A comparison of four South Island robin habitats

Abstract

Aspects of the vegetation, invertebrates present, tier preferences and habitat use of the South Island robin (Petroica australis australis) are compared at Te Kakaho (Marlborough Sounds), Lake Daniells (Buller), Saltwater Forest (South Westland) and Flagstaff Forest (Otago).

Canopy cover, tree density and litter cover are compared and all were found to be significantly different (Kruskal Wallis one way test of variance, p<0.001, p<0.001 and p<0.0001 respectively) between these study sites. It is suggested that none of these variables alone could be used to delineate robin habitat. The relationship between robin density and canopy cover, tree density and litter cover is also examined. A moderate correlation was found between robin density and both canopy cover and tree density (spearman rank coefficient r=-0.5245 n=56 and r=-0.5449 n=20 respectively), but only a weak correlation was found between robin density and litter cover (r=0.0151, n=800). The relationship between robin density and both canopy cover and tree density is thought to be caused by altitude. Litter cover at these sites is not thought to be affecting robin density.

At each site invertebrates were collected over five nights using pitfall and pan traps in summer, autumn and winter. The availability of invertebrates is thought to affect the robins habitat use. Competition for invertebrate prey by other insectivorous birds and mammals with robins is also discussed. Their affect on the invertebrate prey taken by robins is thought to be negligible.

Tests for forest tier preferences were made. Robins at all sites were found to use the ground tier more frequently than expected by chance alone. Over 80% of all observations were observed in the vegetation below 2m. Robins at Saltwater Forest and Flagstaff Forest used the shrub tier more frequently than expected by chance.

The diurnal use of habitat by robins is also described. Robins were found to use the ground most frequently for foraging. The shrub tier was the next most important tier for foraging. Most maintenance and social behaviour was observed in the shrub tier. It is suggested that the ground is important for the prey it provides and the shrub tier for perches.

It is suggested that neither vegetation type nor vegetation structure alone exclude robins from using an area. It is thought that the most important determinant of where robins occur is the availability of invertebrates and suitable perches. A horizon percent organic C and N values (n=175) were highly correlated (R2=0.94). There were significant differences in mean C and N values between parent materials (P<0.001) but classification by parent material explained only 24% and 29% variance in C and N values, respectively. Digital terrain attributes explained only 6% and 5% of variance in C and N values, respectively. Lowest summer rainfall microclimate data explained only 4% and 5% of C and N variance, respectively. The poor correlation between %C or %N and terrain attributes precluded a SLM being developed on the basis of those attributes. A SLM based on parent material alone could have been produced but, generally, variances of %C and %N within parent materials were sufficiently high to make this of little value.