Skylarks trade size and energy content in weed seeds to maximize total ingested lipid biomass

International audience

English. tThe trade-off between forage quality and quantity has been particularly studied in herbivore organisms,but much less for seed eating animals, in particular seed-eating birds which constitute the bulk of win-tering passerines in European farmlands. The skylark is one of the commonest farmland birds in winter,mainly feeding on seeds. We focus on weed seeds for conservation and management purposes. Weedseeds form the bulk of the diet of skylarks during winter period, and although this is still a matter fordiscussion, weed seed predation by granivorous has been suggested as an alternative to herbicides usedto regulate weed populations in arable crops. Our objectives were to identify whether weed seed traitsgovern foraging decisions of skylarks, and to characterize key seed traits with respect to size, which isrelated to searching and handling time, and lipid content, which is essential for migratory birds. Wecombined a single-offer experiment and a multiple-offer one to test for feeding preferences of the birdsby estimating seed intake on weed seed species differing in their seed size and seed lipid content. Ourresults showed (1) a selective preference for smaller seeds above a threshold of seed size or seed sizedifference in the pair and, (2) a significant effect of seed lipid biomass suggesting a trade-off betweenforaging for smaller seeds and selecting seeds rich in lipids. Skylarks foraging decision thus seems to bemainly based on seed size, that is presumably a ‘proxy’ for weed seed energy content. However, thereare clearly many possible combinations of morphological and physiological traits that must play crucialrole in the plant–bird interaction such as toxic compound or seed coat.