HOW WELL DO ARTIFICIAL NESTS ESTIMATE SUCCESS OF REAL NESTS

Artificial nests frequently are used to assess levels and patterns of nest predation, but how well these nests measure rates of predation or trends in predation rates at real nests is unclear. We compared preda tion rates between 58 active Wood Thrush (Hylocichla mustelina) nests paired with 58 artificial nests designed to resemble Wood Thrush nests . Paired nests were available to the same predator community both spat ially and temporally. Rates of nest predation were significantly lower for active Wood Thrush nests (33%) than for artificial nests (64%). R ates of parasitism by Brown-headed Cowbirds (Molothrus ater) also diff ered between the two groups. Twenty-six percent of active nests and no ne of the artificial nests were parasitized by cowbirds. During 1993 a nd 1994, we conducted three artificial nest trials on six study sites per year. Rates of predation were highest in small woodlots and declin ed with increasing forest patch size consistent with the trend reporte d for active nests on the same sites. Within sites, rates of predation varied among trials with the amount of variation highest on sites wit h high predation rates. Our results suggest that although artificial n ests should not be used to measure actual rates of nest predation or p arasitism, they may be valuable for detecting trends in rates of preda tion. However, because there are many potential biases associated with the use of artificial nests that may make interpretation of trend dat a difficult, we recommend using artificial nests primarily in pilot st udies or in conjunction with active nests.