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When interpreting counts of forest birds it is seldom possible to distinguish the effects of changing density from those of changing conspicuousness; these often arise from the birds’ singing and calling. To investigate this, birds first seen were recorded separately from those first heard when counting birds in forest of the Orongorongo Valley, Wellington, New Zealand. Apparent changes in the frequency of Paradise Duck (Tadorna variegata) in the river valley, and of the Song Thrush (Turdus philomelos), Whitehead (Mohoua albicilla), Grey Warbler (Gerygone igata) and Chaffinch (Fringilla coelebs) in the forest coincided with changes in the frequency of their singing or calling; they were considered suspect. Changes in the frequency of the Black-backed Gull (Larus dominicanus) on the riverbed, and of N.Z. Pigeon (Hemiphaga novaeseelandiae), Blackbird (Turdus merula), Fantail (Rhipidura fuliginosa), Silvereye (Zosterops lateralis), Bellbird (Anthornis melanura), Tui (Prosthemadera novaeseelandiae), and Australian Magpie (Gymnorhina tibicen) did not show a similar correlation. Special study of the ways in which birds either advertise or conceal themselves is needed. Correction factors may never compensate completely for the effects of the birds’ varying conspicuousness.