1 SPECIAL: ANECDOTES IN BEHAVIOUR

2 Killing behaviour of adult brood parasites

3 Šulc M.1,*, Štětková G.1,2, Jelínek V. 1, Czyż B.3, Dyrcz A.3, Karpińska O.4,

4 Kamionka-Kanclerska K.4, Rowiński P.4, Maziarz M.5, Gruszczyński A.5, Hughes A.E.6,

5 Honza M. 1

61Institute of Vertebrate Biology of the Czech Academy of Sciences, Brno, Czech Republic

72Department of Botany and Zoology, Faculty of Sciences, Masaryk University, Brno, Czech

8Republic

93Department of Behavioural Ecology, University of Wrocław, Wrocław, Poland

104Department of Forest Zoology and Wildlife Management, Warsaw University of Life

11Sciences (SGGW), Warsaw, Poland

125Museum and Institute of Zoology, Polish Academy of Sciences, Warsaw, Poland

136Department of Psychology, University of Essex, Colchester, U.K.

14*corresponding author: Michal Šulc ([email protected])

15

16Summary

17Decades of studies have revealed the striking adaptations of avian brood parasites for 18their unique reproductive lifestyle. Several have reported that adult brood parasites 19sometimes kill host nestlings, although the reasons for this behaviour remain unclear. 20Using continuous video-recording and camera traps, we observed the same behaviour in 21the common canorus, showing that both host and parasite nestlings can 22be killed. The latter has never previously been observed in any avian . 23Here, we review this phenomenon and discuss possible explanations. 24 25Keywords: nestling infanticide, chick ejection, farming, Mafia, co-evolution

26

27Main text 28Infants are an obvious target for extermination because of their vulnerability, therefore they 29are often exploited by various predators (Weidinger 2009). Apart from this relatively simple 30predator–prey relationship, there is also the phenomenon of infanticide when a young 31offspring is killed by an adult animal of the same . This has been observed in 32numerous species from diverse taxonomic groups including , , amphibians, 33and mammals (Hrdy 1979; Hausfater & Hrdy 2017). Although this behaviour was originally 34considered to be pathological (especially cases of filial infanticide when a parent kills their 35own offspring), we now know that it may be an adaptive behaviour, e.g. reducing competition 36for limited resources (Hoogland 1985) or allowing monopolization of reproduction (Haines et 37al. 2018).

38 Over the last 130 years, adults of several avian brood parasitic species have been also 39observed killing host nestlings (Table 1) for reasons that are less understood. This behaviour 40has predominantly been observed in the two most frequently studied parasitic species, the 41common cuckoo Cuculus canorus (15 nests) and the brown-headed Molothrus ater 42(20 nests) but also in the Himalayan cuckoo Cuculus saturatus (one nest) and the Shining 43bronze cuckoo Chrysococcyx lucidus (two nests). Indirect evidence for this behaviour has also 44been found in the glandarius. It appears to be a relatively rare 45phenomenon; we found only 24 studies (including this one) reporting about 41 events (Table 461). Many reports have involved only circumstantial evidence; however, more recent studies 47have recorded this behaviour on video-cameras (Table 1). Using continuous video-recording 48and camera traps, we observed this killing behaviour in the (hereafter 49cuckoo) in four different hosts: the Erithacus rubecula, the 50Phylloscopus sibilatrix, the arundinaceus and the reed 51warbler Acrocephalus scirpaceus. Here, we present five pieces of evidence (four videos and 52photos; Videos 1-4 and Figure 1) showing the adult ejecting nestlings out of host 53nests. Three of these cases are particularly interesting because the nestlings were young 54cuckoos (Videos 1 and 2, Figures 1 and 2). All events are independent observations recorded 55in four geographically separate breeding areas in the Czech Republic and Poland and 56therefore were performed by different cuckoo females. We believe that our literature review 57and video evidence demonstrating parasitic behaviour during these incidents may help to 58explain this peculiar behaviour of brood parasites.

59 All studies apart from one (Igl 2003) showed that only females of parasites exhibit this 60behaviour which is supported also by our recordings. The act cannot be seen as predation 61because parasites never preyed on nestlings (but see, Wyllie 1975). Since there appear to be 62no cases where parasites ejected nestlings from nest of non-host species (Weidinger 2009, and 63>1000 video-monitored nests on non-host species, K. Weidinger, unpubl. data), it seems that 64parasitic killing is aimed only at their hosts. From behaviour of parasites while ejecting 65nestlings it seems that it is an intentional act. Parasitic females grabbed the nestling and tossed 66it out of the nest immediately after arrival at the nest (Videos 1 and 2). If there were multiple 67nestlings in the nest, parasites systematically removed them one at a time (Video 4 and see 68also video published at Youtube by P. Elliott; Elliott 1999). In some cases, they tried to eject 69even under host attacks (Videos 3 and 4). Moreover, in two of our videos (Videos 3 and 4) it 70is possible to see that parasites visited nests repeatably despite initially being attacked and 71flushed away by the hosts. The number of ejected nestlings varied from one to the whole 72clutch of six (Table 1). No study reported after the ejection event occurred (but see, 73Sheppard 1996) which could indicate that parasites were aware that the host nests were not 74suitable for parasitism. From the above, and due to the fact that observed ejection behaviour is 75identical in phylogenetically distant parasitic species of cuckoos and and convergent 76evolution could take place, we suggest that this behaviour is adaptive (Losos 2011).

77 Birds of several species have been reported to kill immature birds of the same or other 78species in contexts other than predation: for example, to compete for nesting sites (Kattan 792016), mates (Freed 1986), paternal investment (Veiga 1990) or food (Belles-Isles & Picman 801986; Freed 1987). In brood parasites, two other hypotheses have been proposed to explain 81the killing of host nestlings by adult brood parasites: the “Mafia” hypothesis and the 82“farming” hypothesis (Soler et al. 2017). The Mafia hypothesis proposes that parasites cause 83nest failure to punish a host that ejected a parasitic and hence to enforce its compliance in 84the future. This tactic is presumably only effective in parasitic species where offspring do not 85kill host nestlings, as only in these cases may the host parents still benefit by raising their own 86young while also accepting a parasite egg or nestling (Zahavi 1979). Thus, this strategy could 87evolve in e.g., the great spotted cuckoo and the brown headed cowbird, but not the common 88cuckoo (and other Old World cuckoos) where the young cuckoo chick usually evicts all host 89eggs or chicks (Reboreda et al. 2017). In contrast, the farming hypothesis could relate to all 90parasite species. It suggests that parasites cause failure of the host nests to force the host to re- 91nest and so to increase the opportunity for future parasitism. Both hypotheses seem to be 92valid, however, only in the context of destroying (Soler et al. 1995; Hoover & Robinson 932007) not ejecting nestlings. 94 The reason why these two interesting hypotheses have not been tested is because 95nestling ejection by adult brood parasites is relatively rare. Brown-headed cowbirds ejected 96host nestlings at 11 of 334 video-monitored host nests (summary data from Granfors et al. 972001; Stake & Cavanagh 2001; Stake et al. 2004), and we recorded this behaviour at five of 98311 host nests for the common cuckoo (summary data from four different localities, for 99details, see Supplementary material). It therefore seems to be similarly prevalent in both these

2 100parasitic species (chi-square test: χ 1=1.26, P=0.26). Moreover, we found that nestling 101ejection cannot be predicted by parasitism rate because it occurred both in rarely (e.g. the 102European robin Erithacus rubecula, see Supplementary material) and frequently parasitized 103host species (e.g. great reed warbler Acrocephalus arundinaceus, see Supplementary 104material).

105 Because of the small sample sizes involved, at present we can only speculate about the 106validity of these hypotheses in explaining killing behaviour. In cowbirds, we would expect 107killing behaviour to occur at nests where hosts rejected the parasitic egg if the Mafia 108hypothesis were true (see above). However, observations at ten video-recorded host nests did 109not support this prediction because all of them were assessed as non-parasitized (Granfors et 110al. 2001; Stake & Cavanagh 2001). Therefore, it seems that cowbirds do not kill host 111nestlings to punish hosts for their non-cooperation.

112 For the farming hypothesis, we had two predictions; 1) parasites should kill all 113nestlings to make the host re-nest, and 2) the killing parasite should benefit from the host re- 114nesting and parasitize the replacement nest. We found that ejection of all nestlings occurred 115only in seven of 19 and 11 of 14 nests in cowbirds and cuckoos, respectively (Table 1). 116Therefore, it seems that parasites (especially cowbirds) often do not succeed in making hosts 117re-nest. However, it is possible that the low success of ejection is because hosts can attack 118parasites which may put the parasite in danger (reviewed in Šulc et al. under review) and 119prevent nestlings from being ejected (Video 3 and 4). Finally, there is scarce evidence about 120replacement nests and if they were parasitized or not. There are only two studies reporting 121that one replacement nest was parasitized (Wyllie 1975) and the other one was not (Kinoshita 122& Kato 1995). Therefore, due to small sample size further studies are required.

123 Interestingly, we recorded killing both host (Videos 3 and 4) but also parasitic 124nestlings (Videos 1 and 2, Figure 1). Although killing parasitic chicks seems even rarer than 125host nestlings, we speculate that targeted killing of the cuckoo chicks might also bring an 126adaptive benefit. For example, the adult cuckoo could eliminate a potential rival female that 127could compete for host nests in future. However, genetic analysis showed that one of the 128killed cuckoo chicks was male. Alternatively, the cuckoo female might take advantage of 129parasitizing ‘naïve’ hosts. Hosts with a cuckoo chick apparently accepted a cuckoo egg, 130giving a signal indicating their low frontline defence and poor ability to recognize a parasitic 131egg. Since the host response towards parasitism tends to be repeatable (Honza et al. 2007), 132there may be a second reproductive advantage for cuckoos farming hosts raising cuckoo 133chicks, in that these hosts may be more likely to accept the new parasitic egg. However, 134testing this hypothesis would require clear identification of parasites at host nests; without 135this, we are unable to confirm even whether parasites are killing the offspring of another 136female. Therefore, a final explanation might be that this behaviour is simply maladaptive 137(especially if females are found to kill their own offspring) because it does not bring any 138benefit and takes time that could be used for another activities, e.g. feeding.

139 To conclude, ejection of host nestlings by adult brood parasites has been observed 140relatively rarely but in two non-related taxa which may indicate that this behaviour is 141adaptive. Moreover, video-recordings from host nests allow us to observe this behaviour in 142detail, perhaps suggesting that it may be intentional. However, it is difficult to make strong 143conclusions, especially because we do not have any evidence that killing the nestlings benefits 144brood parasites. We suggest that more studies using video-cameras in brood parasites will 145help us to better understand this interesting and understudied phenomenon.

146

147Acknowledgments

148We are obliged to the management of the Fish Farm Hodonín and local conservation 149authorities for permission to conduct the fieldwork in the Czech Republic. The kind 150cooperation of the Białowieża National Park administration is also acknowledged. We thank 151M.M. Abraham, R. Beňo, V. Brlík, J. Koleček, L. Kulísek, R. Valterová-Poláková, M. 152Požgayová, P. Procházka, B. Prudík, K. Sosnovcová, J. Studecký, F. Svoboda and K. 153Žabková for their assistance in the field. This work was supported by a project from the 154Czech Science Foundation (grant no. 17-12262S) and the funds of the National Science 155Center, Poland (research project No. 2016/23/N/NZ8/03374). B.C. and A.D. were supported 156by the University of Wrocław (project no. (501)0410/2990/18-IBS). The wood warbler nest 157monitoring in 2019 was funded by the National Science Centre, Poland (project no. 2017/26/ 158D/NZ8/01063). 159Tab. 1. Incidences of brood parasites killing host or parasite nestlings

Observation Nb of ejected Parasite species Host species Short description and reference method nestlings E Clamator glandarius Corvus corone Nestling killed in the nest in captivity (Zahavi 1979) 1 of 1 One nestling found dead with hematomas on the ground (Soler et al. E* Clamator glandarius Pica pica 1 of NA 2017) Cuckoo flushed from the nest by the author and all three nestlings E Cuculus canorus Anthus pratensis 3 of 3 found outside the nest, two in blood and dead (Milburn 1915) Three just hatched wagtail nestlings and a cuckoo egg found, the E* Cuculus canorus Motacilla alba cuckoo egg collected. Two days after all nestlings ejected out of the 3 of 3 nest (Headley 1919). Cuckoo heard at the day of the host eggs hatching, the nest checked E Cuculus canorus Prunella modularis immediately after the cuckoo calling, the nest empty and one nestling 1 of 1 out (Jourdain 1919) Cuckoo observed in the bush with a wagtail nest, nest empty and five E Cuculus canorus Motacilla flava 5 of 5 nestlings (seven-days old) scattered around the nest (Gurney 1897) All six still warm four-days old nestlings ejected about 20 cm out of the E Cuculus canorus Anthus pratensis 6 of 6 nest (Vincent 1933) One of two nestlings eaten in the same manner (shaking head) as E Cuculus canorus Acrocephalus scirpaceus cuckoo eats ; the other nestling disappeared; the re-nest NA parasitized (Wyllie 1975) Cuckoo repeatedly visited host nest with an egg and six nestlings (two- days old), all nestlings were ejected out of the nest bleeding internally, E Cuculus canorus Saxicola torquatus 6 of 6 the egg disappeared, the re-nest not parasitized (Kinoshita & Kato 1995) Matsuda et al., unpubl. data with no details; cited in (Kim & Yamagishi E Cuculus canorus Acrocephalus arundinaceus NA 1999) Two of six nestlings (eight-days old) pecked and ejected out of the nest V/P Cuculus canorus Paradoxornis webbiana 2 of 6 (Kim & Yamagishi 1999) E* Cuculus canorus Acrocephalus scirpaceus Five nine-days old nestlings stuck in the reed under the nest (this study) 5 of 5 V/P Cuculus canorus Acrocephalus scirpaceus Seven-days old cuckoo chick ejected (Video 1 in this study) 1 of 1 V/P Cuculus canorus Acrocephalus scirpaceus 13-days old cuckoo chick ejected (Video 2 in this study) 1 of 1 Acrocephalus V/P Cuculus canorus Six-days old cuckoo chick ejected (Figure 1 in this study) 1 of 1 arundinaceus V/P Cuculus canorus Erithacus rubecula One of eight nestlings (eight-days old) ejected (Video 3 in this study) 1 of 8 V/P Cuculus canorus Phylloscopus sibilatrix Three of seven nestlings (five-days old) ejected (Video 4 in this study) 3 of 7 All six nestlings systematically ejected by the cuckoo within a minute V/P Cuculus saturatus Uroshena squameiceps 6 of 6 (Kawaji 2009) Two nests both with two nestlings (ca 10-days old), all pecked and 2 of 2 V/P Chrysococcyx lucidus Gerygone igata ejected (Briskie 2007) 2 of 2 When flushed cowbird carried a very young nestling from the nest. E Molothrus ater Melospiza melodia Three others (eyes not yet open) ejected (ca 20 cm from the nest) and 4 of 4 wounded (Du Bois 1956) Parasitized nest with a cowbird and three host nestlings of about three- E Molothrus ater Setophaga virens 1 of 4 days old; one host nestling ejected by a cowbird female (Tate Jr 1967) Five nestlings (seven-days old) ejected sequentially from the nest E Molothrus ater Setophaga pinus 5 of 5 (Beane & Alford 1990) All three nearly naked nestlings ejected out of the nest (Scott & E Molothrus ater Polioptila caerulea 3 of 3 McKinney 1994) Parasitized nest; three of four host nestlings (two-three days old) with E Molothrus ater Opopornis formosus open wounds, one of them outside the nest; cowbird nestling (about 1 of 4 four days old) in the nest and also wounded (Sheppard 1996) Newly parasitized nest; one nestling dangling from the nest, others E* Molothrus ater Icteria virens 1 of NA inside (number NA) (Averill-Murray et al. 1997). Two cases. First, all five (six-days old) dead with open wounds below the nest. Second, cowbird female flushed away from the nest; one E* Setophaga petechia 5 of 5 nestling disappeared. E Molothrus ater Setophaga petechia 1 of 4 Six nestlings (six-days old) pecked and thrown out of the nest within a V/P Vermivora pinus 6 of 6 minute, then cowbird female examined the empty nest several seconds (Elliott 1999). Three of four just hatched bobolink nestlings killed (Granfors et al. Dolichonyx oryzivorus 2001); 3 of 4 V/P Molothrus ater Passerculus sandwichensis All four-days old sparrow nestlings pecked and ejected out of the nest, 3 of 4 one rolled back and survived (Granfors et al. 2001) V/P Molothrus ater Vireo atricapillus Eight nests, altogether 18 nestlings (six to 12 days old) ejected, for 4 of 4 Setophaga chrysoparia 2 of 4 2 of 4 2 of 3 2 of 4 details see Table 1 in (Stake & Cavanagh 2001) 1 of 3 1 of 3 4 of 4 Male cowbird flew with the recently hatched nestling in its , landed at the fencepost, dropped and held it down with its foot and pecked its E Molothrus ater unidentified NA head until the nestling died. Then the cowbird flew off with the dead limp nestling in its beak and dropped it 100 m away (Igl 2003). V/P Molothrus ater Setophaga chrysoparia No details specified (Stake et al. 2004) NA 160E=eyewitnessed; V/P=video-recorded or photographed; *indirect observation (the dead nestlings out of the nest); cases when parasite chick 161killed in bold 162References

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222 223 Figure 1 Photographs of the adult rufous cuckoo

224 female ejecting 6-days old cuckoo chick.

225

226 227Figure 2 Screenshots from Video 2. The cuckoo female landed at the reed warbler (Acrocephlaus scirpaceus) nest and immediately ejected the 228cuckoo chick out of the nest. Later, the female searched in the empty nest and then left. The whole action took less than 20 seconds. Video- 229recordings and detailed descriptions of all events can be found in the Supplementary material. 230 231 232 233 234