Territory choice during the breeding tenure of male sedge warblers

Abstract

A territorial male can shift the location of its territory from year to year in order to increase its quality. The male can base its decision on environmental cues or else on its breeding experiences (when territory shift is caused by breeding failure in previous seasons). We tested these possible mechanisms of territory choice in the sedge warbler (Acrocephalus schoenobaenus), a territorial migrating passerine that occupies wetlands. This species bases its territory choices on an environmental cue: tall wetland vegetation cover. We found that the magnitude of territory quality improvement between seasons (measured as the area of tall wetland vegetation) increased throughout the early stages of a male's breeding career as a result of territory shifts dependent on the earliness of arrival. The distance the territory was shifted between seasons depended negatively on the previous year's territory quality and, less clearly, on the previous year's mating success. On the other hand, previous mating or nesting success had no influence on territory quality improvement between seasons as measured in terms of vegetation. The results imply that tall wetland vegetation is a long-term, effective environmental cue and that a preference for territories in which this type of landcover prevails has evolved into a rigid behavioral mechanism, supplemented by short-term individual experiences of breeding failure.

Fig. 1

Example of measurement of consecutive territory shifts of a sedge warbler male. In 2001, the first territory of the male coded OWO was occupied in the most peripheral western part of study plot HT. It achieved mating, but nesting failed. In the next year, it was observed close to a territory that is occupied very early in each breeding season. It mated and raised a successful brood. In 2003, it occupied this best site (with a common reed patch), where it mated and probably lost the clutch in an early stage of incubation. Every year, all its records during the first day after arrival were plotted on the GIS map, then their midpoint was found, and the distances between midpoints in consecutive seasons were measured. Gray patches cattail, dark gray common reed, black dots songposts during the first day after settlement, open dots midpoints of the songposts in consecutive seasons

Fig. 2

Differences between consecutive seasons of sedge warbler male breeding careers in: a tall wetland vegetation cover within territories, b occupancy of territory, c arrival date, d distance between territories, e nesting success (number of eggs laid, black boxes; number of fledged young, gray boxes) with the respective t test statistics shown for number of eggs laid (upper part) and for number of fledged young (lower part), f number of recruits. All t tests are for matched pairs

Fig. 3

Relationship between the change of males' arrival dates between seasons and the corresponding change in the proportion of tall wetland vegetation at their sites between seasons (a first and second seasons, b second and third seasons)

Fig. 4

Differences in territory shift distance between categories of fitness change: 1–0 males successful in the first season and unsuccessful in the second, 0–0 males unsuccessful in both seasons, 0–1 males unsuccessful in the first season and successful in the second, 1–1 males successful in both seasons. a Shift distance differences between males differing in mating success, b shift distance differences between males differing in nesting success, c shift distance differences between males differing in recruitment success, d vegetation cover differences between males differing in mating success, e vegetation cover differences between males differing in nesting success, f vegetation cover differences between males differing in recruitment success