Reproductive consequences of autumnal singing in black redstarts (Phoenicurus ochruros)

Auk, The, Jan 2000 by Weggler, Martin

ABSTRACT.-In a migratory population of Black Redstarts (Phoeniourus ochruros) in central Europe, males were territorial and sang in autumn between the end of molt in early September and the abandonment of territories in October. Participants in autumn singing were adult males past their first potential breeding season; subadults rarely defended territories in autumn. Prior to the autumn singing period, unmated males and males after their first breeding season often dispersed to new locations within the study site. Pair associations with experienced female breeders still present on the breeding ground were preformed. Low winter mortality, site dominance, and fidelity to autumn territories allowed the reformation of 59% of autumnal pair bonds in the following spring. The mating pattern was assortative by age because the initiation of territory acquisition and pair formation was temporally segregated by more than six months between subadult and adult breeders. Males benefitted from mating with experienced females because they started breeding earlier and initiated more breeding attempts per season. Autumnal singing and territoriality, a phenomenon that has not attracted much attention, may play a key role in the understanding of age-related reproductive asymmetries in Black Redstarts. Age-related reproductive performance may be the underlying cause for the evolution of delayed plumage maturation in this species. Received 16 November 1998, accepted 5 May 1999.

THE REPRODUCTIVE CYCLE of most north-temperate bird species starts in spring and ends in late summer (Murton and Westwood 1977). Preparation for nesting is thought to be displayed only a few weeks or months before nest building actually starts. Among the few exceptions to this rule are cavity-nesting ducks that prospect for breeding opportunities for the next year during the present breeding season (Patterson and Makepeace 1979, Eadie and Gauthier 1985, Zicus and Hennes 1989), and Spotted Sandpipers (Actitis macularia) that inspect potential breeding territories for next year during late-summer movements (Reed and Oring 1992).

The autumnal song of several passerines also may yield benefits for breeding in the subsequent year. For example, Hegner and Wingfield (1986) showed that territorial activities of resident House Sparrows (Passer domesticus) in autumn may enhance next year's breeding success. Autumnal song of nonmigratory Northern Mockingbirds (Mimus polyglottos) serves to attract mates, and birds that are paired in autumn may benefit from a longer breeding season the next year (Logan and Hyatt 1991).

Lawn (1994) suggested that nonbreeding Willow Warblers (Phylloscopus trochilus) that settle late in the breeding season intend to stake out next year's breeding territories rather than breed in the current year, and Yamagishi (1991) revealed that resident male Meadow Buntings (Emberiza cioides) advertise their settlement or site tenacity by singing in autumn.

Male Black Redstarts (Phoenicurus ochruros) breeding in central Europe sing intensively during two phases: (1) from late March immediately after their arrival on the breeding territory until the onset of molt in July, and (2) from mid-September until mid-October before the birds leave their breeding grounds to migrate to their winter quarters (Glutz and Bauer 1988, Schmidt 1992). It is not known whether males that sing in autumn are mainly transient birds or males on their breeding territories, and possible reproductive consequences of autumnal singing heretofore have not been investigated.

Here, I present evidence that autumnal singing in a migratory population of Black Redstarts helps male breeders to secure access to territories and high-quality females for the following breeding season, Because subadult males (i.e. hatched in the current calendar year) rarely sing in autumn, autumnal singing by older mates may help to explain age-specific reproductive performance and the evolution of delayed plumage maturation in this species. My aim was to (1) identify the origin and age of autumnal singers and (2) explore relationships between autumnal singing and behaviors related to subsequent breeding attempts and reproductive performance.

METHODS

Study species and population.-In central Europe, the breeding habitat of Black Redstarts consists of alpine cliffs and rocky slopes (primary habitat), as well as human settlements (secondarily). Within human settlements, nests are located in walls or roofs of dwellings and huts. The species is multibrooded and regularly produces two (rarely up to three) broods per season.

Between 1994 and 1996, 1 studied a breeding population of Black Redstarts in two villages (167 dwellings and huts in total) in the Swiss Alps. The annual number of birds fluctuated between 25 and 28 males and 20 and 29 females (Table 1). Males were strictly territorial, and territorial arrangement varied only little between years. Mean territory size was 0.48 ha (range 0.12 to 1.32 ha, 28). Most (77%) of the 78 males that were recorded as breeders at least once were mated monogamously, 10% were bigamous, and 13% were unmated. These numbers refer to the situation on 10 May of each season. However, because summer mortality of females and males was high (Table 1), and the breeding season was long, pair bonds and mating systems changed frequently in the course of the breeding season.