88 ARDEOLA 43flb 1996 INTRODUCTION The negative eííect that brood parasites have on the breeding success of their hosts is Avian brood parasites exploit hosts by usually very high. Local depression and ex- laying eggs in their nests, and leaving paren- tinction of host bird populations caused by a tal care of the parasitic offspnng to the host. parasite have been reported (see review in The iitness cost of parasitism to hosts is often Payne, 1977). However, sometimes parasitism high because before laying parasitic females may have little eflect on host-young survival usually remove or damage at least one host (Weatherhead, 1989). This is the case for the egg and the parasitic young eject al1 host viduine finches, where parasites do not remo- oííspring or outcompete most of them during ve host eggs and the young of both parasite the nestling penod (Payne, 1977; Rothstein, and bost are reared together (Payne. 1977). 1990). The least costly parasitism occun in the pre- Avian brood parasitism has been conside- cocial Black-Headed Duck (Heteronetta ani- red a model system for the study of coevolu- capilla) in which hatchlings develop without tion. Heavy host losses may impose very parental cate (Payne, 1977; Rothstein, 1990). strong selection pressures on hosts, which The Great Spotted Cuckoo (Clamatot enhance the likelihood that coevolution will glandarius) is an obligate brood parasite that occur and be detectable (Rothstein, 1990). in Europe parasitizes mainly Magpies (Pica Many workers have reponed tests of predic- pica) (Cramp, 1985), although sometimes also tions related to coevolutionary mechanisms other corvid species, in al1 cases larger than (Rothstein, 1975, 1982; Brook & Davies, the parasite (Valverde, 1971; Cramp, 1985; 1988; Davies & Brooke, 1988; 1989; Cruz & Soler, 1990). The Great Spotted Cuckoo nes- Wiley, 1989; Moksnes & Ruískaft, 1989; tling does not eject the eggs or young of the Brown et al., 1990; Soler & Mller, 1990; host (Valverde, 1971; Alvarez & Arias de Briskie et al., 1992; Soler et aL, 1994a). Bree- Reyna, 1974; Soler, 1990). As a rule, Great ding success of host species in parasitized and Spotted Cuckoo eggs hatch several days be- unparasitized nests is an important topic be- fore the Magpie's eggs, and the reproductive cause the diííerence between both indices of success of the host is markedly reduced by breeding success will indicate the strength of intense competition for food between the lar- the selection pressures involved. However, ge, fast-growing cuckoos and the smaller despite the great importance of this topic, Magpie chicks (Cramp, 1985; Soler, 1990; So- studies on breeding success of host species in ler & Soler, 1991). Adult Great Spotted Cuc- parasitized and unparasitized nests are scarce koos often damage some bost eggs during the and never have been studied experimentally, act of parasitism, thereby inílicting severe re- surely, as a consequence of parasitism rate productive losses ammg hosts (Soler, 1990; being usually very low (Rothstein, 1990). Soler et al., MS). Adult Greai Spotted Cuc- The means by which nesting success is de- koos damage the eggs of their Magpie host pressed vanes with the tactics of the parasite. without removing or eating them; 68.5 % of Female cuckoos and cowbirds may destroy 206 parasitized nests contained between 1 or remove and eat at least one of the host's and 8 damaged Magpie eggs (Soler et al., eggs (Sealy, 1992; Ruískaft et al., 1993; Soler et MS). The damaged eggs of the Magpie were al., MS). Honeyguides and at least one cuc- pecked, crushed or cracked (Soler, 1990). koo species stab host nestlings to death with As a rule, the breeding success of a parasite special mandibular hooks (Rothstein, 1990). species tends to be max&ed when the bree- In most cuckoos, nestlings push al1 host eggs dine success of the host reaches MIO IRoth- and young out of the nest (Payne, 1977; steiñ, 1990). and this is also the case ror the Rothstein, 1990). Finally, cowbirds and some Great Spotted Cuckoo-Magpie system (see cuckoos do not eject host young, but the below). Hence, adult and nestling parasites parasite chick competes successfully with should behave in such a way that generally host young for food (Payne, 1977; Rothstein, no host chicks survive. In fact, the reproduc- 1990). Sometimes outcompetes, or at least tive failure of the host species is the conse- «outsizes» some host nestlings, particularly quence of egg-destruction khaviour of adult in small hosts (Weatherhead, 1989). parasites and compeiition between nestling 90 ARDEOLA 4x1). 1996 mentally introducing one recently hatched (O- nests have been considered as independent 1 days old) cuckoo chick into 15 unparasiti- data. Analyses of these indices of success has zed Magpie nests where the oldest Magpie been made using ANOVA o Mann-Whitney chick was between one and seven days old. U-test. Three experimentally parasitized nests that The breeding success of the Magpies in were depredated were not considered. In ex- relation to time of parasitism during the perimentally parasitized nests, the cuckoo breeding cycle was analyzed using clutch size chick was on average 5.3rC0.6 (n= 12) days of Magpies and number of damaged Magpie younger than the oldest Magpie chick, while eggs as covariates in the analysis of variance in naturally parasitized nests, the cuckoo because both parameters could innuence the chick was on average 3.6f 1.94 (n=S nests breeding success of Magpies. Hatching suc- where the difference was determined exactly) cess and breeding success were arcsine-trans- days older than the oldest Magpie chick. formed and clutch size and number of dama- This difference was statistically signiiicant ged eggs were square-root transformed to (Mann-Whitney U-test, z=2.8, P=0.005). fuliill the requirement of a normal frequency distribution of the variables. b) Experiment 2 In 1992 we experimentally parasitized 17 unparasitized Magpie nests with one Great Spotted Cuckoo egg that had been laid one, Breeding success of the Magpie two or three days after than the iirst Magpie All reproductive parameters differed signi- egg of the experimental nest (simulating the ficantly in parasitized compared with unpa- situation when the parasite egg is laid early rasitized nests (Table 1). and signiiicant diffe- in the host laying cycle). We parasitized rences were also found with respect to Magpie nests with cuckoo eggs instead of hatching, fledging and breeding success cuckoo chicks because when the difference in (Table 1). laying date between the fírst Magpie egg and Most parasitized Magpie nests did not the cuckoo egg is less than three days, the produoe any Magpie chicks (77.2 %, n= 206). cuckoo chick hatches earlier than the first An average of 0.6 Magpie chicks fledged per Magpie chick due to the incubation period of nest, a rate signiiicantly lower than the mean the cuckoo egg being approximately four of 3.5 chicks produced in unparasitized nests days shorter than that of the Magpie egg (Table 1). At least one Magpie chick nedge (Soler, 1990). Hatching dates of Magpie and from only 47 parasitized Magpie nests, but in Great Spotted Cuckoo chicks were not signi- six of these nests the parasite egg was ejected ficantly different in naturally and experi- and in another 15 the cuckoo egg did not mentally parasitized nests (Mann-Whitney hatch. Thus, Magpie chicks plus a cuckoo U-test, U=36, n,= 14 and n,=6, P=0.62). chick were raised in only 26 of 185 parasiti- Four experimentally parasitized nests that zed Magpie nests (14.1 %) in which at least were depredated, and four others naturally one Great Spotted Cuckoo egg hatched parasitized by Great Spotted Cuckoos, were (Fig. 1). In these nests, very often no Magpie not considered. chicks nedged (85.9 %), but sometimes up to four chicks left the nest (Fig. 1). Most Mag- pies fledged when only one cuckoo chick hat- Sratistical analyses ched in a nest. In three nests out of 59 (5.1 %) where more than one cuckoo egg hatched did The statistical methods of Sokal & Rohlf some Magpie chicks iiedge. (1989) and for non-parametric statistics the There were major differences in number of methods described by Siegel (1988) were fo- eggs hatched between parasitized and unpa- Ilowed. Values given are meansf SD. rasitized nests as a consequence of egg des- Analyses of hatching, iiedging and bree- truction by the parasite (Table 1). The num- ding success have been made by calculating ber of chicks dying per nest was signiiicantly the success rate for each nest and individual lower in parasitized nests (Table 1). EFFECTS OF BROOD PARASiTlSM BY THE GREAT SPOlTED CUCKOO Breeding success ol !he Magp~ein parasitized and unparasltized nesis. [Exm rrproduzrnr de /u U~racacn nidos pnrasiradoc y no paravtradnc.] Parasirized nesrs Unparasiiized nesis n X SE n X SE - Clutch size .................................. 192 5.5 0.1 1 98 6.9 0.1 1 *** Eggs hatched ............................... 183 1.3 0.14 88 5.0 0.22*** Eggs unhatched ............................ 168 3.9 0.17 85 2.0 0.21 *** Chicks fledged .............................. 206 0.6 0.09 106 3.5 0.14*** Chicks dead ................................ 176 0.7 0.10 85 1.5 0.16*** Hatching suceess ........................... 171 23.5 2.48 85 71.8 2.99*** Fledging success ............................ 75 40.9 4.96 85 74.4 2.57*** Breedtne success ............................ 185 10.6 1.57 97 50.1 2.18*** Only nerts wherc al Icari onc chiek (magpie or cuckool fledgcd vere conridercd. Paras8Iiwd ncslr hsve been included il ihe cuckoo eeg did nat hateh or war cjecled. Stiiirtical Icsir uwd are Student r.tnts benwnn mcanr and Mann. Whiincy U-lesi on penenlager. n=nuniber oí nnts .---P<0.00001.