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Agricultural waste grains are significant for providing nutrients for wintering waterfowl in California. Rice and corn comprise 56% of their nutrient needs in the Central Valley and changes to agricultural practices, such as postharvest treatments, could impact these food resources. Currently, limited data exist on how postharvest treatments in rice and corn fields affect the abundance of waste grain, yet these data are essential to determine the carrying capacity of agricultural lands for wintering waterfowl. To address this knowledge gap, we estimated the abundance of waste grain (kg/ha) using dry field transects, dry field soil cores, and flooded field (wet) soil cores. In 2016 and 2017, we sampled 84 rice fields and 47 corn fields. Our results indicate that the abundance of waste rice varied significantly among postharvest treatments. Fields that received no postharvest treatment (stubble left standing; no incorporation of straw) had the greatest amounts of waste rice, whereas fields that were disced, disced and rolled, or burned provided the least amount of waste rice. The average abundance of waste rice across all postharvest treatments was 320 kg/ha in dry fields (arithmetic mean; geometric mean = 228 kg/ha; soil core samples). Estimates of waste rice in flooded fields averaged only 169 kg/ha (geometric mean 98 kg/ha; soil core samples), significantly lower than in the same fields prior to flooding. Variation in the abundance of waste corn was greater than that of waste rice. Fields that did not receive any postharvest incorporation had the greatest abundance of waste corn, 233 kg/ha on average (arithmetic mean; geometric mean = 72 kg/ha), whereas fields that were incorporated (disced or disced and rolled) contained significantly lower abundance of waste corn, averaging 50–60 kg/ha (arithmetic mean; geometric mean = 5–10 kg/ha). The average, across all postharvest treatments, was 159 kg/ha of waste corn (geometric mean = 25–34 kg/ha). Our results suggest that postharvest practices affect the abundance of waste grain in rice and corn fields; changes in these practices could impact food availability for wintering waterfowl. Our results also indicate that the method of sampling waste grain can influence estimates of residual grain abundance.

Environmental contamination is widespread and can negatively impact wildlife health. Some contaminants, including heavy metals, have immunosuppressive effects, but prior studies have rarely measured contamination and disease simultaneously, which limits our understanding of how contaminants and pathogens interact to influence wildlife health. Here, we measured mercury concentrations, influenza infection, influenza antibodies and body condition in 749 individuals from 11 species of wild ducks overwintering in California. We found that the odds of prior influenza infection increased more than fivefold across the observed range of blood mercury concentrations, while accounting for species, age, sex and date. Influenza infection prevalence was also higher in species with higher average mercury concentrations. We detected no relationship between influenza infection and body fat content. This positive relationship between influenza prevalence and mercury concentrations in migratory waterfowl suggests that immunotoxic effects of mercury contamination could promote the spread of avian influenza along migratory flyways, especially if influenza has minimal effects on bird health and mobility. More generally, these results show that the effects of environmental contamination could extend beyond the geographical area of contamination itself by altering the prevalence of infectious diseases in highly mobile hosts.

Obligate brood parasites depend entirely on other species to raise their offspring. Most avian obligate brood parasites have altricial offspring that require enormous amounts of posthatching parental care, and the large fecundity boost that comes with complete emancipation from parental care likely played a role in the independent evolution of obligate parasitism in several altricial lineages. The evolution of obligate parasitism in the black-headed duck, however, is puzzling because its self-feeding precocial offspring should not constrain parental fecundity of a potential brood parasite in the way that altricial offspring do. We used an experimental nest predation study to test the idea that high nest predation rates played a role in the evolution of brood parasitism in this enigmatic duck. Experimental duck eggs in untended nests suffered massive rapid predation, while eggs in tended nests of the three main hosts, all aggressive nest defenders, had very high success, illustrating the benefits of parasitizing these 'bodyguard' hosts.