NIGHT MOVEMENTS OF YOUNG REED WARBLERS (ACROCEPHALUS SCIRPACEUS) IN SUMMER: IS IT POSTFLEDGING DISPERSAL?

Auk, The, Jan 2004 by Mukhin, Andrey

ABSTRACT.-I studied summer movements of juvenile Reed Warblers (Acrocephalus scirpaceus) marked as nestlings during four field seasons (1999-2002). To control for birds' movements, nocturnal playback of songs and daytime mist-netting were done. Captures by song playback suggest the existence of nocturnal postfledging movements in Reed Warblers. Birds' age (days) during such movements was found to be 39-52 days. I analyze the temporal schedule and physiological condition of the birds during this period and discuss the mechanism of nocturnal postfledging dispersal and its background and relationships with other events of the annual cycle during the premigratory period. Received 17 October 2002, accepted 4 October 2003.

DISPERSAL BEHAVIOR is one of the fundamental features of an organism and is a major determinant of many of the most basic patterns and processes characterizing an organism (Walters 2000). Many contributions devoted to various aspects of dispersal behavior have been published recently (Newton and Rothery 2000, Martin et al. 2000, Arguedas and Parker 2000). However, the mechanisms behind this complicated behavior are poorly studied because of the methodological difficulty in observing spatial locations of individuals (Baker 1993, Bardin 1993, Vega Rivera et al. 1998).

Reed Warblers (Acrocephalus scirpaceus) are associated with reed stands and possess adaptations to the intrinsically fragmented nature of such stands. If the area covered by a single reedbed is large enough, birds can undoubtedly satisfy their urge to disperse within it. However, when birds fledge in small reedbeds, they must leave them during their postfledging movements. When reed patches are connected by canals with reeds, birds may use them (Nielsen and Bensch 1995). However, it remains unclear how postfledging dispersal is achieved when distant reedbed fragments are situated at a considerable distance from one another. It may be expected that the type of postfledging movements in the Reed Warbler may be qualitatively different from the pattern observed in the species that inhabit continuous habitats.

Another possible feature of Reed Warbler dispersal movements was first described by Herremans (1990). he captured juvenile Reed Warblers at night by playback of songs in early August and suggested that "some of the nocturnal activity of juveniles might be related to dispersal rather than true migration."

Studies of the diel rhythm of locomotory activity carried out with caged young Reed Warblers have revealed the existence of an early nocturnal restlessness that has another source than that of the typical zugunruhe (Mukhin 1999, Chernetsov and Mukhin 2001).

Here, I studied postfledging dispersal in Reed Warblers and how it is realized. By controlling the movements of juveniles, I show (1) how postfledging movements between separated reedbeds occur in this species and (2) whether Reed Warblers show nocturnal movements during their juvenile dispersal and what the features of those movements are.

METHODS

The aim of the present study was to capture young Reed Warblers banded as nestlings during postfledging movements outside reedbeds. A playback site was set up on sandy dunes 6 km from the nearest reed stands. By playback of species-specific songs, birds were captured flying from one reedbed fragment to another, fn one reed stand (Rybachy), additional daytime and nocturnal trapping was carried out without playback.

Banding of nestlings. - Fieldwork was carried out on the Courish Spit on the Baltic Sea (55°00-09'N, 20°34-51'E) in f 999-2002. For each year, in three reedbed fragments called hereafter "Lake", "Rybachy", and "Museum" (Fig. f), I searched for nests and banded Reed Warbler pulli. A total of 1,751 nestlings were banded.

Nocturnal playback.-Playback of songs was carried out at a specially designed field station (playback site) in 1999-2002. The station was at roughly equal distance between the two main nestling-banding areas (Rybachy and Museum), in a transition gap between forest and high sandy dunes covered with willow scrub. Birds were captured in standard mist nets (81-110 m in different years) that were set up in squares, either one (1999) or two (2000,2001), or placed in a line (2002). In addition, high nets (Bolshakov et al. 2000) were placed between the two mist-net squares (2000-2002) (Fig. 2). AH nets were opened at sunset and closed 30-40 min after sunrise (1999) or 20 min before sunrise (2000-2002).

Each acoustic system was centered in a mist-net square and consisted of a cassette tape player with two 20 W loudspeakers that were oriented northeast and southwest. In 1999, songs of two species, Reed Warbler and Marsh Warbler (A. palustris), were played continuously (45 min sessions of each species) between sunset and sunrise. In 2000 and 2002, Reed Warbler songs were played; in 2001, Reed Warbler and Sedge Warbler (A. schoenohaenus) songs were played. In optimal conditions, hearing distance was approximately 700-1,000 m on ground level.