Nestling feeding-space strategy in Arabian Babblers

ABSTRACT.-Maintaining individual space is a foraging tactic widely used by many adult animals. However, this behavior has not been described for altricial nestling birds. In this study, nestling Arabian Babblers (Turdoides squamiceps) defended individual spaces around themselves and obtained food in relation to the size of these spaces. Each nestling created two circular feeding zones around itself: (1) an internal one (termed exclusive space), within which it usually succeeded in preventing nest mates from obtaining food; and (2) an external one (shared space), within which the nestling competed for food with siblings according to its relative strength. The first indication of the imminent death of a nestling was when siblings began receiving food inside its exclusive feeding space. Stability of feeding spaces was tested by adding a fifth nestling to the nest. None of the exclusive feeding spaces decreased, and feeding rates within them remained constant. In contrast, feeding rates in the shared feeding spaces diminished as a consequence of their reduction in size. It seems that a nestling's survival depends on its ability to defend its exclusive feeding space. This tactic may enable Arabian Babbler nestlings to obtain food and survive in their cooperative as well as competitive nest environment in a relatively nonaggressive manner. Received 22 December 1997, accepted 13 November 1998.

AVIAN BROOD REDUCTION has been the subject of much empirical and theoretical research since the appearance of O'Connor's (1978) seminal paper. So-called altruistic behaviors, as well as selfish ones, were explained through brood reduction using a wide range of theories such as inclusive fitness, the handicap principle, parent-offspring conflict, and honest signaling.

Unlike its theoretical explanations, the mechanisms of brood reduction have attracted less attention. In nonpasserines, sibling aggression is very common and leads to both uneven food distribution and brood reduction (Cash and Evans 1986, Mock et al. 1987, Anderson 1990, Forbes 1991, Drummond 1993, Ploger 1997). In passerines, however, aggressive interactions among nest mates are rare. Although nonaggressive brood reduction has been widely documented (Magrath 1989, Teather 1992, McRae et al. 1993, Kacelnik et al. 1995, Price and Ydenberg 1995, Leonard and Horn 1996, Ostreiher 1997), its mechanisms are still enigmatic. Begging calls (Harper 1986, Smith and Montgomerie 1991), jockeying for a better position (Gottlander 1987, McRae et al. 1993), body postures (Kilner 1995), or a combination of these behaviors (Kacelnik et al. 1995, Kilner 1995) have been considered as the main manifestations of sibling competition. All of these behaviors, however, seem to be directed toward the feeding adults. In the Blue-footed Booby (Sula nebouxii; Drummond and Garcia Chavelas 1989), Great Blue Heron (Ardea herodias; Mock 1985, Mock and Parker 1986), Great Egret (Ardea alba; Mock 1984, 1985), American Kestrel (Falco sparverius; Anderson et al. 1993), Arabian Babbler (Turdoides squamiceps; Ostreiher 1997), and others, food distribution among nest mates is determined by the outcomes of nestling interactions. It seems unlikely that in passerines, no direct interactions have been developed among the nest mates during the evolution of nestling competition.

Arabian Babblers are cooperative breeders (Zahavi 1990). The modal clutch size is four, and incubation usually starts after the last egg has been laid such that broods hatch almost synchronously, with only 6 to 34 h passing between hatching of the first and last chicks. Feeding rate is negatively correlated with hatching order, nestlings compete for food, and brood reduction is frequent (Ostreiher 1997). Pecking among the nestlings has never been seen (based on more than 1,100 h of observation from hatching to fledging in 42 nests), but nest mates may struggle by pushing each other. The aim of the current research was to study mechanisms of sibling rivalry used by Arabian Babbler nestlings.

METHODS

The study was carried out at the Shezaf Nature Reserve in the Arava Valley and around the Hazeva Field Study Center, about 30 km south of the Dead Sea, in southeastern Israel. These babblers are part of an ongoing study that was started in 1971 (Zahavi 1990). The current study was conducted over the five breeding seasons from 1 February to 1 June, 1992 to 1996.

The study area contained about 35 groups comprising more than 240 individuals. The birds are accustomed to human presence and tolerant of close observations. Each group was observed at least twice a week, and groups that had started nest building were observed daily. Nests were visited every day, and each egg was marked to establish laying order. During the last two days of the 14-day incubation period, nests were checked every hour and were observed continuously from first to last hatching. Each nestling was marked with a colored dot immediately after hatching. A colored collar was fitted to its neck and a colored wire to one leg on day 4 after hatching. The collars were exchanged among nestlings at the end of each observation to prevent possible feeding bias associated with certain colors. Each nestling was banded with a four-color combination at 9 or 10 days old, and the collar was removed on day 12 or 13, one or two days before fledging.