Several avian brood parasites appear to be generalists at the species level, utilizing a range of host species. Such parasites, however, may consist of several host specific races (also called gentes), each utilizing one or a few host species (de Brooke and Davies 1988; Moksnes and Røskaft 1995; Davies 2000; Gibbs et al. 2000). Each gens has often evolved host-specific adaptations as a response to anti-parasite adaptions in their particular host, of which egg mimicry is one of the most prominent parasite adaptations (de Brooke and Davies 1988; Moksnes and Røskaft 1995; Davies 2000; Starling et al. 2006; Antonov et al. 2010; Fossøy et al. 2011, 2016). So far, most of the research on gentes-specific adaptations has involved studies on Common Cuckoos (Cuculus canorus), while our knowledge of the existence of such adaptations in many other members of the Cuculidae family is much more fragmentary (but see e.g. Starling et al. 2006; Spottiswoode 2010; Feeney et al. 2014). For poorly known brood parasites, the first key information is to provide background data on host use in various parts of their range.
The Plaintive Cuckoo (Cacomantis merulinus) is an interspecific obligatory brood-parasitic bird, with a wide range in south and south-east Asia (Becking 1981; Yang et al. 2012). There are four recognized subspecies: C. m. merulinus in Philippines, Sulawesi and Borneo; C. m. querulus distributed from India and China through Bangladesh, Bhutan, Myanmar, Laos, Thailand, Vietnam and occasionally Nepal and Sri Lanka; C. m. threnodes in Malay-Peninsula and Sumatra; and C. m. lanceolatus in Java (Payne 2005; Erritzøe et al. 2012).
There exists some information on host use and egg appearance of Plaintive Cuckoos in parts of their range, but in general adaptations to their parasitic lifestyle is poorly known. Baker (1942), however, reports that Plaintive Cuckoo chicks evict host eggs or young similarly to the Common Cuckoo.
The Plaintive Cuckoo mostly parasitizes host species with domed nests with a narrow entrance (Erritzøe et al. 2012). The Common Tailorbird (Orthotomus sutorius) is a common host for the Plaintive Cuckoo (Payne 2005), with records from China and India (Baker 1942; Becking 1981; Payne 2005; Erritzøe et al. 2012; Yang et al. 2012, 2016). In south-western China, a few records have recently been obtained from other Orthotomus species (Huang et al. 2015). Some other hosts have been identified in the other parts of the cuckoo’s range, such as Grey-backed Tailorbird (Orthotomus derbianus) in the Philippines, different species of prinias (Prinia spp.) and Zitting Cisticola (Cisticola juncidis) in India, different species of tailorbirds and Eastern Crimson Sunbird (Aethopyga siparaja) in Java, Dark-necked Tailorbird (Orthotomus atrogularis) and Yellow-bellied Prinia (Prinia flaviventris) in Malay-Peninsula, and Grey-breasted Spiderhunter (Arachnothera modesta), Dark-necked Tailorbird, Common Iora (Aegithina tiphia) and Green Iora (A. viridissima) in Borneo (Baker 1942; Becking 1981; Payne 2005; Erritzøe et al. 2012; Yang et al. 2012).
In India, three different types of Plaintive Cuckoo eggs have been found depending on host and locality (Becking 1981). In Ashy Prinia (Prinia socialis) nests, the cuckoo eggs are reported to be uniformly chestnut colored, sometimes with faint darker chestnut spots around the blunt end. In Plain Prinia (Prinia inornata) nests, the cuckoo eggs have pale blue ground color boldly marked with clear blotches and twisted lines of red, dark brown or purple and greyish underlying markings. The third type of cuckoo eggs have pinkish or bluish white ground color with reddish brown blotches and dots often forming a ring around the blunt end and found in Common and Olive-backed Tailorbird (Orthotomus sepium) and Zitting Cisticola nests (Becking 1981; Erritzøe et al. 2012; Yang et al. 2016). Hence, these observations from different parts of Asia indicate that the Plaintive Cuckoo may consist of host-specific tribes similar to the Common Cuckoo in Eurasia (de Brooke and Davies 1988; Moksnes and Røskaft 1995; Davies 2000; Antonov et al. 2010; Fossøy et al. 2011, 2016).
The Plaintive Cuckoo is a resident bird in Bangladesh and there are many potential host species available, for example, Prinia spp. and Cisticola spp. (Grimmett et al. 1999). However, there are no previously confirmed records of parasitism from this area. Our aim was to disclose host species use by Plaintive Cuckoos in Central Bangladesh and discuss host use and basic egg characteristics in relation to findings in other areas. By doing so, we provide another piece of the puzzle to disclose the spatial variation in host use and parasite adaptations (here egg appearance), and set the stage for more thorough analyses of existence of host-specific gentes using e.g. DNA-analyses of eggs sampled in the field and stored at museums (see e.g. Gibbs et al. 2000; Fossøy et al. 2016).