Margaret A. Rubega

NEST SITE SELECTION AND NESTING SUCCESS IN SALTMARSH BREEDING SPARROWS: THE IMPORTANCE OF NEST HABITAT, TIMING, AND STUDY SITE DIFFERENCES

Abstract We examined nest-site selection and nesting success in Saltmarsh Sharp-tailed (Ammodramus caudacutus) and Seaside Sparrows (A. maritimus), at seven sites in Connecticut. We found 160 Saltmarsh Sharp-tailed Sparrow nests and 23 Seaside Sparrow nests, and compared characteristics of their locations to each other and to random locations. We tracked success of all nests, quantified nest productivity and causes of nest losses, and tested for habitat differences between successful and unsuccessful nests. Saltmarsh Sharp-tailed Sparrows nested in higher than average locations, where the vegetation was taller and more dense than at random locations, where there was a deep layer of thatch, and where saltmeadow cordgrass (Spartina patens) dominated the vegetation. There was little evidence that habitat characteristics influenced the success of nesting birds, but the timing of nest initiation relative to spring tides was important. Seaside Sparrow nests occurred in even taller vegetation, that was more sparse than average and dominated by the tall form of smooth cordgrass (S. alterniflora). Habitat influenced the success of Seaside Sparrow nests, but timing did not; on average, successful nests occurred in taller vegetation. Model comparisons suggest that vegetation structure influences site selection more than species composition or inherent differences among marshes. Overall, our results indicate that nest flooding is a major threat to successful reproduction in both species, but they have different strategies to avoid flooding. Saltmarsh Sharp-tailed Sparrows time their reproduction to avoid especially high tides, while Seaside Sparrows avoid flooding spatially by nesting in tall vegetation.

INVASIVE FRUITS, NOVEL FOODS, AND CHOICE: AN INVESTIGATION OF EUROPEAN STARLING AND AMERICAN ROBIN FRUGIVORY

Abstract We compared the feeding choices of an invasive frugivore, the European Starling (Sturnus vulgaris), with those of a native, the American Robin (Turdus migratorius). Using captive birds, we tested whether these species differ in their preferences when offered a choice between a native and an invasive fruit, and between a novel and a familiar food. We examined willingness to eat fruits of selected invasive plants and to select a novel food by measuring the time elapsed before feeding began. Both species demonstrated significant preferences for invasive fruits over similar native fruits in two of three choice tests. Both starlings and robins ate autumn olive (Elaeagnus umbellata) fruits significantly more willingly than Asiatic bittersweet (Celastrus orbiculatus). Starlings, but not robins when choosing between a novel and a familiar food, strongly preferred the familiar food. We found no differences in willingness of birds to eat a novel food when it was the only food available. These results suggest that some fleshy-fruited invasive plants may receive more dispersal services than native plants with similar fruits, and that different frugivores may be seed dispersers for different invasive plants.

The hummingbird tongue is a fluid trap, not a capillary tube

Hummingbird tongues pick up a liquid, calorie-dense food that cannot be grasped, a physical challenge that has long inspired the study of nectar-transport mechanics. Existing biophysical models predict optimal hummingbird foraging on the basis of equations that assume that fluid rises through the tongue in the same way as through capillary tubes. We demonstrate that the hummingbird tongue does not function like a pair of tiny, static tubes drawing up floral nectar via capillary action. Instead, we show that the tongue tip is a dynamic liquid-trapping device that changes configuration and shape dramatically as it moves in and out of fluids. We also show that the tongue–fluid interactions are identical in both living and dead birds, demonstrating that this mechanism is a function of the tongue structure itself, and therefore highly efficient because no energy expenditure by the bird is required to drive the opening and closing of the trap. Our results rule out previous conclusions from capillarity-based models of nectar feeding and highlight the necessity of developing a new biophysical model for nectar intake in hummingbirds. Our findings have ramifications for the study of feeding mechanics in other nectarivorous birds, and for the understanding of the evolution of nectarivory in general. We propose a conceptual mechanical explanation for this unique fluid-trapping capacity, with far-reaching practical applications (e.g., biomimetics).

Developing Dynamic Mechanistic Species Distribution Models: Predicting Bird-Mediated Spread of Invasive Plants across Northeastern North America

Species distribution models are a fundamental tool in ecology, conservation biology, and biogeography and typically identify potential species distributions using static phenomenological models. We demonstrate the importance of complementing these popular models with spatially explicit, dynamic mechanistic models that link potential and realized distributions. We develop general grid-based, pattern-oriented spread models incorporating three mechanisms—plant population growth, local dispersal, and long-distance dispersal—to predict broadscale spread patterns in heterogeneous landscapes. We use the model to examine the spread of the invasive Celastrus orbiculatus (Oriental bittersweet) by Sturnus vulgaris (European starling) across northeastern North America. We find excellent quantitative agreement with historical spread records over the last century that are critically linked to the geometry of heterogeneous landscapes and each of the explanatory mechanisms considered. Spread of bittersweet before 1960 was primarily driven by high growth rates in developed and agricultural landscapes, while subsequent spread was mediated by expansion into deciduous and coniferous forests. Large, continuous patches of coniferous forests may substantially impede invasion. The success of C. orbiculatus and its potential mutualism with S. vulgaris suggest troubling predictions for the spread of other invasive, fleshy-fruited plant species across northeastern North America.

Prey switching in red-necked phalaropes Phalaropus lobatus: Feeding limitations, the functional response and water management at Mono Lake, California, USA

The ability of red-necked phalaropes Phalaropus lobatus to switch prey under conditions of changing prey abundance at Mono Lake, California, was tested in order to predict the potential effects of continued water diversions there on migratory waterbird populations. Red-necked phaloropes cannot switch prey because they are incapable of surviving on a diet of just the hardier of two invertebrates threatened by salinity increases at Mono Lake. Individuals experimentally limited to brine shrimp Artemia monica as a primary prey source lost mass rapidly until death ensued, or until they were offered other prey. These data show that a marked preference for one prey may indicate important physiological limitations in a predator. Such limitations may partly explain Murdochs empirically supported prediction (Ecol. Mongr. (196), 39, 335-54) that prey switching will not occur where strong prey preferences exist. These results also indicate that strong prey preferences of migratory birds can, and probably should, help determine water management policy at wetlands of importance to shorebirds, especially in cases where initial prey diversity is low.

Egg and Chick Fates During Tidal Flooding of Saltmarsh Sharp-Tailed Sparrow Nests

Abstract We determined nest attendance patterns of Saltmarsh Sharp-tailed Sparrows (Ammodramus caudacutus) in Connecticut using temperature data-loggers. In this species, only females incubate and feed the young. Females maintained a stable thermal environment in their nests during incubation by modifying their attendance behavior in relation to ambient temperature; during cooler conditions, females made shorter, but more frequent, trips away from their nests to feed. Once eggs hatched, average nest temperature increased significantly. The data-loggers also recorded information on rare and unpredictable events, such as nest flooding, depredation, and fledging. Eggs and nestlings apparently tolerated nest inundation for periods averaging more than 90 min. Nestlings fledged from their nests soon after sunset on the flooding tide. Detailed insight into nest attendance behavior and the circumstances surrounding rare events such as flooding are especially important for this species of high conservation concern in which tidal inundation is the major cause of breeding failure.

How Well can We Model Numbers and Productivity of Saltmarsh Sharp-Tailed Sparrows (Ammodramus Caudacutus) Using Habitat Features

Abstract Habitat models are often used to describe species distributions, but they need to be tested to evaluate their predictive performance. We investigated the importance of model evaluation in a study of habitat selection by Saltmarsh Sharp-tailed Sparrows (Ammodramus caudacutus), a species of conservation concern in eastern North America. We estimated the number of birds, nests, and fledglings produced in thirty 1-ha study plots spread across multiple marshes and used an information-theoretic approach to select among explanatory habitat models. Model performance was evaluated using both the original data set and data from another 30 plots from the same set of marshes. Our models indicate that both female and male Saltmarsh Sharp-tailed Sparrows increase in abundance as one moves away from the marshs upland edge and in areas where there is deep thatch and uniform vegetation height. In contrast to the results for adult birds, number of nests and production of fledglings were positively associated with Saltmeadow Rush (Juncus gerardii). These models suggest that where adults spend the most time is influenced by the vertical structure of the vegetation but that nesting activity is determined more by plant compositions that indicate subtle variations in marsh elevation. Despite the fact that we found good internal consistency for our models during model evaluation, model performance worsened considerably when used to make predictions about new sites. Thus, although our study identifies several new factors influencing habitat selection in Saltmarsh Sharp-tailed Sparrows, it also highlights the need to be cautious when making predictions from habitat models.

Feeding mechanisms: Hummingbird jaw bends to aid insect capture

The upper jaws of birds, unlike those in many tetrapods, move relative to the skull and are often flexible along their length, whereas the lower jaw (mandible) is usually a rigid structure formed by the fusion of several bones, flexing only where it meets the skull. Here we describe a previously unnoticed mandibular bending movement in hummingbirds, in which the distal half of the mandible is actively flexed downwards and the gape widens to catch flying insects. The hummingbird is thought to have developed a long narrow bill as it specialized in feeding on floral nectar, but the birds need to supplement its diet with insects must have contributed to the surprising flexibility of its jaw.

Does frugivory by European starlings (Sturnus vulgaris) facilitate germination in invasive plants?1

Abstract Fleshy-fruited plants are an important component of the invasive flora of the northeastern United States, but few studies have examined how avian frugivory may benefit such plants. European starlings (Sturnus vulgaris) are one of the most abundant avian frugivores in New England, and if effective seed dispersers for fleshy-fruited invasive plants, starlings may participate in a mutualistic interaction which benefits both the invasive plants and birds involved. Here, we investigate the role of European starlings in facilitating the germination of three invasive, fleshy-fruited plants with which they co-occur: Elaeagnus umbellata, Celastrus orbiculatus, and Rosa multiflora. For each plant species examined, less than 20% of the seeds ingested by captive starlings were not voided, and assumed to be destroyed as the result of passage through the digestive system. Starlings retained the seeds of E. umbellata 29 (mean) ± 19 (SD) min, C. orbiculatus 43 ± 20 min, and R. multiflora 27 ± 9 min. We also examined whether ingestion of seeds by starlings affected the germination of E. umbellata and C. orbiculatus seeds. Seeds that were cleaned by hand or regurgitated by birds had the same likelihood of germinating, and were significantly more likely to germinate than were seeds contained in intact fruit. Defecated C. orbiculatus seeds germinated significantly less well than hand-cleaned or regurgitated seeds, but better than those that were contained in intact fruits. We also found that C. orbiculatus seeds ingested by starlings required significantly less time to germinate than those contained in intact fruits. This study shows ingestion by starlings improves germination for both E. umbellata and C. orbiculatus seeds, and that starlings retain seeds long enough for seed dispersal to occur. Studies to determine the extent to which starlings feed on these plants, and the distances which seeds are moved, are needed.

Sexual size dimorphism in red-necked phalaropes and functional significance of nonsexual bill structure variation for feeding performance

Sexual size dimorphism is widespread in shorebirds, yet no tests of the assumption that such size dimorphism extends to functionally significant dimensions of the bill exist. This report presents tests of: (1) the assumption that sexual size dimorphism extends to the feeding structures in sexually size dimorphic bird, and (2) the hypothesis that bill‐size variation influences feeding performance in Phalaropus lobatus, the red‐necked phalarope. Discriminant function analysis revealed that the sexes of this species can be distinguished on the basis of five body size/bill length variables, but with low accuracy in sexing of females because of misclassification of small females as males. In the shorebird literature, the assumption is generally made that in the absence of selection to the contrary, bill size scales to body size and hence sexual size dimorphism extends to bill size. However, discriminant function analysis of measures from red‐necked phalaropes failed to separate the sexes on the basis of either external or internal bill dimensions other than length. Nonetheless, internal dimensions of the upper jaw combined with exposed culmen length explained 86% of the variance in feeding performance of phalaropes; high feeding performance depends on a wide, shallow, complex internal bill structure. This study provides evidence that internal bill dimensions determine feeding performance in a manner consistent with the mechanics of surface tension transport of prey. These results suggest that some dimensions of bill size may be constrained by performance demands and demonstrate that variation in bill morphology has functional consequences.