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1. Due to reductions in winter food resources, newly sown cereal seeds have become a key component of many bird species' diets, but these seeds are often treated with pesticides that may cause toxic effects. To complete an appropriate risk assessment, data on treated seed toxicity need to be combined with information about the risk of exposure of birds in the field and the factors that modulate such exposure. 2. We studied the abundance of pesticide-treated seeds available for birds in the field, the pesticides and their concentrations in treated seeds, and the bird species observed in the field that were feeding on these pesticide-treated seeds. The exposure of red-legged partridge to treated winter cereal seeds was characterized through the analysis of crop and gizzard contents of hunted individuals (n = 189). Moreover, we measured the contribution of cereal seeds in the autumn-winter diet of partridges in order to assess the potential risk of exposure to pesticide-treated seeds. 3. Density of treated seeds on the soil surface after sowing (11Á3 AE 1Á2 seeds m À2 in the centre of field and 43Á4 AE 5Á5 seeds m À2 in the headlands) was enough to provide, in an area between 6 and 50 m 2 , doses of six active ingredients above those indicating acute (i.e. a dose capable of killing 50% of individuals of a sensitive species) and / or chronic (no observed effect level) toxicity. 4. Up to 30 bird species were observed consuming treated cereal seeds in recently sown fields. Corn bunting was identified as an appropriate focal passerine species for the risk assessment of pesticide-treated seeds. 5. We found that treated seeds were an important route of pesticide ingestion for red-legged partridge; pesticide residues (six fungicides and two insecticides) were found in 32Á3% of crops and gizzards. Cereal seeds represented more than half (53Á4 AE 4Á3%) of total biomass consumed by partridges from October to February. 6. Synthesis and applications. The field exposure data combined with previous studies about the toxicity to partridges of using pesticide-treated seeds point to an unacceptable risk of this practice to farmland birds. Our results suggest that the prophylactic use of pesticide-coated seeds should be avoided, with the approval of this treatment considered on a case-by-case basis and accompanied with specific measures to minimize risks of adverse effects on avian communities.

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The mechanisms involved in the production of red carotenoid-based ornaments of vertebrates are still poorly understood. These colorations often depend on enzymatic transformations (ketolation) of dietary yellow carotenoids, which could occur in the inner mitochondrial membrane (IMM). Thus, carotenoid ketolation and cell respiration could share biochemical pathways, favoring the evolution of ketocarotenoid-based ornaments as reliable indices of individual quality under sexual selection. Captive male red crossbills (Loxia curvirostra Linnaeus) were exposed to redox-active compounds designed to penetrate and act in the IMM: an ubiquinone (mitoQ) or a superoxide dismutase mimetic (mitoTEMPO). MitoQ can act as an antioxidant but also distort the IMM structure, increasing mitochondrial free radical production. MitoQ decreased yellow carotenoids and tocopherol levels in blood, perhaps by being consumed as antioxidants. Contrarily, mitoTEMPO-treated birds rose circulating levels of the second most abundant ketocarotenoid in crossbills (i.e., canthaxanthin). It also increased feather total red ketocarotenoid concentration and redness, but only among those birds exhibiting a redder plumage at the start of the study, that is, supposedly high-quality individuals. The fact that mitoTEMPO effects depended on original plumage color suggests that the red-ketocarotenoid-based ornaments indicate individual quality as mitochondrial function efficiency. The findings would thus support the shared pathway hypothesis.

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