Testing competing hypotheses for the seasonal variation in nesting success of a late-nesting waterfowl
Pokley, Kalen John
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Populations of lesser scaup and greater scaup have been below the North American Waterfowl Management Plan goal of 6.3 million since 1984. As of 2013, scaup populations are down 12% from their long term average (1955-2013) of 4.7 million birds (Walker 2005). Nesting success has shown to be an important factor in determining population growth. If the mechanisms of nesting success can be identified wildlife managers can make the necessary changes to increase scaup nesting success and thus increase the population. Nests were located during nest searches conducted from May through July and monitored until fate was determined. Nest age, nest location, vegetation height, distance to water and depth of water were recorded at each nest. Program MARK was used to determine known fate models. The model that incorporated both nest age and the effect of nest date showed the greatest support. I found a greater influence of date on nesting daily survival rate than age during this study, although both positively influenced scaup nest daily survival rate. The positive relationship between scaup nest daily survival rate and date, provided support for the nest concealment hypothesis. This posits that increasing vegetation height and density throughout the nesting season decreases predation. Sugden and Beyersbergen (1987) found similar results that artificial nests in tall, dense nesting cover escaped predation from crows for longer than those in sparse cover. The positive effect of nest age on nest survival supports the nest heterogeneity hypothesis, i.e., that low quality nests are depredated at a higher rate than nests of higher quality. Although these results have been supported by others (Klett and Johnson 1982) it has not received unanimous support. It is possible that these results might be influenced by the lack of heterogeneity in vegetation and differences in predator community at Red Rock Lakes National Wildlife Refuge compared to other sites.