Blue-throated Bee-eater (Merops viridis)(Linnaeus, 1758)
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Blue-throated Bee-eater. Photo by Tee Yu Xun, used with permission.

1. Overview

There are 25 species of Bee-eaters (Meropidae) worldwide [16]. They are a family of colourful birds related to the similarly colourful kingfishers and rollers. They are so called because their diet contains bees and wasps. They feed on other insects as well. Like swifts and swallows, they catch their prey on the wing.Singapore has 2 species, Blue-throated bee-eater, and Blue-tailed bee-eater. The blue-throated bee-eater is recognisable by its bright blue throat, chestnut head and back, and black eye stripe. Its upperparts and wing feathers are predominantly green, while its lower back,rump, and tail are a brilliant blue. Adults have distinctive long tail feathers. Juveniles lack the long tail feathers and have a bluish green head. Adults measure about 28cm in length, including their tails.Their flight is jerky and undulating, with a few rapid wing beats followed by a long glide. Their call sounds like “berek berek”, giving rise to its Malay name, and is frequently uttered in flight. Both Blue-tailed and Blue-throated Bee-eaters have long streamer tails, have similar calls, live in the same habitat. Both have blue tails, but the Blue-throated Bee-eater has a blue throat, whereas the Blue-tailed Bee-eater has a rufous throat. And the latter lack the chestnut head of the former. Also, they are found in Singapore at different times of the year with a slight overlap: Blue-throated from April to September, Blue-tailed from August-March [2].

2. Morphology:

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Morphological annotations of Blue-throated Bee-eater. Photo by Tee Yu Xun, used with permission.
As a member of the Coraciiformes, the blue-throated bee-eater possesses syndactyl feet whereby the second and third digits are joined. Males and females do not possess any significant visible sexual dimorphism. Juveniles (bottom), however, have a deep green crown instead of chestnut brown and lacks the blue throat. The green feathers are shed during the post-juvenile moulting and replaced with chestnut brown feathers over time. “Patchy” (individual on the left) appearance of the crown and mantle is indicative of a younger age.
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Juvenile Blue-throated Bee-eater. Photo by rusli, permission pending.

3. Vocalization:

[3]
The call of the blue-throated bee-eater is usually a sharp high-pitched “be-rek”. This is usually heard during flight. [2]

4. Biology:

4.1 Feeding:

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Blue-throated Bee-eater knocking an Asian Honey Bee. Photo by Alan Chou, used with permission.

As its name suggests, bee-eaters prey on bees along with other invertebrates, mostly insects such as wasp, dragonflies, and butterflies. Blue-throated Bee-eaters are aerial insectivores [4]. They hunt and capture invertebrate prey mid-air using their beak. This is followed by the knocking of the prey on a hard surface which removes the stinger (below). During this process, the prey is highly stressed causing it to release its venom before it is being consumed (above). Such a behaviour can be observed in other members of the Coraciiformes such as kingfishers and rollers[5].
Blue-throated Bee-eater FIM@ Punggol End 17 Jun 2016
Video of Blue-throated Bee-eater feeding. Video by Alan Chou, used with permission.

4.2 Bathing:

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A Blue-throated Bee-eater dust bathing. Photo by Graeme, permission pending.
Like many other birds, blue-throated bee-eaters are host to various parasites[6][7]. In order to keep themselves healthy, they take daily baths to remove parasites such as chewing lice and to keep their feathers clean. Bathing involves fluffing of feathers while beating at the water while dipping its head into it every now and then. Following which, the bee-eater removes excess water off by shaking away the remaining droplets and flying off to further dry itself. If water is not available, bee-eaters can also indulge in sand bathing. They roll about in loose sand fluffing their feathers and shaking vigorously. It is believed that the sand particles help absorb oil, removes dry skin and parasites that can damage the feathers or suck the blood of the birds[8].

Video of a juvenile Blue-throated Bee-eater preening itself. Original Video from YouTube by Leong Tzi Ming.
Grooming of feathers usually comes after bathing. It is crucial to keep each filament of the feather, free and clean for flight. Thus the bird preens itself by passing each primary through the beak cleaning it in the process[8] (above).

4.3 Courtship

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A male offering food gift to a younger female. Photo by Tee Yu Xun, used with permission.
Courtship often begins with males calling out and raising of throat and wing feathers as a form of display [9]. This is usually followed by an offering of food prey as a form of "nuptial gift". Once the gift is accepted, copulation follows. More often than not, multiple gifts are required before the female accepts her mate. The pair would be perched in proximity and the male slide up to the female which most likely lean forward to facilitate mounting. The male then mounts the female and use his bill to push her head down. As copulation begins, the male will mount the female while flapping his wings to maintain balance. At the same time, females will hang tightly onto the perch so as not to be displaced. This entire process is completed rather quickly in a matter of seconds [10].

4.4 Nest Building

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Diagram of a Blue-throated Bee-eater burrow. Image by Mickey Lim, permission pending.
Blue-throated Bee-eaters build their nest in underground tunnels on soft soil (above). The birds fly directly onto the ground, dig the soft soil with their feet to slowly excavate the tunnels. As the depression deepened, they use their beaks to dislodge the soil and feet to displace the loosened particles backwards. Each bout of digging took a few minutes before the birds flew off to rest. Excavation continued until the nests were ready. While their beaks make good picks for digging, their small feet is not suitable for such an activity. As such, beaks are used to break up the soil while the feet rapidly kick back loose earth, with the curved wings forming a ‘half-pipe’ to elevate the legs allowing ease of movement and to direct the loose soil. Each time the birds approached the nest entrance to start the digging, they would look out around for dangers before digging (below) [6][11].
Blue-Throated Bee-eater Digging hole @ Punggol End 20 Jun 2016
Video of a Blue-throated Bee-eater digging. Video by Alan Chou, used with permission.

4.5 Egg Laying

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A clutch of Blue-throated Bee-eater chicks. Photo by Mickey Lim, permission pending.
As the time for egg laying arrives, females would retreat into the nest to lay the eggs. A clutch of approximately 2-7 eggs is laid over a few days apart from which zero to three young were raised. This is to facilitate asynchronous hatching resulting in chicks of variable size and age from a single clutch. Sometimes, larger clutches of eggs can be observed. These clutches were suspected of containing eggs dumped by extra-pair females. Incubation takes around 30 days during which time the male will feed his partner while she incubates the eggs. Bee-eater broods hatched over periods of up to 10 days [6].

4.6 Brood Care

When the chicks hatch they are supplied by a minimum of the two parents. Most nests, however, were supplied by 3-5 birds. Along with the parents, feeding was carried out by older birds who may have lost a partner and in particular by genetically related younger birds from the last brood. These younger birds could be identified by a lack of the central, elongated, tail feathers as well as remnants of green feathers on their heads giving a patchy appearance. These young adults seem to be highly efficient hunters given the high frequency by which they returned to the nest with food [6][11]. Losses of chicks occurred throughout nesting period but it’s reported to be most frequent during the early stages. Due to the nature of asynchronous hatching, chicks varied in sizes. Mortality often begins with smaller and younger individuals due to the lack of food and attacks from their larger siblings. This behaviour has inferred from peck or puncture wounds on the upper body and head of naked or downy chicks[6].


5. Distribution

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Distribution ranges of Blue-throated Bee-eater. Figure adapted from IUCN Red List.
The blue-throated bee-eater is a widespread Southeast Asian species that can be found in countries such as Brunei, Cambodia, China (vagrant), Hong Kong (vagrant), Indonesia, Laos, Malaysia, the Philippines, Singapore, Taiwan (vagrant), Thailand, and Vietnam [2][3][4].

In Singapore, their habitats include secondary forest, scrub, old plantation and occasionally parklands. Birds nest communally, excavating nest burrow in sandy banks on quarry faces and man-made sand piles. Birds usually breed in Singapore from March to June but migrate to Indonesia from September on [2].

6. Conservation

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Conservation status of Blue-throated Bee-eater. Figure adapted from IUCN Red List.
Conservation status of the Blue-throated Bee-eater is listed as least concern owing to its large geographical range and availability of habitats that are not threatened. However, this may change in the future as urbanisation may result in the loss of these habitats [12].


7. Taxonomy and Systematics


7.1 Type Specimen

The holotype of the Blue-throated Bee-eater was collected from Myanmar and described by Carl Linnaeus in 1758 under Systema Naturae.
It is currently deposited at Museum für Naturkunde, Berlin [13].
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Holotype of the Blue-throated Bee-eater. Image adapted from Global Biodiversity Information Facility

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Original description by Carl Linneaeus in Systema Naturae [14].
More Details of Type specimen: http://www.gbif.org/occurrence/1038124674

7.2 Taxonomic hierarchy


Kingdom:
Animalia
Phylum/Division:
Chordata
Class:
Aves
Order:
Coraciiformes
Family:
Meropidae
Genus:
Merops
Species:
M. viridis

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Blue-throated Bee-eaters belongs to the order Coraciiformes which falls under Coraciimorphae along with woodpeckers, hornbills, trogons, cuckoo-roller and mousebirds [15]. Image adapted from Jarvis et al. (2014)

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Bayesian consensus tree. Bayesian probabilities are above the nodes, likelihood bootstrap proportions are below the nodes on the reduced data set consensus tree. Inset box shows likelihood bootstrapping proportions for an alternate arrangement of taxa [16]. Image adapted from Marks et al. (2007)
Single most parsimonious tree (length = 2636, CI = 0.516, RI =0.58) shown with bootstrap values from 100 bootstrap replicates above the nodes [16].
Image adapted from Marks et al. (2007)
Phylogenetic trees (above) were constructed using both mitochondrial and nuclear genes (ND2, BFib5 and TGFb2). Both trees placed M. viridis under Clade B which is a group of bee-eaters that are migratory and widely spread across the Old World [16]. While both phylogenetic trees indicated a subspecies of Merops viridis as M. v.americanus, recent taxonomic revisions have elevated the subspecies, M. v. americanus to a full species of M. americanus commonly known as the Rufous-crowned Bee-eater [1].

7.3 Blue-throated Bee-eater vs Rufous-crowned Bee-eater

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Comparision of Blue-throated Bee-eater and Rufous-crowned Bee-eater. Photo on the left by Tee Yu Xun, used with permission. Photo on the right by Chris Chafer, permission pending.
The Rofous-crowned Bee-eater (Right) is restricted to the Philipines while the Blue-throated Bee-eater (Left) is widespread across Southeast Asia. Morphologically, It differs from M. viridis in having blue of throat and upper breast reduced to slight tinge spreading from malar area, so that underparts appear virtually all green; crown to mantle rich rufous rather than dark chestnut; wing feathers and coverts with little or no metallic mid-blue coloration ; much broader and unfraying vanes of central rectrices, shafts remaining black (vs shading to white), vanes retaining intense blue of rest of tail (vs shading to pale greenish or fraying to nothing), and broadly and squarely tipped black (vs tapering to two bare points) [17].


7.4 Other Closely Related Species

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Chestnut-headed Bee-eater (Merops leschenaulti). Photo by Sulakna Obeysekara, permission pending.
Blue-tailed Bee-eater (Merops philippinus). Photo by Richard Lloyd, permission pending.
Green Bee-eater (Merops orientalis). Photo by Shakyasom Majumder, permission pending.
Forehead, crown, nape, lower face and ear-coverts bright chestnut ; lores black, continued as a band under the eye and ear-coverts ; wing-coverts, lower back and tertiaries green, the latter tipped with bluish; rump and upper tail-coverts pale shining blue; primaries and secondaries green, rufous on the inner webs, and all tipped dusky ; central tail-feathers bluish on the outer, and green on the inner webs ; the others green, margined on the inner web with brown and all tipped dusky ; sides of face, chin and throat yellow ; below this a broad band of chestnut extending to the sides of the neck and meeting the chestnut of the upper plumage ; below this again a short distinct band of black and then an ill-defined band of yellow ; remainder of lower plumage green, tipped with blue, especially on the vent and under tail-coverts [18].
A greenish bee-eater with a prominent orangy-brown throat. Black eye-stripe with white patch from base of bill. Upperparts mostly green. Underparts lighter green, grading into blue on the lower belly and vent. Lower back, flight feathers and tail rich blue. Juveniles are less strongly marked [2].
Like other bee-eaters, this species is a richly coloured, slender bird. It is about 9 inches (16–18 cm) long with about 2 inches made up by the elongated central tail-feathers. The sexes are not visually distinguishable. The entire plumage is bright green and tinged with blue especially on the chin and throat. The crown and upper back are tinged with golden rufous. The flight feathers are rufous washed with green and tipped with blackish. A fine black line runs in front of and behind the eye. The iris is crimson and the bill is black while the legs are dark grey. The feet are weak with the three toes joined at the base. South-east Asian birds have rufous crown and face, and green underparts, whereas Arabian beludschicus has a green crown, blue face and bluish underparts. The wings are green and the beak is black [19].

7.4 DNA Barcode

DNA sequences of both nuclear and mitochondrial genes can be found here.

8. References

1. Collar, N. J. (2011). Species limits in some Philippine birds including the Greater Flameback Chrysocolaptes lucidus. Forktail, 27, 29-38.
2. Yong Ding Li, L. K. (2013). A Naturalist's Guide to the Birds of Singapore. Singapore: John Beaufoy Publishing.
3. Xeno-canto. (2016, November 20). Retrieved from Xeno-canto: http://www.xeno-canto.org/explore?query=merops+viridis
4. Blue-throated Bee-eater (Merops viridis). (2016, November 20). Retrieved from Handbook of the Birds of the World: http://www.hbw.com/species/blue-throated-bee-eater-merops-viridis
5. Line, L. (1993). A bug's worst nightmare. International Wildlife, 23(4), 30.
6. Bryant, D. M., & Tatner, P. (1990). Hatching asynchrony, sibling competition and siblicide in nestling birds: Studies of swiftlets and bee-eaters. Animal Behaviour, 39(4), 657-671. doi:10.1016/S0003-3472(05)80377-X
7. Siefferman, L., Yuan-Jyun Wang, Yi-Ping Wang, & Yuan, H. (2007). Sexual Dichromatism, Dimorphism, and Condition-Dependent Coloration in Blue-Tailed Bee-Eaters. The Condor, 109(3), 577-584. Retrieved from http://www.jstor.org.libproxy1.nus.edu.sg/stable/4500993
8. Bird Ecology Study Group. (2016, November 20). Retrieved from Sun and Dust Bathing: http://www.besgroup.org/2006/12/23/sun-and-dust-bathing/
9. Burt, D. (2002). Social and Breeding Biology of Bee-Eaters in Thailand. The Wilson Bulletin, 114(2), 275-279. Retrieved from http://www.jstor.org.libproxy1.nus.edu.sg/stable/4164452

10. Bird Ecology Study Group. (2016, November 20). Retrieved from Blue-throated Bee-eater: 1. Courtship and mating: http://www.besgroup.org/2009/07/05/blue-throated-bee-eaters-courtship-and-mating/
11. Malaysian Wildlife Photography. (2016, November 20). Retrieved from Blue-throated Bee-eater nesting in Penang: http://malaysianwildlifephotography.blogspot.sg/2012/07/blue-throated-bee-eater-nesting-in.html
12. The IUCN Red List of Threatened Species. (2016, November 20). Retrieved from Merops viridis : http://www.iucnredlist.org/details/22725900/0
13. Global Biodiversity Information Facility. (2016, November 20). Retrieved from GBIF 1038124674: http://www.gbif.org/occurrence/1038124674
14. Linné, Carl von (1758). Systema naturæ per regna tria naturæ, secundum classses, ordines, genera, species, cum characteribus, differentiis, synonymis, locis (p. 460). Göttingen.
15. Jarvis, E.D., et al. (2014). Whole-genome analyses resolve early branches in the tree of life of modern birds. Science, 346(6215), 1320-1331. doi:10.1126/science.1253451. PMC 4405904. PMID 25504713.
16. Marks, B. D., Weckstein, J. D., & Moyle, R. G. (2007). Molecular phylogenetics of the bee-eaters (aves: Meropidae) based on nuclear and mitochondrial DNA sequence data. Molecular Phylogenetics and Evolution, 45(1), 23-32. doi:10.1016/j.ympev.2007.07.004
17. Handbook of the Birds of the World. (2016, November 20). Retrieved from Rufous-crowned Bee-eater (Merops americanus): http://www.hbw.com/species/rufous-crowned-bee-eater-merops-americanus
18. Bishop, K. D. (1999). Birds of the indian subcontinent by richard grimmet, carole inskipp and tim inskipp. Emu, 99(2), 153. doi:10.1071/MU99018A_BR
19. Popular handbook of indian birds. (1942). Ibis, 84(3), 449-450. doi:10.1111/j.1474-919X.1942.tb05725.x