. The Canadian field-naturalist. 2000 Wright, Bruner, Li, Jarvis, and Dowlan: Harlequin Ducks 193 Table 3. Proportions of counted Dicosmoecus gilvipes according to substrate type in Quartzville Creek, Oregon, in 1995, 1996, and 1997 (n = total number of observations made in each year). Year Large Small Bedrock Boulder Boulder Cobble Gravel Sand Silt 1995 (n = 2591) 0% 1996 (n = 372) 1997 (n = 272) 0% 0% Table 4. Average densities (#/m2) (±1SD) of Dicosmoecus gilvipes, total benthic macroinvertebrat
. The Canadian field-naturalist. 2000 Wright, Bruner, Li, Jarvis, and Dowlan: Harlequin Ducks 193 Table 3. Proportions of counted Dicosmoecus gilvipes according to substrate type in Quartzville Creek, Oregon, in 1995, 1996, and 1997 (n = total number of observations made in each year). Year Large Small Bedrock Boulder Boulder Cobble Gravel Sand Silt 1995 (n = 2591) 0% 1996 (n = 372) 1997 (n = 272) 0% 0% Table 4. Average densities (#/m2) (±1SD) of Dicosmoecus gilvipes, total benthic macroinvertebrates, and EFT (Ephemeroptera, Plecoptera, and Trichoptera) taxa from 1995-1997 in Quartzville Creek, Oregon. Year D. gilvipes Total Benthic Invertebrates EPT taxa 1995 1996 1997 ()* () () 482 (53) 308(96) 237 (52)=* 106(11) indicates a significant difference between years at the level. associated with bedrock may be unfavorable to small- er herbivores, reducing potential competition for large caddisflies (Lamberti et al. 1995; Li and Gregory 1989). The shallow habitat created by bedrock glides also may be unfavorable to fish large enough to eat D. gilvipes, but these appear to be optimal foraging sites for insectivorous birds. At the temporal scale, our results suggested that Harlequin Ducks switched preferred prey items in late June when: (1) harlequin broods were generally 10 £? 6 - < 2 I fii [ni 2nd Instar 3rd Instar 4th Instar Instar 5th Instar 1995 1996 mmm 1997 EiGURE 4. Average densities (#/m2) (with 1 SD) of the indi- vidual Dicosmoecus gilvipes life stages in 1995, 1996, and 1997 in Quartzville Creek, Oregon. first observed, and (2) D. gilvipes were developing from third to larger-bodied fourth instar. Other stud- ies have recognized that invertebrates are crucial in the diets of ducklings (, Baldassarre and Bolen 1994). In fact, hatching peaks for ducks often coin- cide with periods when invertebrates are especially abundant (Baldassa
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