Brood-parasitic cowbirds are hypothesized to search for and locate host nests within a relatively constant area, as this is presumed to facilitate the monitoring of nests over time and the synchronization of parasitism with host laying. We tested this hypothesis in Shiny Cowbirds (Molothrus bonariensis) and Screaming Cowbirds (M. rufoaxillaris), two cowbird species that differ in host specificity and, apparently, in social mating system, by radio-tracking females and males for 3-6 consecutive days and determining individual daily morning ranges and cumulative morning ranges. In Shiny Cowbirds, the mean size of morning daily ranges and cumulative morning ranges was larger for males than for females, but we did not find a difference between the sexes in range size for Screaming Cowbirds. In both species, there was extensive overlap in the morning ranges of individual females between consecutive days, and the addition of new area to their ranges decreased over time. For both Shiny and Screaming cowbirds, morning ranges of conspecific females radio-tracked the same day overlapped, indicating lack of territoriality. Male and female Screaming Cowbirds that were trapped together were also spatially associated during radio-tracking, indicating social monogamy. Most radio-tracked Shiny and Screaming cowbirds used mainly one roost, relatively close to their morning ranges, which was maintained throughout the breeding season. Our results show that Shiny and Screaming cowbird females use relatively constant areas for nest searching and that Screaming Cowbirds are socially monogamous.Areas de acción de hembras y machos de Molothrus bonariensis y M. rufoaxillaris durante la búsqueda de nidos de hospedadores RESUMEN Pusimos a prueba la hipótesis que los tordos parásitos de cría buscan y localizan nidos de hospedadores dentro de un área relativamente constante, ya que esto facilitaría el monitoreo de nidos a través del tiempo y la sincronización del parasitismo con la puesta del hospedador. Testeamos esta hipótesis en Molothrus bonariensis y M. rufoaxillaris, dos especies que difieren en su especificidad de uso de hospedadores y, aparentemente, en su sistema de apareamiento social, mediante radio telemetría de hembras y machos durante 3-6 días consecutivos, determinando las áreas de acción diarias y las áreas de acción acumuladas. En M. bonariensis, el tamaño medio de las áreas de acción diarias y de las áreas de acción acumuladas fue mayor para los machos que para las hembras, pero no encontramos diferencias entre sexos en M. rufoaxillaris. En ambas especies, hubo un extenso solapamiento de las áreas de acción de cada hembra en días consecutivos y la incorporación de área nueva disminuyó a través del tiempo. Tanto para M. bonariensis como para M. rufoaxillaris, las áreas de acción de hembras conespecíficas monitoreadas en un mismo día se solaparon, indicando ausencia de territorialidad. Los machos y hembras de M. rufoaxillaris que fueron capturados juntos se mantuvieron asociados espacialmente durante el monitoreo, indicando mo...

We studied visits to potential host nests by two avian brood parasites, the host generalist shiny cowbird, Molothrus bonariensis, and the host specialist screaming cowbird, M. rufoaxillaris, in the periods preceding and overlapping the laying period of their hosts. Our goal was to examine the hypothesis that during pre-laying visits cowbird females form a dynamic memory library of laying opportunities, which they deploy to target suitable nests at a later pre-dawn period. We recorded presence of radio tagged females within a fixed area around nests of chalked-browed mockingbirds, Mimus saturninus (a common host of shiny cowbirds) and baywings, Agelaioides badius (the main host of screaming cowbirds), using proximity data-loggers placed at nests during prelaying, laying and early incubation. Our data confirmed that females of both species visit potential host nests prior to laying and that parasitic events occurred before dawn, earlier in screaming than shiny cowbirds but with little chance of host nests having been discovered on the laying day. There were interesting species differences: visits were less frequent in shiny than screaming cowbirds and the former rarely returned after laying, while screaming cowbirds visited nests repeatedly after laying and occasionally showed repeat parasitism. The higher frequency of re-visiting by screaming cowbirds to baywing nests is consistent with the uncommonly long and variable baywing prelaying period, and the incidence of repeat parasitism may reflect low availability of baywing nests and greater flexibility of its parental care potential thanks to it being a social breeder. Significance StatementAvian brood parasites synchronize their laying with that of their hosts, as this reduces egg rejection and optimizes hatching time. They also avoid parasitizing nests repeatedly, thus preventing harm to their own previously laid eggs and competition among their offspring.3 Further, they lay at dawn, so that location of target nests must be known from previous days' exploration. It has been argued that these adaptations must depend on memory for the location and status of host nests within their home range, a memory feat known as "bookkeeping". We study nest prospecting in a host specialist and a host generalist parasitic cowbird, using a combination of proximity radio tracking and video recordings. Our results confirm the prospecting hypothesis, report previously unknown interspecies differences and illustrate how cognitive adaptations can be studied in the context of field behavioural ecology.

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