The efficiency and costs of Common Moorhen Gallinula chloropus parasitism on the Little Bittern Ixobrychus minutus: an experimental study

Abstract

Capsule The Little Bittern is highly suitable as Common Moorhen host in terms of accepting and incubating parasitic eggs, however, the survival of Common Moorhen chicks remains disputable.

Facultative brood parasites usually raise their own brood but occasionally lay some eggs also in other birds’ nests to augment their reproductive success. This type of parasitism occurs both within species (intraspecifically) where females parasitize nests of their own species, and between species (interspecifically) where females of one species parasitize nests of the other species. Intraspecific brood parasitism is relatively common among birds, occurring in at least 234 bird species (Yom-Tov 2001), whereas facultative interspecific parasitism was documented in only about 42 species, especially in waterfowl and galliforms (Lyon & Eadie 1991, Krakauer & Kimball 2009). The vast majority of facultative interspecific brood parasites are also intraspecific parasites suggesting facultative parasitism between species is an adaptive extension of parasitism within species (Lyon & Eadie 1991).

The Common Moorhen Gallinula chloropus is a well-known intraspecific brood parasite of the family Rallidae (see e.g. Gibbons 1986, Ueda et al. 1993, McRae 1995, 1997, 2011, Post & Seals 2000) that has recently been found to rarely lay eggs also in the nests of other bird species (for review see Meniaia et al. 2014), including two closely related birds of the family Ardeidae, the Little Bittern Ixobrychus minutus and Yellow Bittern Ixobrychus sinensis (Ueda & Narui 2004, David et al. 2005, Pardo-Cervera et al. 2010, Samraoui et al. 2012). Although cases in which moorhens parasitize nests of bitterns are relatively scarce (note that due to their secretive and shy habits, bitterns remain one of the least studied marsh-dwelling birds, Pardo-Cervera et al. 2010), previous studies have suggested that frequency of such parasitism might be higher than reported in the literature, and this behaviour could represent an alternative breeding tactic of Common Moorhens to enhance their reproductive output (Ueda & Narui 2004, David et al. 2005). However, more detailed experimental study is needed to clarify the adaptive value of this phenomenon.

Here, I examined the efficiency and costs of the Common Moorhen parasitism on Little Bitterns experimentally by introducing real, freshly laid Common Moorhen eggs into the active nests of Little Bitterns. I also documented the natural rates of Common Moorhen parasitism in a Little Bittern population. The objectives of this study were threefold. First, to determine responses of Little Bitterns towards Common Moorhen eggs. Second, to evaluate the hatching success of accepted parasitic eggs and the fate of hatched chicks. Third, to assess the cost of parasitism to the host. Given that Common Moorhens do not remove host eggs during parasitism and their chicks leave the nest soon after hatching (within 1–2 days), the main cost to Little Bittern hosts should involve the cost of incubation of the parasitic egg(s) (see also Ueda & Narui 2004). Therefore, I evaluated hatching success of Little Bittern eggs in experimentally parasitized nests and compared it with hatching success of non-parasitized nests. Lower hatching success of host eggs in parasitized nests may indicate some incubation costs.

The study was conducted on a local population of Little Bitterns breeding at fishponds near Štúrovo (47°51′N, 18°36′E, 115 m a. s. l.), south-western Slovakia. Little Bitterns and Common Moorhens breed here in relatively narrow strips of littoral vegetation surrounding the fishponds. The Little Bittern population consists of 15–20 breeding pairs (Trnka & Grim 2014), and the breeding population of the Common Moorhen comprises about 10–15 pairs. Nesting habitat use by both species overlaps both spatially and temporally in the study area. The frequency of natural Common Moorhen parasitism in Little Bittern nests was investigated in 2012–14; the experimental part of the study was done in 2014. Each year, nests were searched for systematically at 3–5-day intervals between May and late July. Of 27 Little Bittern nests found in 2014, 15 were experimentally parasitized during the egg laying stage (when the clutch contained 3 or 4 eggs) by introducing one natural Common Moorhen egg; the rest remained as control nests. The overall mean clutch size (± sd) of the studied Little Bittern population in 2014 was 4.8 ± 0.9 (n = 27 clutches, range 3–6 eggs). I used freshly laid Common Moorhen eggs obtained from active nests with multiple eggs found in the study area. For ethical reasons and due to legal restrictions, only one egg was removed from each nest. None of nests was abandoned or predated after handling of eggs. Timing of parasitism and the number of parasitic eggs corresponded to that observed in naturally parasitized nests (see David et al. 2005, Pardo-Cervera et al. 2010, own observations, and Ueda & Narui 2004 for Yellow Bittern). All tested nests were first nests built by Little Bittern pairs in the breeding season. The species breeds only once per year in the study area and replacement nests (e.g. after predation) are located in the same territory, close to the first nests (Cramp & Simmons 1977, Hudec 1994). In 2014, only three nests (11%) of Little Bitterns were predated (for high breeding success of this species in the study area, see also Trnka & Grim 2014). Therefore, although the study population was not marked completely, the probability that there was pseudoreplication due to repeated sampling of the same Little Bittern pair was very low. Experimental nests were first checked 24 h after parasitism and thereafter at 4–5-day intervals until the parasitic egg hatched. Expected hatching date of Common Moorhen eggs was predicted by assuming a minimum incubation period of 16 days (Cramp & Simmons 1980, Hudec 1994, Kushlan & Hancock 2005) from adding the egg into the nest of Little Bittern. The introduced egg was considered accepted if it remained undamaged in the nest until at least one egg, either host or parasitic, hatched. If the experimentally introduced egg hatched, the nest was checked twice a day (early morning and afternoon) until the parasitic chick left (jumped out of) the nest. Hatching success was calculated as the percentage of the total eggs in a clutch that hatched and the overall hatching success as the mean of the hatching successes in individual nests. One parasitized nest was depredated during incubation and therefore it was excluded from subsequent analysis. The remaining nests successfully fledged at least one host young suggesting the research activity did not influence the nesting success of tested birds. After jumping out of the nest, some Common Moorhen chicks were caught and transferred to active Common Moorhen nests (see below).

All the Little Bittern pairs that were tested accepted and incubated the Common Moorhen egg introduced in their nests. More importantly, in 12 of 14 cases the parasitic egg also successfully hatched (the remaining 2 eggs were sterile), almost always synchronously with host eggs (± 4 days from the hatching of the first host egg). Hatched Common Moorhen chicks remained in the host nest for 1–2 days, rarely for 3 days (two cases), and then left the nest. Although some isolated cases of successful hatching of Common Moorhen eggs in the nests of Little Bitterns have also been reported in the literature (Hudec 1994, Pardo-Cervera et al. 2010), this study is the first to show experimentally that incubation of Common Moorhen eggs by Little Bitterns is highly effective, and thus that this species is highly suitable as host for the Common Moorhen in terms of accepting and incubating parasitic eggs. On the other hand, incubation of parasitic Common Moorhen eggs probably did not impose significant costs to the Little Bittern hosts because no difference was found in the hatching success of Little Bittern eggs between experimentally parasitized (n = 14) and unparasitized nests (n = 12, ANCOVA, treatment: F_1,25 = 1.7, P = 0.21; treatment*clutch size: F_1,25 = 1.5, P = 0.16).

I found only 2 of 49 Little Bittern nests (4.1%) that were naturally parasitized by the Common Moorhen in the study area in 2012–14 (one in 2013 and one in 2014, Fig. 1). This corresponds to the frequency of the Common Moorhen parasitism reported in the literature for this and a closely related species the Yellow Bittern (1.5–4.2%, Ueda & Narui 2004, Pardo-Cervera et al. 2010). This figure was likely not an underestimate because of host rejection behaviour because the Little Bitterns were found to not discriminate against Common Moorhen eggs, that is, parasitic eggs were unlikely to be ejected by the Little Bittern parents before parasitism was detected (Brooke et al. 1998). Thus, the observed frequency of natural parasitism events should represent a reasonable sample of true frequency of Common Moorhen parasitism in Little Bitterns.

Figure 1. Naturally parasitized nest of the Little Bittern by the Common Moorhen, 18 June 2014, Štúrovo (photo by A. Trnka).

Why is parasitism of the Little Bittern nests by Common Moorhens so rare if there are few costs? The reason may lie in the survival of hatched Common Moorhen chicks that, although precocial, require supplemental feeding and other care from their parents (Huxley & Wood 1976) that Little Bitterns are not able to provide to them because of substantial differences in their biology. Based on observations of successfully parasitized nests of the closely related Yellow Bitterns, Common Moorhen chicks may receive post-hatching parental care from Common Moorhen parents nesting nearby parasitized bittern nests (Ueda & Narui 2004). In my study, I transferred five (2–3 days old) Common Moorhen chicks caught immediately upon jumping out of Little Bittern nest to three Common Moorhen nests containing newly hatched young. Their fate, however, remained unknown as chicks immediately dispersed into the surrounding vegetation after leaving the nest. Adoption of unrelated young is common phenomenon among shorebirds and waterfowl (Avital et al. 1998, Kalmbach et al. 2005), including Common Moorhens (McRae 1996). Therefore, it is quite likely that a lone Moorhen chick would be adopted into an existing Moorhen brood if they can find one.

Another explanation for the low incidence of Common Moorhen parasitism in Little Bitterns could be that female moorhens mistake nests of Little Bitterns for conspecifics (note that nests of these two species are very similar and located often close together) and thus, observed parasitism by Common Moorhens is only the result of misidentification of host nests by parasitic females. Common Moorhen females frequently parasitize nests of conspecific females located in the close proximity to their own nests (Gibbons 1986, McRae 1998). Similarly, two naturally parasitized Little Bittern nests found in the study area were situated within 3 m of active Common Moorhen nests. Alternatively, the Common Moorhen eggs found in nests of the Little Bittern might be laid by floater females that were unable to locate and parasitize nests of conspecifics and thus they were forced to parasitize the nests of other species (McRae 1998, Peer 2006). Such errors may be an early evolutionary route to more complicated co-evolutionary dynamics of brood parasitism. However, further experimental studies are needed to better clarify the causes of this intriguing phenomenon.

ACKNOWLEDGEMENTS

I thank P. Procházka and M. Požgayová for valuable comments on the manuscript. I also thank P. Prokop for some statistical analyses. The fishpond keepers allowed me to work in their private fishpond area. The comments of two referees and W. Cresswell substantially improved the manuscript. The experiments were conducted in compliance with the law of Slovakia. The licence to conduct the research and bird ringing was issued by the Ministry of Environment of the Slovak Republic.