98 98 CHAPTER 8 CONSERVATION SOLUTIONS FOR THREATENED AND ENDANGERED COWBIRD (MOLOTHRUS SPP.) HOSTS: SEPARATING FACT FROM FICTION S I. R 1,3  B D. P 1,2,4 1 Department of Ecology, Evolution and Marine Biology, University of California, Santa Barbara, California 93106, USA; and 2 Molecular Genetics Laboratory, National Museum of Natural History, Smithsonian Institution, 3001 Connecticut Avenue, Washington D.C. 20008, USA A.—Brood parasitism by Brown-headed Cowbirds (Molothrus ater; hereaer “cow- birds”) reduces the reproductive output of individual hosts and has been the subject of con- siderable research and management activity. Many current management practices aimed at controlling parasitism are based on outdated perceptions. Among these are that cowbirds are increasing in range and abundance; cowbirds are relatively new to North America; cowbird parasitism reduces or limits population sizes of host species; new host populations are defense- less against parasitism; and cowbird control increases reproductive output and populations of hosts. Instead, cowbird numbers have declined significantly in recent decades. Cowbirds have been in North America for at least a million years, and their numbers likely peaked when mam- malian megafauna were present during the Pleistocene. Thus, effects on hosts have occurred over a long period, and any recent extinction threats posed by cowbirds are likely aributable to anthropogenic changes. Because of this long history of contact with cowbirds, many hosts that nest in nonforested habitat have had long periods to evolve adaptations against parasit- ism, and evidence clearly demonstrates that they retain those defenses from past bouts of para- sitism and, hence, are oen well defended when parasitism is renewed. Cowbird parasitism is only one of many factors that can limit avian populations, and despite such factors, avian populations can still produce enough young to remain stable. There is no evidence that cow- birds are a serious threat to the survival of any passerines other than several taxa that are listed as federally endangered species. Cowbird control does not always translate into increased population sizes for those endangered species, and habitat loss is the most critical factor limit- ing their populations. Current management programs based on these outdated perceptions have shied the focus away from the most critical issue threatening avian populations, namely anthropogenic habitat disturbance. A serious shortcoming of cowbird control programs is that they are open-ended; this is because local cowbird numbers in control areas stay roughly con- stant year aer year as a result of extensive dispersal of new cowbirds into those areas. In some cases, control programs have become excessive and have been co-opted by special interests, particularly in Texas, where a broad statewide program to control cowbirds has been used to legitimize environmentally harmful actions that can hinder recovery of endangered species and affect avian diversity in general. We provide a series of questions that should be addressed before the initiation of cowbird control programs. Most importantly, we encourage managers to determine whether cowbirds are an important pressure on the host populations they over- see, whether resources used for control programs could have greater management benefits if used differently, and whether it is possible to define conditions that will indicate that annual cowbird control is no longer needed. R.—El parasitismo de cría por Molothrus ater reduce el éxito reproductivo de los individuos hospederos, por lo que ha sido el sujeto de considerable número de actividades de 3 E-mail: rothstei@lifesci.ucsb.edu 4 Present address: Department of Biology, Simpson College, 701 North C Street, Indianola, Iowa 50125, USA. Ornithological Monographs Volume (2005), No. 57, 98–114 © The American Ornithologists’ Union, 2005. Printed in USA.