Evaluation of the impact of time of day, weather, vegetation density and bird movements on outcomes of area searches for birds in eucalypt forests of south-western Australia

We assessed the influence of sampling biases on estimates of absolute density. Using area searches for birds on 1-ha plots, we showed that time of day, weather and vegetation density (logged v. unlogged forest) had no significant impact on the detection rates of birds. As a consequence these environmental variables did not affect estimates of total bird density or species richness. Amount of cloud cover was positively correlated with abundance of three species (white-browed scrubwren, inland thornbill and white-naped honeyeater) but negatively correlated with counts of the western gerygone. Radio-tracking of western yellow robins showed that about half of all birds present on a plot were detected during area searches. However, robins moved over an area twice as large as the plot searched. These errors compensate to give an accurate estimate of density for this species. Collectively, these results indicate that area searches give a reliable estimate of density in both logged and unlogged eucalypt forest.