Dirk E. Burhans

Predation of Songbird Nests Differs By Predator and Between Field and Forest Habitats

Our understanding of factors affecting nest predation and ability to mitigate high nest predation rates is hampered by a lack of information on the importance of various nest predator species in different habitats and landscapes. We identified predators of songbird nests in old-field and forest habitats in central Missouri, USA, with miniature video cameras. We used an information-theoretic approach to evaluate support for hypotheses that the importance of predator species varied among habitats and nest stage. We monitored 165 nests with cameras and 272 nests without cameras during 1997-1999, and identified predators at 61 of 74 depredated nests monitored by cameras. Model selection indicated the most support for a model with seperate rates for predation by birds, mammels, and snakes in field and forest habitats. Predation by snakes was greater than predation by mammels and birds in old fields; predation by mammels (mostly rccoons [Procyon lotor]) was greater than by snakes and birds in forest. We found little support for the hypothesis that monitoring nests with cameras effects predation. Nests could not be assigned reliably to a predator group based on condition of the nest. We believe that knowledge of the identity and abundance of dominant predators in a habitat or landscape is neccessary to target conservation efforts to reduce nest predation or to interpret results of research on factors affecting nest success.

Factors affecting predation at songbird nests in old fields

We determined the effects of microhabitat, year, weather, time of season, stage of the nesting cycle, and brood parasitism on nest predation from a 7-year dataset on field sparrows (Spizella pusilla) and indigo buntings (Passerina cyanea) in central Missouri, USA. Year, site, and the interaction of species and 2-week interval of the season were important factors explaining nest predation. The only microhabitat variable that consistently explained predation was nest height: nests over 3 m high almost always fledged. Validation of the model parameters on an independent set of nests resulted in proper categorization (e.g., lost or not lost to predation) of 61.5% of nests. In models testing weather and temporal effects, year was related to daily survival for indigo buntings, and 2-week intervals of the season explained daily survival for both species. Nest predation was higher overall in the nestling stage than in the incubation stage for indigo buntings, and indigo buntings parasitized by brown-headed cowbirds (Molothrus ater) experienced higher predation than nonparasitized buntings. Temporal patterns within the breeding season were consistent between years, and between-year variance appeared to be important, whereas microhabitat was generally unimportant. Research on the mechanisms underlying temporal variability in nest mortality due to predation may identify management options to reduce nest predation.

BREEDING AND POST-BREEDING HABITAT USE BY FOREST MIGRANT SONGBIRDS IN THE MISSOURI OZARKS

Abstract We compared habitat use by forest migrant songbirds during the breeding and post-breeding periods in four Missouri Ozark habitats: mature upland forest, mature riparian forest, 9- to 10-year-old upland forest, and 3- to 4-year-old upland forest created by clearcutting. Adult forest-ground species showed a decrease in abundance in all habitats during the post-breeding period, but hatching-year birds of one of the two forest-ground species were most abundant in early-successional forest during this time. Adults of the two forest-canopy species tended to increase in abundance in 3- to 4-year-old forest from breeding season to post-breeding season. During the breeding season, some forest species were detected with mist-nets in the two early-successional habitats, but infrequently or not at all with point counts in those habitats. Forest birds captured in early-successional habitats during the breeding season may have been nonbreeding floaters, or may have been foraging there from nearby territories in mature forest. Dense shrubs or young trees in early-successional forest may provide habitat for nonbreeding and post-breeding forest migrant songbirds in the Missouri Ozarks.

Effects of Time and Nest-Site Characteristics on Concealment of Songbird Nests

We studied the effect of time and nest-site characteristics on nest concealment measurements and analyzed differences in concealment between parasitized, nonparasitized, depredated, and fledged nests. Mean concealment at nests of three old-field bird species was best explained by bird species, nest plant, and height of the nest. Nests lost concealment over time, particularly those placed high in shrubs or roses (Rosa spp.). Mean and minimum concealment did not explain occurrence of predation or brood parasitism for any of the three bird species, and concealment at parasitized versus unparasitized nests and depredated versus fledged nests did not change differently over time. A literature review showed that most studies of real passerine nests using visual nest concealment have taken measurements after nest termination, and few studies indicated that concealment was important in explaining nest predation or brood parasitism. Late concealment measurements may be an additional source of error in nesting studies, especially if predation or parasitism is more likely to occur at nests sharing similar vegetation characteristics.

Habitat and microhabitat features associated with cowbird parasitism in two forest edge cowbird hosts

I examined the relationship of habitat and nest microhabitat features of Field Sparrows (Spizella pusilla) and Indigo Buntings (Passerina cyanea) to brood parasitism by the Brown-headed Cowbird (Molothrus ater) in central Missouri. In old field habitats, Indigo Buntings were more frequently parasitized than Field Sparrows, but Indigo Buntings nesting in forested habitat were parasitized at higher frequencies than buntings in old fields. Logistic regression models showed that nest concealment best explained parasitism for all Indigo Buntings and field-nesting Indigo Buntings, with poorly concealed nests more likely to be parasitized. However, side concealment was not related to parasitism for heavily-parasitized Indigo Buntings in forested habitats. Microhabitat variables did not explain parasitism at Field Sparrow nests, but their nests were lower and better concealed than field-nesting Indigo Buntings. Results suggest that nest microhabitat features may influence probability of parasitism, but species and habitat characteristics may override microhabitat in explaining frequency of parasitism.

Patterns of Snake Predation at Songbird Nests in Missouri and Texas

Abstract Snakes are frequent predators of eggs and nestlings, but general patterns of snake predation at bird nests are not well known. We reviewed 84 video observations of snakes visiting nests of four songbird species in Texas and Missouri to identify patterns of predatory behavior. Eastern Ratsnakes (Elaphe obsoleta) were the most common species, and coachwhips, racers, kingsnakes, and a garter snake also were recorded. Snakes almost always removed all nest contents during a single visit but sometimes force-fledged nestlings that were old enough to escape. During many visits late in the nestling period, snakes pinned their prey in the nest while feeding, thereby preventing many of the young from escaping. Snakes spent an average of 13 min and 23 sec at each nest (1 min and 52 sec before striking), and the duration of nocturnal visits exceeded the duration of diurnal visits. Snakes sometimes returned to empty nests after they caused failure but only after nestlings were depredated. Visits by Texas Ratsnakes (Elaphe obsoleta lindheimeri) were mostly nocturnal, whereas visits by Black Ratsnakes (Elaphe obsoleta obsoleta), coachwhips, racers, and kingsnakes were diurnal. Snake predation increased as the nesting cycle progressed with the highest rate occurring in the last few days of the nestling period. Increased predation at the end of the nestling period suggests that avian activity (i.e., feeding visits, nest defense, and nestling movement) contributes to the foraging success of snakes at our sites.

Songbird Abundance And Parasitism Differ Between Urban And Rural Shrublands

Many studies have examined differences in avian community composition between urban and rural habitats, but few, if any, have looked at nesting success of urban shrubland birds in a replicated fashion while controlling for habitat. We tested factors affecting nest survival, parasitism by the Brown-headed Cowbird (Molothrus ater), and species abundance in shrubland habitat in rural and urban landscapes. We found no support for our hypothesis that nest survival was lower in urban landscapes, but strong support for the hypothesis that survival increased with nest height. We found strong support for our hypothesis that cowbird parasitism was greater in urban than rural landscapes; parasitism in urban sites was at least twice that of rural sites. We found strong support for an urban landscape effect on abundance for several species; Northern Cardinal (Cardinalis cardinalis) and Brown-headed Cowbirds were more abundant in urban landscapes, whereas Field Sparrow (Spizella pusilla) and Blue-winged Warbler (Vermivora pinus) were more abundant in rural sites. There was support for lower abundances of Blue-gray Gnatcatcher (Polioptila caerulea) and Indigo Bunting (Passerina cyanea) with increased housing density. For six other species, edge and trail density or vegetation parameters best explained abundance. Lower abundances and greater parasitism in habitat patches in urban landscapes are evidence that, for some species, these urban landscapes do not fulfill the same role as comparable habitats in rural landscapes. Regional bird conservation planning and local habitat management in urban landscapes may need to consider these effects in efforts to sustain bird populations at regional and local scales.

Relationship of songbird nest concealment to nest fate and flushing behavior of adults

Advoiding predation is an important consideration for any potential prey animal. Failure to escape from a predator results in loss of fitness, so there is strong selection for choices and behaviors that result in successful escape (Lima and Dill 1990). In their cost-benefit approach to flight from predators, Ydenberg and Dill (1986) stressed that flight should be optimized rather than maximized, because there is a cost (usually cessation of feeding) incurred by fleeing from predation. Field studies have largely supported their predictions (see Bonenfant and Kramer 1996).

A comparison of point-count and mist-net detections of songbirds by habitat and time-of-season

Abstract We compared the results of point-count and mist-net surveys during the breeding and post-breeding seasons in four Missouri Ozark habitats: mature upland forest, mature riparian forest, 9- to 10-yr-old upland forest and 3- to 4-yr-old upland forest created by clearcutting. We determined whether differences in abundance estimates among habitats or between breeding and post-breeding seasons varied with survey method (i.e., habitat × method, season × method, or habitat × season × method interaction effects). The habitat × method interaction was significant for 13 of 16 species. The general pattern was for canopy or sub-canopy species to be detected by point counts more often in the mature-forest habitats, and to be detected by mist nets more often in the young-forest habitats. The season × method interaction was significant for 6 species largely because there was a greater decrease from breeding to post breeding season in mist-net captures than point-count detections. Two species had a significant habitat × season × method interaction. Differences in the patterns in abundance among habitats and between seasons, by survey method, are indicative of bias in the survey methods. Individuals planning bird surveys should recognize these biases and select the method that best samples the segment of the community they are surveying and that is least likely to confound treatment effects. Often this may require the use of multiple survey methods.

Effects of point count protocol on bird abundance and variability estimates and power to detect population trends

Abstract We used a three-year point-count data set from Hoosier National Forest, Indiana, to evaluate alternative point-count sampling strategies for detecting songbird declines. Repeated-measures ANOVA indicated that mean abundance estimates increased with increasing count radius (P < 0.0001, each species), and coefficients of variation (CVs) decreased. Mean abundance estimates increased with longer count duration (6-, 8-, and 10-min) for only two of 13 species, and CVs did not necessarily decrease. The power to detect a 5% annual population trend increased with more survey points, more visits per point, and more years of surveys. Managers can use observer time most efficiently by employing counts of short duration. Counts using a larger radius will reduce CVs and therefore provide potentially better annual estimates of abundance and power to detect changes over a period of years. The design of the study and nature of the variability in bird abundance will determine whether increasing the number of points, or the number of visits per point, will have greater effect on power to detect a population trend.