Winter flocking behaviour of speckled warblers and the Allee effect

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Abstract

The aggregation of individuals into foraging flocks is one behavioural trait that, if disrupted, can cause the Allee effect, which is a slowing in population growth at low density or small population size, and this can greatly increase the risk of extinction. Here, I describe intraspecific flocking behaviour of a colour-banded population of speckled warblers, Chthonicola sagittata, a species that has declined across a large part of its range in the fragmented temperate woodlands of Australia. I make predictions about the context in which the Allee effect might be expressed and the consequences for the viability of populations living in small habitat remnants. Speckled warblers lived in discrete, stable social groups throughout winter, the nucleus of which was the residents from one or more adjacent breeding territories. The timing and mode of flock formation and the size of flocks varied between two winters, apparently in response to the severity of conditions; thus flocking probably facilitates increased foraging efficiency and predator detection, potentially leading to increased survival in harsh conditions. Because flock territories were up to 30 ha each, and larger territories are likely, birds living in remnants smaller than 40 ha may suffer increased mortality if there are too few birds available to form flocks of an appropriate size to facilitate the benefits of grouping when conditions are most extreme. Further, in small remnants where survival is reduced, dominance behaviour and male–male competition may act to compound the Allee effect by reducing reproductive success. Regardless of these predictions, speckled warbler populations may only be viable in remnants that are large enough to support multiple flocks, to enable rapid recruitment to breeding vacancies and thus provide adequate numbers of birds for flocking.

Introduction

A variety of behavioural traits, subject to particular conditions, can cause the Allee effect, which is a slowing in population growth at low density or small population size (Allee, 1931; Sæther et al., 1996; Courchamp et al., 1999; Stephens and Sutherland, 1999). One of the best-known examples of such behaviour is the aggregation of individuals into foraging groups (Sæther et al., 1996; Courchamp et al., 1999; Reed, 1999; Stephens and Sutherland, 1999; Stephens et al., 1999). Grouping is thought to facilitate increased foraging efficiency and predator detection and therefore acts to increase survival in harsh conditions (Pulliam, 1973; Ekman, 1987; Hogstad, 1988; Lima and Dill, 1990; Bednekoff and Lima, 1998). The benefits of grouping increase with group size, up to a point, and group size is varied in response to the severity of conditions (Elgar, 1989). A reduction in population size that depletes the number of individuals available for grouping may therefore lead to a reduction in survival if groups become so small that foraging and anti-predator strategies become inefficient. Thus, the Allee effect may be an important mechanism for population regulation, especially in species that show a high degree of sociality (Sæther et al., 1996; Courchamp et al., 1999; Reed, 1999; Stephens and Sutherland, 1999).

There is evidence that the Allee effect can occur in a wide range of taxa and can greatly increase the likelihood of local and global extinction, but despite this, its importance in population ecology has traditionally been underestimated (Creel, 1998; Courchamp et al., 1999; Reed, 1999; Stephens and Sutherland, 1999). This may be due to the inherent difficulty of demonstrating an imbalance between births and deaths in small populations (Courchamp et al., 1999; Stephens et al., 1999). In addition, the consequences of such an imbalance for population viability may be difficult to predict because outcomes will depend on the prevailing environmental conditions. Nevertheless, recent advances in statistical modelling have led to a better understanding of such interactions and have highlighted the importance of the Allee effect in the dynamics of small populations (e.g., McCarthy, 1997; Amarasekare, 1998).

Habitat fragmentation is one form of anthropogenic disturbance that has driven many populations to small sizes or low densities, potentially placing them at risk from the Allee effect (Courchamp et al., 1999; Reed, 1999). Habitat fragmentation alters the amount, configuration in the landscape and quality of suitable habitat available for species, thereby having potential to affect the distribution and abundance of species (Saunders et al., 1991; Fahrig, 1997). Studies of habitat fragmentation have tended to focus on description of the loss of biodiversity and changes to species composition in remnants. Much less is known about mechanisms of decline and this makes it difficult to predict the consequences of environmental change (Sutherland, 1998; Courchamp et al., 1999; Reed, 1999; Anthony and Blumstein, 2000). This limitation has led to a call for behavioural studies, including those that elucidate the causal mechanisms of the Allee effect (Courchamp et al., 1999; Stephens and Sutherland, 1999).

The speckled warbler Chthonicola sagittata is one of a number of declining species from the highly fragmented temperate woodlands of Australia (Garnett, 1992; Robinson and Traill, 1996; Recher, 1999; Reid, 1999). Since European settlement, up to 80% of this habitat type has been cleared to make way for grazing of livestock and agriculture, and this has had considerable impact on the status of species within the region (Ford et al., 2001). Speckled warblers have declined across a large part of their range, and in districts where no habitat fragments larger than 100 ha remain, they appear to be locally extinct (Barrett et al., 1994; Robinson and Traill, 1996; Traill and Duncan, 2000). In addition to area, habitat quality is likely to affect distribution in remnants, since presence of the species appears to be associated with a high degree of habitat complexity (Watson et al., 2001).

Specific causes of the decline of speckled warblers are unknown but thought to be linked with processes associated with the fragmentation of their habitat, rather than a direct result of habitat loss, since populations have continued to decline in areas where land clearing has stopped (Reid, 1999). The species occurs at low population density and is a specialist in terms of foraging and nesting behaviour (Ford et al., 1986; Tzaros, 1996; Gardner, 2002; Gardner et al., 2003), characteristics that are likely to result in small population size in fragmented landscapes and thus make it prone to extinction (May, 1973; Soulé, 1987; Caughley, 1994; Mac Nally and Bennett, 1997; Reed, 1999). One aspect of behaviour that may have consequences for survival as a result of the potential for the Allee effect is the inclination to form intraspecific flocks as well as join mixed species flocks in winter. Habitat fragmentation can influence the dynamics of winter flocks because it can affect the numbers of individuals available for flocking, as well as predator abundance and the availability of food and suitable habitat (Tellerià et al., 2001).

Here, I describe the formation, structure and composition of intraspecific flocks of speckled warblers living in a large (>1000 ha) habitat remnant, based on observations of a colour-banded population over two winters. I conclude that flocking is likely to increase individual fitness and thus, under specific conditions, trigger the Allee effect. I make predictions about the context in which the Allee effect might be expressed and discuss probable consequences for populations living in small remnants of habitat.

Section snippets

Study species and site

The speckled warbler is a small (11–15 g) insectivorous passerine which is a sedentary resident of the temperate woodlands of eastern Australia (Blakers et al., 1984). It is a member of the Acanthizinae within the Pardalotidae (Christidis and Boles, 1994) or Acanthizidae, in which case the species name has reverted to Pyrrholaemus sagittatus (Schodde and Mason, 1999). I use Acanthizinae hereafter. Males are slightly larger than females and can be distinguished on the basis of a black rather

Flock formation and social organisation

Speckled warblers lived in flocks of 2–7 individuals throughout the winter (excluding the single case of a lone male on a territory). The nucleus of each flock was one or more breeding pairs or groups, some of which were joined by immigrants that had dispersed onto the study area (Table 1). Immigrants included juveniles that were banded as nestlings and had dispersed from their natal territories, as well as unbanded birds that had dispersed onto the study site after the previous breeding

Function of flocking

Speckled warblers lived in discrete, stable flocks throughout winter, the nucleus of which was the residents from one or more adjacent breeding territories. Residents were joined by immigrants that had dispersed after the previous breeding season. Boundaries of the flock territory encompassed the breeding territory boundaries of residents so flocks were largely non-overlapping. Exclusive ranges were maintained through avoidance, although when flocks occasionally met at boundaries all members of

Acknowledgements

I thank Rob Magrath, Penny Olsen, Todd Soderquist, Richard Major, Denis Saunders and two anonymous referees for comments on the draft manuscript, and Peter Marsack for assistance in the field. Rainfall data were provided by the Bureau of Meteorology. The study was supported by an ANU Graduate Scholarship, a Stuart Leslie Bird Research Award (Birds Australia) and a Cayley Memorial Scholarship (Gould League of New South Wales). Bird trapping and banding were carried out under license from the

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