Survey Timing and Spring Phenology Influence Scaup and Mallard Population Estimates in the Canadian Western Boreal Forest

Multi-species surveys improve logistical efficiency but may introduce bias when survey timing does not align with species-specific phenology. The Waterfowl Breeding Population and Habitat Survey (WBPHS) is timed primarily for early-breeding species, particularly mallard (Anas platyrhynchos, Linnaeus 1758), raising concerns regarding its suitability for late-breeding lesser and greater scaup (Aythya affinis, Eyton 1838 and Aytha marila, Linnaeus 1761). I hypothesized that survey timing and spring phenology would differentially influence counts, with later-breeding scaup more sensitive to timing than earlier-breeding mallard due to migration-survey mismatch. I used count data from 1982-2019 in the Canadian western boreal forest, to evaluate these relationships. Survey dates increased with latitude (53.4–69.3°N), and northern transects accounted for most scaup. At high latitudes, scaup showed limited sensitivity to timing or phenology, suggesting surveys generally occurred after arrival. In contrast, counts at low to mid latitudes were elevated during late-phenology years when surveys occurred early, consistent with inclusion of migrants. Mallard counts declined with later survey dates across latitudes and were sensitive to spring phenology. Model-based reconstructions indicated mean annual observation error of approximately 8–10% for both species beyond that currently reported, without evidence of long-term directional bias. Overall, WBPHS estimates appear to provide useful broad-scale indices of abundance in the CWBF, particularly for scaup at high latitudes, assuming other unmeasured factors do not introduce systematic bias.