One of the birds I’d hoped to see in Bhutan was the Bhutan Laughingthrush (Trocalopteron imbricatum). I saw this small bird only twice, but since it is a skulker, hiding in undergrowth, I consider myself lucky. In appearance it resembles the Streaked laughingthrush (T. lineatum) which I’d seen several times to the west. Unlike that widespread species, this one is seen only in Bhutan and northeast Assam.
The colours are distinctive, this has gray ear coverts unlike the dull rufous of the T. lineatum, and it has no streaking on its crown. It also lacks the rufous wing panels of the T. lineatum, and has a somewhat different call. Little is known about its diet and foraging behaviour. It breeds between April and June, so we had seen this individual in the middle of the breeding season. But we saw no sign of its mate.
The family of laughingthrushes (Leiothrichidae) is highly speciose, and since many of them are restricted to the Himalayan region, I was curious about their origins. A little search led me to a study of the genetics of this family which indicated that it originated in the middle of the Miocene epoch. The Leiothrichidae have weak flights and do not migrate. So the geologically rapid speciation of this family must be due to rapid changes in climate which disconnected and reconnected biomes multiple times. The reasons for these climatic changes have to be sought in the geology of the time.
The Miocene is the geological epoch between 23 million and 5.3 million years ago. The major climate changes during this period were globally warmer climates than today, and an increase in aridity. One of the major factors in this was the separation of the Antarctic, and the beginning of the ice sheets over it during the mid-miocene. The other was mountain building: the Sierra Nevada and Cascades were formed in North America, the Andes in South America, the rifting of the African plate leading to the East African highlands (where primates began to diverge), and the upthrust of the Tien Shan mountains. This last change, along with the separation of the Mediterranean and Tethys seas caused by the northward drift of the African-Arabian mass and its joining with Eurasia, created aridity in the Himalayan region.
Globally the changes led, on land, to the retreat of forests and the growth of grasslands, and, in the oceans, to the growth of kelp forests. Around the mid-miocene the Tethys sea dried up, the Tibetan plateau was uplifted, and rapid climate changes began in the homeland of the laughingthrushes. Subsequent uplift of the Tien Shan allowed a monsoon circulation to set in, and this region began to approach its modern climate. These changes not only allowed the laughingthrushes to speciate, but also to use the newly opened Arabian pathway to reach Africa.
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Don't hold your breath 
