Responses of potential hosts of Asian cuckoos to experimental parasitism

Ibis (2012), 154, 363–371

Responses of potential hosts of Asian cuckoos to experimental parasitism 1

SAJEDA BEGUM, 1,2 ARNE MOKSNES, 1 EIVIN RØSKAFT 1 * & BA˚ RD G. STOKKE 1 Department of Biology, Norwegian University of Science and Technology, Realfagbygget, NO-7491 Trondheim, Norway 2 Jahangirnagar University, Savar, Dhaka, Bangladesh

In the arms race between avian brood parasites and their hosts, several adaptations and counter-adaptations have evolved. The most prominent host defence is rejection of parasitic eggs. We experimentally parasitized nests of 10 potential host species breeding in sympatry with four different cuckoo species in an area in Bangladesh using differently coloured model eggs to test host responses. In four species we introduced both mimetic and non-mimetic eggs. Black Drongos Dicrurus macrocercus, hosts of the Indian Cuckoo Cuculus micropterus, rejected all model eggs. Common Mynas Acridotheres tristis and Jungle Babblers Turdoides striata accepted all eggs regardless of mimicry. These two species are parasitized by Asian Koels Eudynamys scolopaceus, Common Hawk-cuckoo Hierococcyx varius and, in the case of Jungle Babblers, Jacobin Cuckoos Clamator jacobinus. Pied Mynas Gracupica contra, with no records of parasitism in our study area, also accepted all eggs regardless of mimicry. In the six remaining species, all of which lay spotted eggs, we introduced only non-mimetic eggs. Black-hooded Orioles Oriolus xanthornus rejected all model eggs, even though we have found no records of natural parasitism. Long-tailed Shrikes Lanius schach and House Crows Corvus splendens, hosts of Asian Koels, rejected 75 and 9.1% of model eggs, respectively. Large-billed Crows Corvus macrorhynchos, apparently not used as hosts in our study area, accepted all blue but rejected all brown model eggs. Oriental Magpie-Robins Copsychus saularis and Red-vented Bulbuls Pycnonotus cafer accepted all non-mimetic model eggs. In Black Drongos, Long-tailed Shrikes and Black-hooded Orioles, all model eggs were ejected within 24 h of introduction. The results show considerable variation in egg rejection rates among various species, providing baseline data for further investigation of co-evolutionary interactions between brood parasites and hosts in this region. Keywords: Asia, co-evolution, cuckoo, egg rejection, experimental parasitism, model eggs. Avian brood parasites depend on their hosts for successful reproduction. Because host reproductive success is often greatly reduced, brood parasitism inflicts high costs on the host (Davies 2000). Due to these costs, natural selection will favour the evolution of host defences (Davies & Brooke 1989, Moksnes et al. 1991b). For example, many host species are able to discriminate and reject eggs that are unlike their own, abandon parasitized clutches

*Corresponding author. Email: [email protected]

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or bury the parasitic egg in the nest lining (Davies 2000). Some hosts may even desert or eject the cuckoo chick (Grim et al. 2003, Langmore et al. 2003, Sato et al. 2010). However, these host adaptations have resulted in the evolution of counteradaptations in parasites. Sophisticated deception strategies, such as egg colours that mimic those of their hosts or even the production of young that mimic host offspring, have evolved to overcome host defences (Baker 1942, Southern 1958, Brooke & Davies 1988, Moksnes & Røskaft 1995, Davies 2000, Langmore et al. 2003). In response, hosts may produce eggs with low intra- or high interclutch variation that more readily enables them to

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discriminate against mimetic parasitic eggs (Øien et al. 1995, Stokke et al. 2002, 2007). Similarly, hosts may also evolve adaptations at the chick stage, such as intricate gape patterns or other characteristics, that make chick mimicry a more difficult task for the parasite (Davies 2000). The resulting evolutionary arms race between the brood parasite and its host(s) leads to increasingly complex and sophisticated adaptations and counteradaptations (Davies 2000, Stokke et al. 2005, Grim 2011). Studies of experimental parasitism using artificial eggs have been pivotal in generating a better understanding of the co-evolutionary mechanisms in the arms race between brood parasites and their hosts (Davies 2000, Schulze-Hagen et al. 2009), and a substantial number of studies have been undertaken experimentally to investigate host defences against parasitic eggs. Despite the heavy costs of parasitism, there is considerable variation in rejection abilities towards even non-mimetic parasitic eggs among hosts of brood parasites (e.g. Davies & Brooke 1989, Moksnes et al. 1991b, Alvarez 1999, Stokke et al. 1999, 2008, Peer & Sealy 2004, Langmore et al. 2005, Antonov et al. 2006a, 2006b, Vikan et al. 2010). Research on behavioural responses to experimental brood parasitism can help clarify why some species are susceptible to parasitism whereas others are able to recognize and reject the parasite’s eggs or abandon their nest. This knowledge may in turn also disclose whether or not individual species have been involved in arms races with avian brood parasites. Most studies of rejection of experimentally added eggs have been carried out in Europe, America and Australia but very few such studies have been conducted in Asia (Japan, Korea, India and China only; Higuchi 1989, Marchetti 1992, 2000, Lotem et al. 1992, 1995, Nakamura et al. 1998, Lee & Yoo 2004, Takasu et al. 2009, Yang et al. 2010). We assessed the responses of 10 potential host species to experimental parasitism with artificial eggs. All species tested were passerines (Passeriformes): Long-tailed Shrike Lanius schach, Largebilled Crow Corvus macrorhynchos, House Crow Corvus splendens, Black-hooded Oriole Oriolus xanthornus, Black Drongo Dicrurus macrocercus, Oriental Magpie-Robin Copsychus saularis, Common Myna Acridotheres tristis, Pied Myna Gracupica contra, Red-vented Bulbul Pycnonotus cafer and Jungle Babbler Turdoides striata. In four species that lay completely or partially unmarked eggs

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(i.e. Black Drongos, Jungle Babblers, Common and Pied Mynas), we introduced both mimetic and non-mimetic eggs. In the six remaining species, which lay spotted eggs, we introduced only nonmimetic eggs. We also recorded time to rejection of the experimental egg. Ten species of parasitic cuckoos of the family Cuculidae have been recorded in Bangladesh, but so far no experimental study has been undertaken for any of their hosts. In our study area, close to the capital Dhaka, we have previously recorded host use by four sympatric cuckoo species: Asian Koel Eudynamys scolopaceus, Common Hawk-cuckoo Hierococcyx varius, Jacobin Cuckoo Clamator jacobinus and Indian Cuckoo Cuculus micropterus. Brood parasitism was recorded in five of the investigated passerines in our study area: Common Mynas, Long-tailed Shrikes and House Crows are parasitized by Asian Koels, Jungle Babblers by Common Hawk-cuckoos and Jacobin Cuckoos, and Black Drongos by Indian Cuckoos (Begum et al. 2011a). Begum (2011) found that the Common Hawkcuckoos, Jacobin and Indian Cuckoos all laid eggs that showed, at least according to the human eye, excellent mimicry with those of their hosts, and all were accepted. Asian Koel eggs showed good mimicry with House Crows, but poor mimicry with its two other hosts. In spite of this, most Asian Koel eggs were accepted, although desertion from parasitized as opposed to non-parasitized nests was significantly higher in Common Mynas (Begum et al. 2011a). METHODS Study area The study was conducted on the campus of Jahangirnagar University, 2.5 m above sea level, 32 km north of Dhaka in the central region of Bangladesh (3016¢N, 9052¢E). The 200-ha study site consists of a mosaic of habitats including pockets of different species of fruit trees, wetlands, grasslands and open woodland. There are also monoculture plantations, agricultural lands, orchards and botanical gardens in and around human settlements (Begum et al. 2011b). In total, 34 passerine species are breeding residents (Mohsanin & Khan 2009). Experiments were carried out from 2008 to 2010. We systematically searched for nests of different potential host species during the breeding

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of synthetic plastic ‘Creal-Therm’, following the procedure of Bártol et al. (2002) and Antonov et al. (2009). We used three colours of acrylic paint for the experimental eggs: (1) pale blue, (2) dark brown and (3) white (Fig. 1). Each host nest was used only once, and only one model egg type was introduced to each nest. Unmarked (immaculate or uniformly coloured) eggs were selected to avoid the potential confounding effects of spotting patterns in the analyses of egg rejection behaviour, and three egg types to obtain decent sample sizes for each experimental group. For the four species that lay unmarked eggs (see introduction), we investigated rejection of both mimetic and nonmimetic eggs. For the remaining six species, we only investigated rejection of non-mimetic eggs. Blue model eggs resembled eggs of Common

season from January to August each year. Only nests that were located before or during clutch initiation were used for experimental manipulation; nests found in the incubation stage were not used for experimental parasitism. The nest types were classified as open or in holes. All actual or potential host species occurring in sympatry with the four cuckoo species in the study area were considered for experimental manipulation, but we ended up selecting 10 species due to low breeding densities (and therefore few nests) for the remaining breeding passerine species. Model eggs Model eggs that were different in colour were introduced into host nests. Model eggs were made

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Figure 1. Model eggs used in experiments and three representative eggs from each species tested (all from different clutches). (a) Model eggs, (b) Long-tailed Shrike, (c) Large-billed Crow, (d) House Crow, (e) Black-hooded Oriole, (f) Black Drongo, (g) Oriental Magpie-Robin, (h) Common Myna, (i) Pied Myna, (j) Red-vented Bulbul, (k) Jungle Babbler.

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Mynas, Jungle Babblers and Pied Mynas. Brown model eggs were non-mimetic to the eggs of all species investigated. White eggs were only used in experiments with Black Drongos. This species lays eggs that vary in colour from pure white, to white or pinkish with brownish or black spots (Fig. 1) (Rocamora & Yeatman-Berthelot 2009). Therefore, the white model eggs were mimetic in some nests (where host eggs were pure white) but nonmimetic in others. Artificial eggs had a harder surface than real eggs making egg puncturing by hosts impossible (Martín-Vivaldi et al. 2002). However, the paint coating is very useful for identifying attempts at puncture ejection, because bill-prints are easily detected (e.g. Stokke et al. 1999, Antonov et al. 2006b). No such prints were observed, and all experimental eggs were rejected by desertion or grasp ejection. Furthermore, previous studies clearly indicate that hosts respond to real and model eggs in a similar way (e.g. Davies & Brooke 1989, Peer et al. 2000) and hence hosts should respond in the same manner to model eggs as to real eggs. Two different egg sizes were used in the experimental treatments. All species except the House and Large-billed Crows were treated with small model eggs (mean = 24.96 ± 19.29 mm; the average size of Indian Cuckoo eggs), typical of parasites using these smaller passerines as hosts (even though we have recently found that Long-tailed Shrikes and Common Mynas are also parasitized by Asian Koels in our study area, Begum et al. 2011a). For the House and Large-billed Crow

nests, we used the average egg size of the Asian Koel egg (mean = 30.60 ± 23.10 mm), which is the only one of the four sympatric cuckoo species known to use crows as hosts (Table 1). We followed the general experimental procedures of Moksnes et al. (1991b). All experiments were carried out just before (using average clutch size as a cue), on the same day or on the day after the last host egg was laid. A single experimental egg was added to each active nest without removing a host egg, because this has previously been shown to have no influence on host responses (Davies & Brooke 1988, Moksnes & Røskaft 1989). Eggs were added to the nests throughout the day (06:00–18:00 h CST), as there is no evidence that a host’s response is related to the time of the day that the nest is parasitized (Davies 2000). Asian Koel and Jacobin Cuckoo females may sometimes remove a host egg when laying (Payne 2005), whereas Common Hawk-cuckoo and Indian Cuckoo females remove one or more host eggs (S. Begum unpubl. data). Each experimental nest was inspected daily to assess whether the model egg was ejected and to detect any damage or disappearance of host eggs. Monitoring of the nest continued for six consecutive days, and if the model egg remained in the nest after 6 days and the nest was still active, we considered the egg accepted. If the model egg was missing during any of the first five consecutive visits, we considered it ejected. If the nest was unattended, and the eggs were undamaged but cold for two consecutive days, we considered the nest deserted. In cases of

Table 1. Responses of different actual or potential cuckoo hosts to experimental parasitism by blue and brown model eggs. Blue Species Long-tailed Shrike House Crow Large-billed Crow Black-hooded Oriole Black Drongo Oriental Magpie-Robin Common Myna Pied Myna Red-vented Bulbul Jungle Babbler

Brown

Total

Egg size

A

E

D

R (%)

A

E

D

R (%)

A

E

D

R (%)

n

S L L S S S S S S S

2 10 9 0 0 10 12 11 9 12

8 0 0 6 11 0 0 0 0 0

0 1 0 0 0 0 0 0 0 0

80 9.1 0 100 100 0 0 0 0 0

3 10 0 0 0 10 10 15 12 9

7 1 0 5 17 0 0 0 0 0

0 0 7 0 0 0 0 0 0 0

70 9.1 100 100 100 0 0 0 0 0

5 20 9 0 0 20 22 26 21 21

15 1 0 11 44* 0 0 0 0 0

0 1 7 0 0 0 0 0 0 0

75 9.1 43.8 100 100 0 0 0 0 0

20 22 16 11 44 20 22 26 21 21 223

A, accepted; E, ejected; D, deserted; R, rejected; S, small eggs; L, large eggs. *Includes all 16 rejected small white eggs.

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Responses of Asian host species to parasitism

egg acceptance, the model egg was removed on the day 6 visit. This study design was similar to that of many other relevant studies (e.g. Moksnes et al. 1991b, Amundsen et al. 2002, Bártol et al. 2002, Honza & Moskát 2008). We tested differences in rejection of model eggs of varying colour, and in Long-tailed Shrikes also the difference in rejection behaviour between clutches at two different stages of the breeding cycle using Fisher’s exact tests. Statistical analyses were performed in SPSS 17.0 (SPSS Inc., Chicago, IL, USA). RESULTS Altogether, we experimentally manipulated 223 nests (Table 1). Both the blue and the brown model eggs were accepted by all pairs of Oriental Magpie-Robins, Common Mynas, Pied Mynas, Red-vented Bulbuls and Jungle Babblers (Table 1). In House Crows there was no significant difference in rejection of brown and blue model eggs (Table 1; Fisher’s exact test, P = 1.00). Unexpectedly, Large-billed Crows accepted all blue model eggs but deserted all nests containing brown model eggs (Table 1; Fisher’s exact test, P < 0.0001). All experimental eggs introduced to the Blackhooded Oriole and Black Drongo nests were ejected irrespective of colour (Table 1). Furthermore, Black Drongos ejected all white model eggs (Table 1) irrespective of the appearance of their eggs. Except for House and Large-billed Crows, the Long-tailed Shrike was the only species that did not eject or accept all the eggs at a 100% rate (Table 1). However, there was no statistically significant difference in rejection of blue and brown eggs (Table 1; Fisher’s exact test, P = 1.00). In all four nests where the experimental treatment was carried out when the host had laid its penultimate egg, the model egg was accepted (one blue egg and three brown eggs). However, in 15 of the 16 experiments where the model egg was added on the day that the host laid its final egg (eight of nine blue eggs, and all of the seven brown eggs), the model egg was ejected. The difference in rejection behaviour between incomplete and complete clutches was statistically significant (Fisher’s exact test, P < 0.0001). No host egg was damaged or lost after ejection of model eggs. Furthermore, there was no sign of peck marks on accepted or deserted model eggs.

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All model eggs were ejected within 24 h of their introduction by Black Drongos (n = 44), Longtailed Shrikes (n = 15) and Black-hooded Orioles (n = 11). The mean times to rejection by House and Large-billed Crows were 3.0 days (sd = 1.41, n = 2) and 2.0 days (sd = 1.41, n = 7), respectively. Egg appearance in nine of the known or potential host species was consistent among individuals (Fig. 1). However, Black Drongos laid eggs of three different types (Fig. 1). In 90.9% of the nests the eggs were similarly marked: pinkish with brown and black spots (52.3%), whitish with brown and black spots (20.5%), or unmarked white eggs (18.2%). In 9.1% of the nests, the eggs were nonuniform; both whitish and pinkish eggs with spots were found (n = 44). DISCUSSION Egg rejection rates varied substantially among the 10 host species that were experimentally parasitized. Black Drongos and Black-hooded Orioles rejected all model eggs very quickly (within 24 h). Black Drongos are hosts of the Indian Cuckoo both in our study area (Begum 2011) and elsewhere (Becking 1981, Payne 2005, Lowther 2010), and to the human eye, egg mimicry of drongo eggs by Indian Cuckoos is exquisite (Begum 2011). Indian Cuckoo eggs found were whitish ⁄ pinkish with spots, they were all found in drongo clutches containing whitish ⁄ pinkish host eggs with spots, and all parasite eggs were accepted by the host (Begum 2011). It has been shown that hosts of brood parasites laying mimetic eggs may evolve a high interclutch variation as a defence against parasitism and that parasitism may lead to disruptive selection on egg colour in both hosts and parasites (Øien et al. 1995, Stokke et al. 2002, Kilner 2006, Yang et al. 2010, Vikan et al. 2011). This may also be the case for Black Drongos, even though this has not yet been countered by Indian Cuckoos. In this study, even apparently mimetic white model eggs were rejected by Black Drongos that laid unmarked white eggs, although sample size was low (n = 3). This result does not agree with the findings from natural parasitism events (Begum 2011). One explanation may be that we have based our scores of mimicry on human vision and it is now well established that avian and human visual systems differ in several respects. For instance, many bird

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species have ultraviolet-sensitive photoreceptors, as well as oil droplets that are absent in humans (Goldsmith et al. 1984, Vorobyev et al. 1998). Unfortunately, we have no spectrophotometric measures of host or model eggs, and theoretically the white model eggs may appear non-mimetic to drongos in parts of the colour spectrum, and may therefore be easily recognized and rejected by hosts. This possibility should be explored in future investigations. Black-hooded Orioles are potential hosts for the Asian Koel, since parasitism has been recorded in other oriole species (Ali & Ripley 1981, Payne 1997, 2005, Lowther 2010). However, we found no cases of parasitism in our study area (Begum 2011, Begum et al. 2011a). A potential reason for the apparent absence of parasitism could be that the orioles immediately rejected all naturally laid cuckoo eggs such that parasitism events remained undetected. In any case, the welldeveloped egg rejection behaviour in Black-hooded Orioles strongly suggests that they are or have been involved in an arms race with brood parasites. The closely related Olive-backed Oriole Oriolus sagittatus in Australia is parasitized by both Australian Koels Eudynamys scolopacea cyanocephala and Pallid Cuckoos Cacomantis pallidus (Brooker & Brooker 2005, Lowther 2010). Both Black-hooded Orioles and Black Drongos can be regarded as grasp ejectors, because in three cases (two nests of the Black Drongo and one nest of the Blackhooded Oriole) the model eggs were found within 50 m of the nest without visual damage (S. Begum unpubl. data). House Crows rejected 9.1% of model eggs, with no significant difference between rejection of brown or blue model eggs, whereas Large-billed Crows accepted all blue but deserted all brown model eggs. Furthermore, the crows delayed rejection by 2–3 days. Large-billed and House Crows are reported to be among the most frequently used hosts of the Asian Koel throughout the Indian subcontinent (Ali & Ripley 1981, Payne 1997, 2005, Lowther 2010). In our study area, House Crows were parasitized, but we did not find any cases of parasitism of Large-billed Crows (Begum et al. 2011a). It is intriguing and difficult to explain why House Crows accepted almost all model eggs, even though they were non-mimetic. This is especially intriguing given that Asian Koels have apparently evolved eggs mimetic to crows (Payne 1997, Begum et al. 2011a) and the fact that corvids are favoured hosts (Payne 2005). A possible reason for

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the variation in rejection of blue and brown eggs in Large-billed Crows may be that the blue model eggs differed so markedly from the Asian Koel eggs that they were simply not recognized as parasitic eggs by the two crow species. However, we can find no reasonable explanation for the variation in rejection of brown model eggs between House and Large-billed Crows, other than that perhaps Largebilled Crows have had a longer history of co-evolutionary interactions with brood parasites. The brown model egg has approximately the same colour as the spots of the Asian Koel eggs, which may have triggered rejection of such eggs in Largebilled Crows. However, it is unclear why birds desert nests instead of just ejecting the model egg. Crows should be perfectly able to grasp-eject (see Moksnes et al. 1991a). Future investigation should focus on discovering the details of crow rejection behaviour. Long-tailed Shrikes rejected 75% of model eggs with no significant difference between brown and blue eggs. They also rejected experimental eggs quickly (within 24 h). Asian Koel eggs are apparently poor mimics of Long-tailed Shrike eggs. Despite this, all naturally laid Asian Koel eggs were accepted by the shrike hosts (Begum et al. 2011a). Interestingly, Long-tailed Shrikes accepted model eggs introduced when the clutch was still incomplete, but rejected at a rate of almost 100% those eggs introduced on the day of clutch completion. One reason for this might be that it is easier to detect a foreign egg in the nest when the host ‘knows’ that its clutch is complete. The timing of egg-laying is very important in avian brood parasites, as early hatching offers competitive advantages in terms of food acquisition or eviction of host eggs or chicks (Payne 1977, Davies 2000, Birkhead et al. 2011). Asian Koels always lay their eggs before the host’s clutch is complete (S. Begum unpubl. data), which may explain the difference in rejection rate of naturally laid parasite eggs and those experimentally inserted at clutch completion. Furthermore, Long-tailed Shrike nests are regularly parasitized by smaller cuckoos such as Common Hawk-cuckoos and Jacobin Cuckoos (Yosef 2008). Such cuckoos lay smaller eggs than Asian Koels (Payne 2005). In addition to mimicry (Antonov et al. 2006b, Stoddard & Stevens 2010), egg shape and size may be important cues for egg rejection (Marchetti 2000). This could be relevant for Long-tailed Shrikes because their eggs are considerably smaller than those of Asian Koels (Begum

Responses of Asian host species to parasitism

et al. 2011a) and, with the exception of our study area, no parasitism by Asian Koels has been reported, indicating that Long-tailed Shrikes may be a novel host of this parasite (Begum et al. 2011a). As a result, shrikes may simply lack strong defences against parasitism by the Asian Koel. Furthermore, unlike the model eggs used in the present study, the Long-tailed Shrike may not be able to grasp- or puncture-eject the large Asian Koel eggs, and may therefore be ‘forced’ to accept them, as has been shown in Olivaceous Warblers Hippolais pallida, hosts of the Common Cuckoo (Antonov et al. 2009). Common Mynas, Pied Mynas, Oriental MagpieRobins, Jungle Babblers and Red-vented Bulbuls accepted all model eggs. The Common Myna is utilized by Asian Koels in parts of their range (Payne 2005) and is also a common host in our study area (Begum et al. 2011a). This species may breed both in open nests and in holes, and in a previous study we found that parasitism by Asian Koels exclusively took place in open nests. Nests that were naturally parasitized were deserted significantly more often than unparasitized ones (Begum et al. 2011a). This result contrasts with the lack of egg rejection in the present study. A possible explanation for these differences may be that Common Mynas rely on conditional cues of parasitism, such as the sight of the parasite at the nest (Davies & Brooke 1988, Moksnes et al. 2000). Another explanation may be that Common Mynas often suffer from repeated cases of parasitism, with up to six Asian Koel eggs in a single nest (Begum et al. 2011a), leading to a situation where in some cases parasite eggs outnumber host eggs, which again can trigger desertion by the host. Pied Mynas and Oriental Magpie-Robins may lack the ability to reject eggs due to the absence of a co-evolutionary history with avian brood parasites. Oriental Magpie-Robins usually nest in holes in trees, concrete walls or buildings with small entrance holes (Ali & Ripley 1981), which prevents brood parasites from entering. Nests of the Pied Myna are usually dome-shaped, very large and conspicuous, and are sometimes built in colonies (Ali & Ripley 1981). The entrance of the nest is positioned at one side and is not clearly visible. The small entrance is apparently difficult for parasites to enter. We have found no cases of parasitism of these two species in our study area (Begum 2011), and as far as we know, no cases of parasitism have been reported elsewhere. On the other

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hand, Red-vented Bulbuls are sometimes used as hosts by Jacobin Cuckoos and Fork-tailed Drongocuckoos Surniculus dicruroides (Payne 1997, Lowther 2010). Jungle Babblers are known to be parasitized by Common Hawk-cuckoos and Jacobin Cuckoos in our study area (Begum 2011) and are also favoured hosts of these cuckoos elsewhere (Gaston 1976, Payne 1997, 2005, Gaston & Zacharias 2000). It is therefore still a puzzle why rejection behaviour has not evolved in these two species at our study site. One potential reason could be that costs of rejection are higher than the benefits, as found in Cape Bulbuls Pycnonotus capensis parasitized by Jacobin Cuckoos (Krüger 2011). The blue eggs of Common Hawk-cuckoos and Jacobin Cuckoos appear highly mimetic to the eggs of Jungle Babblers in colour (Begum 2011). A possible explanation is that the appearance of these two cuckoo species’ eggs has evolved as a response to rejection by other host species (e.g. other babblers) that also lay blue eggs. If this is the case, egg mimicry in Jungle Babblers is a fortunate pre-adaptation. We have observed that a pair of Jungle Babblers completely destroyed their nest after observing a Common Hawk-cuckoo near it (S. Begum unpubl. data), indicating that babblers may look upon cuckoos as a threat, making Krüger’s (2011) findings of relevance. Further experiments are necessary to disclose the evolution of anti-parasite behaviour in both Red-vented Bulbuls and Jungle Babblers. This study represents the first experimental investigation of egg rejection abilities in potential and actual hosts spanning a complete host community against avian brood parasites on the Indian subcontinent. The results show considerable variation in egg rejection rates among various species, providing baseline data for further investigation regarding co-evolutionary interactions between brood parasites and hosts in this geographical region. We are indebted to all members and staff in the Wildlife Research Group, Department of Zoology, Jahangirnagar University, for co-operation during data collection in the field. We are furthermore grateful to all the members of the Brood Parasitism Research Group, Department of Biology, Norwegian University of Science and Technology (NTNU), for helpful comments on the manuscript. Stephan J. Schoech and two anonymous referees provided valuable comments that significantly improved a previous version of the manuscript. The study was supported by a Grant through a ‘Quota Scheme’ at NTNU and a research grant from The Norwegian Programme for Development, Research and Education (NUFU).

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