In the northern Great Plains of the United States, conflicts between red-winged blackbirds (Agelaius phoeniceus) and sunflower (Helianthus annuus) growers have intensified since the late 1960s due to the expanded commercial production of sunflowers. We studied the potential population effects of the removal of up to 2 million red-winged blackbirds annually under a 5-year program of baiting during spring with DRC-1339 (3-choloro-4-methalalanine)-treated rice. We also examined whether lethal control, in combination with current levels of breeding-habitat management, would be cost-effective in decreasing depredation of sunflower crops during late summer. We evaluated the cost-benefit ratio for 4 culling scenarios involving (1) variable annual culls, not exceeding 2 million birds, with and without density compensation (i.e., a positive density-dependent response) on adult survival; and (2)culls of 2 millions birds annually with and without density compensation. We constructed a red-winged blackbird population model represented as an age-based matrix and calibrated to stable growth. We assumed a total population of 27 million birds on 1 April (week 1), representing the red-winged blackbird breeding population staging in eastern South Dakota and migrating into North Dakota. Under each culling scenario, we reduced the stable red-winged blackbird population (equally for females and males) and projected the population through week 23 of the annual cycle (2 Sep). We then evaluated the associated costs of the management relative to potential sunflower crop losses, assuming $0.07 in damage per bird and 4%loss to other factors. Variable annual culls, likely the more biologically realistic model scenarios, yielded mean annual removals of 1,240,560 (SE = 28,811) birds without density compensation, with cost-benefit ratios of 1:2.3 and 1:3.6 respectively. Annual intrinsic rates for the model population over the 5-year period ranged from-1.4 to -4.8%. Considering potential variability in the effectiveness of the cull and the combination of direct and indirect costs, we contend that eh realized benefits to sunflower growers by lethal control of red-winged blackbirds via spring baiting, in combination with current nonlethal management efforts, would likely be negligible.
Available at: http://works.bepress.com/george_m_linz/50/