Todd J. Underwood

Grasp-ejection in two small ejecters of cowbird eggs: a test of bill-size constraints and the evolutionary equilibrium hypothesis

Many host species accept the costs of avian brood parasitism even though rejection appears to be the adaptive response. Acceptance is often attributed to an evolutionary lag in the appearance of rejection behaviour. Alternatively, the evolutionary equilibrium hypothesis explains acceptance as adaptive because the costs of rejection outweigh those of acceptance. Equilibrium assumes that bill size limits small hosts to more costly rejection methods, such as puncture-ejecting parasitic eggs. We tested whether warbling vireos, Vireo gilvus, and Baltimore orioles, Icterus galbula, the smallest puncture-ejecters of brown-headed cowbird, Molothrus ater, eggs, are capable of grasp-ejection by using solid, plaster cowbird eggs and video analysis. We also tested whether the depth of oriole nests prevents grasp-ejection. Vireos ejected significantly more plaster eggs than did orioles. Most (91%) plaster eggs were ejected by vireos with no cost and grasp-ejection was confirmed, indicating that bill size does not constrain grasp-ejection. The grasp index of warbling vireos was at least 33% smaller than that of previously known grasp-ejecters. Thus, many species should be capable of grasp-ejection, which indirectly supports evolutionary lag. Orioles ejected an intermediate proportion (45%) of plaster eggs and ejection method was not confirmed. Nest depth was not supported as an explanation for acceptance by orioles. Because a few large and small ejecters of cowbird eggs appear to use both grasp- and puncture-ejection, we suggest that puncture-ejection may have evolved first, representing an initial selection pressure for cowbird eggshell strength.

DEMOGRAPHIC VARIABLES ARE POOR INDICATORS OF WOOD THRUSH PRODUCTIVITY

Abstract We tested the ability of 29 indices of productivity to predict and track actual productivity of a Wood Thrush (Hylocichla mustelina) population for 21 years and to detect year-to-year changes in that productivity. Of 29 indices tested, only the productivity of nests initiated in May showed promise. This index reasonably predicted productivity, correctly tracked two out of three temporal trends in productivity, and detected a significant proportion of year-to-year changes in actual productivity. Although they were not useful for predicting annual productivity, other indices based on nest data tracked trends in productivity moderately well, and all, except percent nest success, detected a significant proportion of year-to-year changes in productivity. Productivity indices based on non-reproductive variables, such as abundance of males or females, return rates, and veteran:novice ratios, did not reflect productivity trends or changes. From these results, we recommend that only an intensive measure of actual productivity or a partial measure of it, such as May productivity, be used to make inferences about annual productivity of a population. Indices based on other nest data should be used only for tracking productivity trends. Non-reproductive indices are unreliable for making any inferences about productivity. Las Variables Demográficas son Malas Indicadoras de la Productividad de Hylocichla mustelina Resumen. Pusimos a prueba la capacidad de 29 índices de productividad para predecir y seguir las variaciones anuales en la productividad real de una población de Hylocichla mustelina por un período de 21 años. De los 29 índices probados, sólo el de la productividad de los nidos iniciados en mayo resultó promisorio. Este índice predijo la productividad razonablemente bien y detectó correctamente dos de tres tendencias temporales en la productividad y una proporción significativa de los cambios anuales en la productividad real. Aunque no fueron útiles para predecir la productividad anual, otros índices basados en datos de nidos siguieron las tendencias en la productividad aceptablemente bien. Con excepción del porcentaje de nidos exitosos, todos estos índices detectaron una proporción significativa de las variaciones anuales de la productividad. Por su parte, los índices de productividad basados en variables no reproductivas como la abundancia de machos y hembras, las tasas de regreso y el cociente de aves veteranas: novatas no reflejaron las tendencias o cambios en la productividad real. Con base en estos resultados recomendamos que para hacer inferencias sobre la productividad anual de una población sólo se utilice una medida intensiva de la productividad, o una medida parcial de ésta (i.e., la productividad en mayo). Los índices basados en otros datos de nidos deben ser utilizados únicamente para monitorear las tendencias en la productividad. Los índices no reproductivos son poco confiables para hacer inferencias sobre la productividad.

Video Recording Reveals the Method of Ejection of Brown-Headed Cowbird Eggs and No Cost in American Robins and Gray Catbirds

Abstract. Despite the importance of knowing the method and cost of ejection in understanding the persistence of brood parasitism, anecdotal records of witnessed ejections of real Brownheaded Cowbird (Molothrus ater) eggs exist for only eight of ∼30 ejecter species. The probability of a host damaging its own egg while ejecting a parasites egg is thought to be lower for hosts that grasp-eject, but grasp-ejection is an option only for hosts with appropriate bills. For hosts incapable of grasp-ejection, the cost of puncture-ejection may render acceptance adaptive. We video-recorded 12 ejections of real cowbird eggs by American Robins (Turdus migratorius) and 17 by Gray Catbirds (Dumetella carolinensis). With no damage to their own eggs, robins graspejected all cowbird eggs, whereas catbirds grasp-ejected 14 eggs and puncture-ejected three eggs. Our study revealed that a few species use a mixture of ejection methods and even large species may puncture-eject with little cost.

Functional morphology as a barrier to the evolution of grasp-ejection in hosts of the Brown- headed Cowbird (Molothrus ater)

Acceptance of eggs of the Brown-headed Cowbird (Molothrus ater (Boddaert, 1783)) by hosts is enigmatic because hosts usually raise fewer of their own young when parasitized. Ejection may not be adaptive for small hosts because they cannot eject cowbird eggs efficiently. Grasp-ejection apparently has a negligible cost but requires a bill of a minimum length that is not known. In this study, we examined the limits of grasp-ejection of the American Robin (Turdus migratorius L., 1766) and the Gray Catbird (Dumetella carolinensis (L., 1766)). We determined the largest object width that each species is capable of grasping (limit width) by observing individuals grasping models larger than cowbird eggs in 104 video-recorded ejections and one direct observation. We standardized the limit width to the tomial length of each species (limit ratio) and extrapolated to the width of a cowbird egg to predict the minimum tomial length required for grasp-ejection. Our results suggest that the minimum tomial length required ...

Eastern Wood-Pewees as Brown-headed Cowbird hosts: accepters but infrequently parasitized

Abstract Brown-headed Cowbird (Molothrus ater) parasitism on pewees (Contopus spp.) is uncommon or rare in North America, suggesting that pewees sympatric with cowbirds either reject cowbird eggs or are not selected by cowbirds for parasitism, or that cowbirds are unable to parasitize pewee nests. At Delta Marsh, Manitoba, we inspected Eastern Wood-Pewee (C. virens) nests to determine parasitism frequency and experimentally parasitized pewee nests with real and artificial Brown-headed Cowbird eggs to assess their response to parasitism. No parasitism was recorded at 20 unmanipulated nests or at 12 experimentally parasitized nests, whereas all real and artificial cowbird eggs were accepted. The lack of observed parasitism, therefore, was not an artifact of cowbird egg rejection. We suggest the lack of parasitism on Eastern Wood-Pewees at Delta Marsh reflects a partial asynchrony between the parasites and hosts laying seasons and possibly the high placement of pewee nests. However, these factors may not explain the lack of parasitism at all other sites in North America or on other Contopus species.

Behavior of Warbling Vireos Ejecting Real and Artificial Cowbird Eggs

Abstract We videotaped nine ejections of real (n  =  5) and artificial (n  =  4) Brown-headed Cowbird (Molothrus ater) eggs by Warbling Vireos (Vireo gilvus). All eggs were ejected within 6 min. There were no significant differences in time used for any ejection behavior by egg type, although artificial eggs were probed longer before ejection. Eight vireos ejected the cowbird egg using visual cues only because none sat on its nest before ejection. One male ejected the cowbird egg after sitting on the nest for a few minutes; consequently, both visual and tactile cues were available for its decision to eject the cowbird egg. Most vireos identified the cowbird egg by sight and, in most cases, rapid ejection of the cowbird egg precluded the possibility of using tactile cues. Grasp-ejection was the only ejection method confirmed for real and artificial eggs. Two male vireos ejected cowbird eggs at two nests, which is the first documentation of successful ejection by male Warbling Vireos, and the third cowbird host for which males are known to eject cowbird eggs. The ability of males to eject cowbird eggs has important implications for the evolution of ejection behavior.

Conflicting Temporal Changes in the Frequency and Intensity of Cowbird Parasitism on Four Common Hosts in Ontario over 130 Years

Abstract We tested the hypothesis proposed by Friedmann (1963) that multiple parasitism (nests with more than one parasitic egg) by Brown-headed Cowbirds (Molothrus ater) is a recent phenomenon associated with increased densities of cowbirds. We used Ontario Nest Records Scheme data to quantify frequencies of parasitism and multiple parasitism on four common host species over the last 130 years. Frequency of multiple parasitism significantly increased over all decades only in Chipping Sparrows (Spizella passerina). We also analyzed data separately before and after 1970 because at about this time cowbird abundance began to stabilize and then decline. Pre-1970, multiple parasitism significantly increased on Chipping Sparrows and Song Sparrows (Melospiza melodia), but neither had significant trends post-1970. There were no significant trends in multiple parasitism on Red-eyed Vireos (Vireo olivaceus) or Northern Cardinals (Cardinalis cardinalis). For all time periods analyzed, there were no significant trends in overall frequency of parasitism on any species. Our results provide limited support for the hypothesis that temporal variation in parasitism patterns has followed changes in cowbird density over the past century.

The effectiveness of bird feeder cleaning methods with and without debris

ABSTRACT Although feeders provide supplementary food to wild birds, they can be a site of disease transmission. Periodic cleaning of bird feeders is recommended to prevent disease transmission, but little is known about which cleaning methods are most effective. We determined the effectiveness of 3 cleaning methods (scrubbing with soap and water, bleach soak, and scrubbing with soap and water followed by a bleach soak) in removing Salmonella from feeders with debris from normal field use and without debris. Feeders were inoculated with Salmonella enterica in the lab and then swabbed before and after cleaning to determine the percent reduction of Salmonella colony forming units (CFU/mL). All cleaning methods effectively reduced levels of Salmonella on feeders without debris, but the presence of debris significantly lowered the percent log reduction of Salmonella CFU/mL on feeders. The bleach soak and the scrubbing with soap and water plus bleach soak methods had a significantly higher percent reduction in Salmonella CFU/mL than the scrubbing with soap and water method overall. A significant interaction between debris and cleaning method was noted, however, indicating that the presence of debris greatly lowered the percent reduction of Salmonella CFU/mL on feeders cleaned with the scrubbing with soap and water method compared to other methods. Overall, we recommend either scrubbing with soap and water or a bleach soak to clean feeders with minimal debris, but suggest a combination of these 2 cleaning methods if feeders have heavy debris or if diseased birds are known to be in the area. Received 20 September 2016. Accepted 17 January 2017.

Tufted Titmouse Entangled in the Burrs of Beggar's Lice (Hackelia virginiana)

Abstract We discovered a live Baeolophus bicolor (Tufted Titmouse) entangled in the burrs of Hackelia virginiana (Beggars Lice) on 28 September 2012 in Kutztown, PA. We extracted and released the titmouse because it appeared to be unable to free itself from the plant. To our knowledge, this is the second published report of a bird becoming entangled in Beggars Lice and the first documentation of a Tufted Titmouse becoming entangled in any plant. Entanglement in Beggars Lice is unusual because of the small size (3–4 mm) of its burrs.