Behav Ecol Sociobiol (1999) 45: 33±45 Ó Springer-Verlag 1999 ORIGINAL ARTICLE Javier VinÄ uela Sibling aggression, hatching asynchrony, and nestling mortality in the black kite (Milvus migrans) Received: 9 March 1998 / Accepted after revision: 8 August 1998 Abstract In siblicidal species, hatching asynchrony high-quality chicks, but a manifestation of a parent- could act to reduce sibling rivalry or promote the death ospring con¯ict over brood size. of last-hatched chicks. The pattern of hatching asyn- chrony was experimentally altered in the black kite Key words Hatching asynchrony á Sibling aggression á Milvus migrans. Hatching asynchrony in control broods Nestling mortality á Parent-ospring con¯ict á was intermediate between those of experimentally syn- Milvus migrans chronised and asynchronised broods. Sibling aggression and wounds on the chicks were more commonly ob- served early in the nestling period and in synchronous nests. Serious injuries were observed on last-hatched Introduction chicks in asynchronous nests, as were observations of intimidated or crushed chicks. Sibling aggression was Sibling aggression and hatching asynchrony are com- related to food abundance, but some chicks died at an mon in raptors (Newton 1979), and may promote sib- early age in nests with abundant food (cainism). Cainism licide (Simmons 1988). Hatching asynchrony could was more commonly found in asynchronous nests. For reduce sibling aggression between size-matched chicks species with facultative siblicide, moderate hatching (Hahn 1981), allowing the establishment of a size hier- asynchrony could be a compromise between reducing archy with a minimum of aggressive interactions. Al- sibling rivalry and avoiding large size dierences be- ternatively, it could promote the death of last-hatched tween sibs that would result in cainism. Female black chicks by establishing large size dierences between kites preferentially fed the smallest chicks and exhibited siblings, since in obligately siblicidal species (those in behaviours to reduce sibling aggression, contrary to which >90% of last-hatched chicks die; Simmons 1988), observations in other siblicidal species. In a highly op- hatching asynchrony is higher than in facultative species portunistic forager such as the black kite, a strategy may (those in which siblicide occurs in <90% of nests; exist to protract the life of all the chicks in the brood, Meyburg 1974; Edwards and Collopy 1983; Anderson waiting for unpredictable situations of food overabun- 1989). These two hypotheses are not mutually exclusive, dance. This would induce the appearance of a parent- and there may exist an optimal intermediate degree of ospring con¯ict over brood reduction, re¯ected in the hatching asynchrony (Mock et al. 1990; Osorno and existence of a possible anticipated response by some of Drummond 1995). The research in Ardeidae, Phalacro- the chicks (cainism) and in the appearance of special coracidae and Sulidae indicates that synchronisation of behaviours by the parents to selectively feed smaller hatching in siblicidal species may escalate the combats chicks or reduce sibling aggression. In this facultatively (Fujioka 1985a; Mock and Ploger 1987; Anderson 1989; siblicidal species, cainism does not seem to be the ®nal Osorno and Drummond 1995), while highly asynchro- stage of an evolutionary trend favouring the raising of nous hatching may result in poor growth of last-hatched chicks, excluded from food transfer by their larger sib- lings (Amundsen and Stokland 1988; Mock et al. 1990; see also Pinajowski 1992). This possible ``cost'' of J. VinÄ uela (&) hatching asynchrony on growth of last-hatched chicks Departamento de Ecologõ a Evolutiva may exist even in Passerines, where sibling aggression is Museo Nacional de Ciencias Naturales, (C.S.I.C.) Jose Gutierrez Abascal 2, E-28006 Madrid, Spain negligible or absent (see Slagsvold et al. 1995). e-mail: [email protected], Siblicide may contribute to food-dependent brood Tel.: +34-1-4111328, Fax: +34-1-564078 reduction if sibling aggression is causally linked to 34 hunger (Lack 1954). Thus, hatching asynchrony would Large size and good condition at ¯edging may be promote the establishment of a size hierarchy (VinÄ uela important factors to assure prebreeding survival and 1996), and sibling aggression would facilitate brood re- future breeding in species with high prebreeding mor- duction when there is no food available to raise all tality and strong competition for nesting territories. chicks. However, experimental evidence suggests that Furthermore, the retention of the second egg may al- much of the mortality induced by hatching asynchrony low parents to track population conditions (Simmons is not adaptive (Amundsen and Slagsvold 1991), but a 1988, 1993): raising only one high-quality chick when negative consequence of an early onset in incubation, population density is high (when territorial competi- which would be favoured for other reasons (review in tion is high), raising two lower-quality chicks when Stoleson and Beissinger 1995). Furthermore, in obligate population density is low. Simmons (1988) proposed siblicidal species, there is no relationship between food several predictions of his hypothesis that could be abundance and sibling aggression (Simmons 1988; tested in long-lived facultatively siblicidal raptors. Gargett 1990; Mock et al. 1990), and it is not clear to Speci®cally, the frequency of appearance of siblicide or what degree sibling aggression is related to food abun- the variation in the factors promoting it (hatching dance in facultatively siblicidal species (Mock et al. asynchrony, long laying intervals, intraclutch egg size 1990; Forbes 1991a). variation) should be correlated with population densi- Sibling aggression could be unrelated to food abun- ties. dance if there is a parent-ospring con¯ict over brood The black kite, Milvus migrans, is a medium-sized, reduction (O'Connor 1978; Parker and Mock 1987; relatively long lived raptor (a maximum of 22 years in Anderson 1990a; Nilsson 1995; Rodrõ guez-Girone s the wild; Newton 1979), exhibiting facultative siblicide 1996). Ospring may tend to commit siblicide under less and cannibalism (Jones and ManÄ ez 1990; VinÄ uela 1991). stringent conditions than those optimal for parents, so Black kites are highly opportunistic predators, obtaining selection would favour the appearance of behavioural their food by a searching strategy, and their broods are patterns allowing parental regulation of siblicide (An- exposed to very variable provisioning rates depending derson 1995). Theoretical models predict that parent- on environmental conditions (see e.g. VinÄ uela and Veiga ospring con¯ict over brood size should be more prev- 1992). This study addresses the following questions. (1) alent among species exposed to highly variable provi- Can hatching asynchrony reduce sibling rivalry? (2) sioning rates (Forbes 1991b, 1993; Forbes and Ydenberg Does hatching asynchrony promote the death of last- 1992; Rodrõ guez-Girone s 1996). However, there is little hatched chicks? (3) Is sibling aggression related to food empirical evidence for such con¯ict (Drummond et al. availability? (4) Is there any evidence suggesting the 1986). Parent raptors do not seem to interfere in sibling existence of a parent-ospring con¯ict over brood re- ®ghts or exhibit any behaviour regulating the brood duction in a species with highly variable provisioning reduction process (Newton 1979; Forbes 1991a; but see rates? (5) Is there any eect of brood size or nestling Gerrard and Bortolotti 1988), but most information has mortality on the ¯edging size or condition at ¯edging of been gathered in obligate siblicidal species (Meyburg dominant chicks? Furthermore, I test some of the pre- 1974; Gargett 1990). Little is known about the mecha- dictions of Simmons (1988) in a steadily increasing nisms, if any, allowing parental regulation of sibling population of a facultatively siblicidal raptor, in which competition (Simmons 1988; Anderson 1995). the breeders were exposed to very dierent population There has been much discussion about the signi®cance densities during a 3-year period. of obligate siblicide, typical of some species laying two eggs (Stinson 1979; Simmons 1988, 1991; Mock et al. 1990; Gargett 1991). The second egg may be an ``insur- ance'' in case the ®rst egg does not hatch (Meyburg 1974; Stinson 1979; Parker and Mock 1987; Anderson 1990b; Methods Mock et al. 1990). However, evidence supporting the ``insurance value'' of last-laid eggs is scant (but see Forbes The study was conducted in Matas Gordas, Northern DonÄ ana et al. 1997). This prompted Simmons (1988) to propose an National Park (south-west Spain, 37°N6°5¢W). This is an open and alternative: obligate siblicide would be the consequence of ¯at area of Mediterranean forest (cork oaks Quercus suber), scrublands and grasslands, close to a seasonally ¯ooded marshland an evolutionary trend favouring the raising of high- (see VinÄ uela and Veiga 1992). quality chicks. Chicks raised in the absence of a compet- The breeding area was visited almost daily (>95% of days) itive sibling would reach a higher size or better condition from the start (mid-March) to the end (end of July) of the breeding at ¯edging than senior chicks competing with siblings for seasons of 1987, 1988 and 1989. Nests were visited daily during laying and eggs were marked with felt pens. Laying was considered food, all else being equal (Stinson 1979; Simmons 1988), to be ®nished when the third egg was found or if 4 days elapsed but few data are available to support this idea. Simmons' since the previous one was found. After laying, nests were not hypothesis is intimately related to recent work supporting visited again until 28±30 days after the date of laying of the ®rst the hypothesis that hatching asynchrony in Passerines can egg. Nests were visited daily during hatching, and the chicks indi- be a mechanism to ensure the rearing of high-quality vidually marked under the wing with felt pens. Brood size at hatching varied between one and three chicks. Nests in which only ospring (Slagsvold et al. 1995; Amundsen and Slagsvold one chick hatched have not been considered, unless otherwise in- 1996, 1998).