Territory Establishment, Size, and Tenacity by Male Red-Winged Blackbirds

Abstract

-MOSt male Red-winged Blackbirds (Agelaius phoeniceus) established their first territories by replacing territory holders that disappeared. These territories were significantly larger than those established either through subdivision of the vacated territory between two or three replacement males, or through insertion of a new territory through displacement of one or more resident males. Almost all returning male Red-wings settled on their former territories in consecutive years. Their territories remained similar in size and location between years. Strong territory tenacity apparently is favored by lower costs of territory defense because of reduced aggression between familiar neighbors and site dominance effects. The lower costs of territory defense presumably also allowed the site-tenacious males to establish larger territories in consecutive years. Strong territory tenacity by returning males could explain much of the settlement pattern of new males. As a result, the spatial arrangement of male territories in the study marsh (i.e. the location of territorial boundaries) was similar in consecutive years. Traditionalism in the male territorial system should influence Red-wing mating patterns. Received 27 January 1986, accepted 5 January 1987. MANY birds return to the same territories in consecutive years and frequently breed with previous mates (e.g. Richdale 1957; Coulson and White 1958, 1960; Krebs 1971; Richdale and Warham 1973; Picman 1981; Bedard and LaPointe 1984; Cuthbert 1985; Gratto et al. 1985). Territory tenacity and mate fidelity may increase the fitness of returning individuals through accelerated pair formation (Lehrman and Wortis 1967, Morris and Ericson 1971, Ericson 1973), increased foraging efficiency, and improved escape from predators (e.g. Hinde 1956, Tinbergen 1957). Hence, site tenacity and mate fidelity play an important role in avian reproductive strategies. Territory tenacity could affect significantly the mating pattern of polygynous species such as the Red-winged Blackbird (Agelaius phoeniceus; Searcy 1979, Picman 1981). Female Red-wings return to their previous territories in consecutive years, regardless of whether their previous mate returned (Picman 1981). As a result, new males may inherit the harems of the previous territory holders (Picman 1981). Previous studies on Red-wings suggest that males also generally return to their territories in subsequent years (Nero 1956, Case and Hewitt 1963, Searcy 1979). Because overwinter return rates of male Red-wings are relatively high (over 50%; Fankhauser 1967, Picman unpubl. data), the distribution of Red-wing territories should be similar between years. This, along with strong female site tenacity, could lead to a high degree of traditionalism in both the distribution and number of male territories, and the distribution of females in a given marsh. Territory tenacity should also be favored because of strong intrasexual competition for limited breeding space (Rohwer 1982). This is because familiarity reduces the level of aggression between neighbors (Yasukawa et al. 1982) and, thus, presumably lowers the costs of territory establishment by returning males. Strong site tenacity by returning males could, in turn, determine the location and size of territories established by new males. In addition, territory tenacity by returning males could influence the size of their territories. With respect to benefits associated with territory tenacity, the less tenacious individuals should experience higher rates of territorial interactions, and these could negatively influence the size of their territories. The lower costs of territory defense derived from the high degree of territory tenacity in returning males, however, should allow an increase in territory size in consecutive years if neighboring territories are vacated. Because breeding space is often limited in marsh-nesting Redwings (e.g. Orians 1961, Hurly and Robertson 1985), acquiring a larger territory may increase the probability of acquiring a larger harem (Picman 1980b). 405 The Auk 104: 405-412. July 1987 This content downloaded from 207.46.13.134 on Fri, 23 Dec 2016 04:49:32 UTC All use subject to http://about.jstor.org/terms 406 JAROSLAV PICMAN [Auk, Vol. 104 Returning to the same territory should be disadvantageous for males that experienced low reproductive success. To evaluate the role of previous breeding experience, we need to determine if territory size, harem size, and fledging success of males affect territorial shifts between years. This information should indicate the relative importance of intrasexual competition in determining the pattern of territory establishment by male Red-wings. Thus, if low reproductive success did not influence the degree of territory shifts, competition among males for breeding space presumably played the major role in favoring the high degree of territory tenacity by males. I report the results of a 5-yr study on male Red-winged Blackbird territoriality. My objectives were to examine the pattern of territory establishment by new males; the degree of territory tenacity in males that returned to the marsh in consecutive years; the effect of territory size, mating success, and reproductive success on shifts of male territories between years; and the effect of territory tenacity on the size of male territories.