Life in the slow lane: Reproductive life history of the White-browed Scrubwren, an Australian endemic

Auk, The, Apr 2000 by Magrath, Robert D, Leedman, Ashley W, Gardner, Janet L, Giannasca, Anthony, Et al

ABSTRACT.-An understanding of geographic and phylogenetic variation in passerine life histories is hampered by the scarcity of studies from the Southern Hemisphere. We documented the breeding biology of the White-browed Scrubwren (Sericornis frontalis), an Australia endemic in the Pardalotidae (parvorder Corvida). Like other members of the Pardalotidae, scrubwrens had a long laying interval (two days), a long incubation period (declining from 21 to 17 days through the season), and a long period of postfledging parental care (6 to 7 weeks). Scrubwrens appeared to be typical of the Australian Corvida in having a small clutch size (three eggs) and a long breeding season (5.4 months), and they also had a long interval between breeding attempts (10 days after a failed attempt, 21 days after a successful attempt). Scrubwrens were multibrooded, often raising two broods successfully and occasionally raising three broods. The breeding biology of scrubwrens adds further support to claims of a distinct life-history strategy for members of the Corvida but also reinforces evidence that some "Corvida ' life-history traits more specifically are those of the Pardalotidae. Received 17 November 1998, accepted 18 October 1999.

MOST STUDIES Of the breeding biology of passerines have been carried out in the Northern Hemisphere, primarily on members of the parvorder Passerida (sensu Sibley and Ahlquist 1990). This biases our understanding of passerine biology because these species are not representative of passerines as a whole. For example, the Australian Corvida have smaller clutch sizes than passerines that breed at similar latitudes in the Northern Hemisphere and Passerida that are resident'in Australia (YomTov 1987, 1994; Rowley and Russell 1991). It has also been claimed that members of the "old endemic" Corvida of Australia have a syndrome of slow reproduction, with small clutch sizes, prolonged periods of parental care at all stages of nesting, and long breeding seasons (Ford 1989, Rowley and Russell 1991). Nevertheless, other than clutch size, most features of the breeding biology of these species are not well known, and comparisons of many life-history attributes are inconclusive owing to the lack of data from the Southern Hemisphere (Martin 1996).

Although the 49 species in the Pardalotidae that are resident in Australia appear to epitomize the syndrome of slow reproduction, only a few species have been studied (e.g. Acanthiza pusilla, A. reguloides, and A. lineata (Bell and Ford 1986]; A. chrysorrhoa [Ford 1963]; Sericornis frontalis [Ambrose and Davies 1989]; Pardalotus spp. [Woinarski and Bulman 1985]). Even for these species, sample sizes often are small, and quantitative data do not cover the whole breeding cycle. Indeed, for each of these species, little is known about postfledging care, duration of the breeding season, and number of breeding attempts per season made by individual females.

Here, we provide a quantitative description of the breeding biology of a population of the White-browed Scrubwren (Sericornis frontalis), a sedentary, cooperatively breeding species in the Pardalotidae that is common in southern and eastern Australia. Ours is the first detailed study of the breeding cycle and breeding season of any member of the genus and is based on data gathered from a color-marked population over five years. Our goal is to provide a detailed account of the breeding cycle of a member of this speciose family of Australian birds as a first step toward disentangling the effects of environment and phylogeny on the life history of passerines.

METHODS

Study species.-The White-browed Scrubwren is a small (ca. 11 to 15 g) passerine that is endemic to mainland Australia (Christidis and Boles 1994). It is one of six members of the genus endemic to Australia. White-browed Scrubwrens are largely sedentary as adults (Brown et al. 1990, Wilson 1994) and breed in diverse habitats from coastal rainforest to alpine heath (Blakers et al. 1984). Scrubwrens feed primarily on arthropods on the ground or in low shrubs, but they also search under bark or in foliage, sometimes in tree canopies (Keast 1978, Ambrose 1985, Cale 1994). The nest is domed with a side entrance and usually is well hidden under leaf-litter or vegetation, on or near the ground. White-browed Scrubwrens are known to breed cooperatively (Bell 1982, Ambrose and Davies 1989), including our study population (see below).

We studied a color-banded population of S. fi frontalis in and adjacent to the Australian National Botanic Gardens (3516'S, 14906'E) in Canberra from 1992 to 1996. The Gardens occupy an area of 40 ha, 27 ha of which are planted exclusively with Australian native plants. Most of the remaining 13 ha are natural woodlands that are contiguous with a large area of natural habitat in which scrubwrens breed. Although breeding extended beyond the end of December in each season, we refer to the season by the year in which breeding began.

White-browed Scrubwrens are resident throughout the year, and during the breeding season we visited territories at least three times a week to document reproductive attempts. Between 35 and 48 breeding groups were present in any one breeding season. Scrubwrens typically bred in pairs (46! of groups) or in trios consisting of a socially dominant pair and a subordinate male (44%), although 10% of the groups had more than one subordinate male (Magrath and Whittingham 1997). Only the female builds the nest and incubates, but both members of the dominant pair, and often the subordinate males, provision the nestlings (Magrath and Whittingham 1997). Neither group size nor provisioning by subordinate males had a detectable effect on the duration of any stage of the reproductive cycle, the interval between nesting attempts, or the reproductive success of the group (Magrath and Yezerinac 1997).