly confused pair in Thailand. Both share similarly whitish head and underparts with variably dark streaking on the head and breast. The upperwings are also similarly greyish. The best way to separate the two is by looking at the upperwing coverts and upper tail. In adult female Pied Harrier, there’s usually some white along the front edge of the upperwing coverts creating a “white-shouldered” look. The upper tail of adult female Pied Harrier is also grey with thin but prominent dark bars, whereas the adult-type upper tail of the male Eastern Marsh Harrier is plain grey without any prominent barring. Adult female Pied Harriers also typically show brown “facial disk” which is lacked in Eastern Marsh Harrier. However, from my experience, it is not uncommon for them to lack such obvious facial disk either. Another useful feature is the black markings on the flight feathers. Male Eastern Marsh Harriers have black bars and smudges restricted mostly to the tips of the primaries, whereas in female Pied Harriers, the black bars are more neatly and evenly spaced throughout the primaries and secondaries.
Female Pied Harrier and female/juvenile Hen Harrier
As mentioned earlier, Hen Harrier is a rare winter migrant in Thailand. Most records are either of an adult female or juvenile, while the adult male is the rarest plumage type here. The main challenge is often identifying the adult female/juvenile Hen Harrier from the adult female Pied Harrier. Both species share a similarly brownish plumage with brown facial disk and white rump patch. The best way to identify the two is by looking at the wing shape, face and wing patterns. Generally, female/juvenile Hen Harrier has a much more distinct facial disk than in Pied Harrier. The upperwing also lacks any obvious white shoulder patch or grey panel on the flight feathers like in female Pied Harrier. When seen from below, the dark bars on secondaries are also typically broader than in female Pied Harrier, with noticeable contrast between the darker secondaries and paler primaries, particularly in juvenile plumage. As mentioned earlier, Hen Harrier also has much broader and more rounded wings when compared to the Pied Harrier.
Plumage Variations
One of the things that make the identification of harriers much more complicated than most bird groups is certainly the plumage variability. It is incredible how variable the plumage of each species can be. Any sex or age group of each species seems to have a wide range of plumage variation. Here are some of the variants that I’ve photographed.
I hope the collection of my photos and description in this blog post give you some more ideas about how to identify these 4 harriers in the field. Of course, there will be individuals that can’t be safely identified, but it’s really a lot of fun once you start looking at them closely and trying to identify them to the species, sex and age level.
ayuwat 
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