Breeding behaviour and ecology of the sexually size-dimorphic brown songlark, Cinclorhamphus cruralis

Abstract

The Australian endemic brown songlark, Cinclorhamphus cruralis, is one of the most sexually size-dimorphic of all birds, and yet its breeding ecology remains poorly documented. Here we redress this situation by describing the breeding activities of brown songlarks over three years (1998)(1999)(2000) in the semi-arid grasslands of south-western New South Wales. Study populations of this nomadic species were selected in late August of each year on the basis of high adult abundance. Adult males at these sites were, on average, 2.3 times heavier than females. Over the three seasons, nesting activities started in early to late August and continued until early November or December. Males were highly polygynous and, on average, occupied territories of about 4.0 ha. Nests were well concealed at the base of small shrubs and grass tussocks or in thick herbage. Clutches ranged in size from 2 to 5 eggs (mean 3.2) and were incubated exclusively by the female for 11-13 days (mean 12.1). Nestlings received a range of invertebrate prey, mainly from the female, for 10-14 days (mean 11.5) before leaving the nest. Only 17% of nesting attempts were estimated to be successful, and each of these nests produced an average of 2.7 fledglings. Predators, including foxes, Vulpes vulpes, and brown snakes, Pseudonaja textilis, were the main cause of nest failure. Some females produced replacement clutches following nest failure, while others laid second clutches after the success of an earlier brood. We speculate that extreme size dimorphism has evolved in this species because (i) males compete physically for breeding territories, and (ii) habitat heterogeneity and excellent visibility of their surroundings allow some males to defend territories of sufficient size to support nesting by multiple females. Z O 0 3 0 3 4 B r e e d i n g b e h a v i o u r a n d e c o l o g y o f t h e b r o w n s o n g l a r k M . J . L . Ma g r a t h e t a l .

Sexual size dimorphism and assortative mating in Carolina Wrens

“…Owens and Hartley (1998) indicated that species, like Carolina Wrens, that are socially and genetically monogamous, and show little sexual difference in parental care, are less sexually dimorphic than species with more intense sexual selection such as polygamous species or species where individuals engage in mixed reproductive strategies (e.g., see Magrath et al 2003). The degree of SSD in Carolina Wrens is less than in polygamous species, and my results suggest possible causations and directions for future work.…”

Estimating expenditure on male and female offspring in a sexually size‐dimorphic bird: a comparison of different methods

“…During the nestling period, broods received a range of invertebrate prey items that were provided almost exclusively by the female. Male adults were observed feeding at just four of 66 broods (6%), and at these four nests contributed an average of only 18% of feeds (Magrath et al . 2003a).…”

“…2003a). Mothers were observed feeding young of both sexes for up to 3 weeks after they left the nest (Magrath et al . 2003a), but the data were too few to determine if the time to independence differed between the sexes.…”

“…Data were collected between September and December in the 3 years from 1998 to 2000. Local population densities across the region varied widely between years (Magrath et al . 2003a), so a different study site was selected each year based on an apparent high density of adults in early September The location and area of these study sites was 33°21′S 144°58′E (60 ha) in 1998, 33°72′S 145°30′E (120 ha) in 1999, and 34°22′S 145°02′E (120 ha) in 2000.…”

“…This species is a member of the Sylviidae (Old World warblers) family, endemic to Australia, and one of the most sexually size dimorphic of all birds (Andersson 1994). In our population, adult males were about 2·3 times heavier than adult females and the mating system was strongly polygynous, with females providing almost all parental care (Magrath et al . 2003a).…”