David E. Andersen

Uncertain nest fates in songbird studies and variation in Mayfield estimation

Abstract Determining whether nesting attempts are successful can be difficult. Yet, current protocols for estimating nesting success do not address how uncertain nest fates should be handled. We examined the problem of nest-fate uncertainty as it relates to Mayfield estimation of nesting success and in analyses of factors that influence success. We used data from Minnesota to illustrate the potential effect of uncertain fate; 40% of Ovenbird (Seiurus aurocapillus; n = 127) nests and 30% of Least Flycatcher (Empidonax minimus; n = 144) nests had uncertain fates. How this uncertainty is incorporated into Mayfield estimates of success varied widely among researchers. In a survey of researchers who use the Mayfield method, 9 of 22 respondents (of 40 contacted) excluded nests with uncertain fate. Excluding uncertain fates is counter to how Mayfield first described his estimator and can result in severe downward bias. The remaining respondents (59%) included nests with uncertain fate but varied in how they terminated the exposure period. We developed a simulation model that calculated Mayfield estimates using different approaches and compared them with a known rate of nesting success. Magnitude of bias in Mayfield estimates varied considerably in our simulations. The approach with the least bias terminated exposure with the last observed active date for nests with uncertain fate, and with the midpoint between last observed active and first observed inactive dates for nests with known fate. In addition, information necessary to interpret and compare Mayfield estimates often is not reported. These values, including variance estimates and the period lengths used to estimate survival rates, should be reported with Mayfield estimates. Finally, nest fate is commonly used as a categorical variable in studies of factors affecting nesting success. In this approach, however, nests with uncertain fate must be excluded. An alternative approach is Cox regression, which incorporates nests with uncertain fate.

EDGE EFFECT ON NESTING SUCCESS OF GROUND NESTING BIRDS NEAR REGENERATING CLEARCUTS IN A FOREST-DOMINATED LANDSCAPE

Abstract Forest fragmentation has been implicated as a cause of population declines of several Neotropical migrant bird species. Fragmentation increases the amount of habitat edge, and reduced nesting success rates near forest edges are well documented in agricultural landscapes (“edge effects”). However, edge effects in predominantly forested landscapes, particularly those related to timber harvest, are poorly understood. This study examines nesting success of ground nesting birds in relation to clearcut edges in a forest-dominated landscape in north-central Minnesota. A total of 383 nests of seven species of ground nesting birds were found and monitored during 1992–1998. Ovenbird (Seiurus aurocapilus; n = 318) and Hermit Thrush (Catharus guttatus; n = 44) nests composed the majority of the sample. Predation accounted for 94% of all nest failures. Brown-headed Cowbird (Molothrus ater) parasitism was low (1.8% for all ground nests). Using proportional hazards regression, distance to nearest clearcut edge was the best predictor of nest failure. For all ground nests, nesting success was 0.18 at 0–100 m, 0.39 at 101–500 m, and 0.52 at 501–954 m from nearest clearcut edge. Source–sink modeling indicated that distances ≤100 m from clearcut edges were sink habitats for Ovenbirds (i.e. recruitment was lower than survival). These results provide strong evidence of a negative edge effect on ground nests, extending 100 m or more from clearcut edges in a forest-dominated area of north-central Minnesota.

Seasonal productivity in a population of migratory songbirds: why nest data are not enough

Population models for many animals are limited by a lack of information regarding juvenile survival. In particular, studies of songbird reproductive output typically terminate with the success or failure of nests, despite the fact that adults spend the rest of the reproductive season rearing dependent fledglings. Unless fledgling survival does not vary, or varies consistently with nest productivity, conclusions about population dynamics based solely on nest data may be misleading. During 2007 and 2008, we monitored nests and used radio telemetry to monitor fledgling survival for a population of Ovenbirds (Seiurus aurocapilla) in a managed-forest landscape in north-central Minnesota, USA. In addition to estimating nest and fledgling survival, we modeled growth for population segments partitioned by proximity to edges of non-nesting cover types (regenerating clearcuts). Nest survival was significantly lower, but fledgling survival was significantly higher, in 2007 than in 2008. Despite higher nest productivity in 2008, seasonal productivity (number of young surviving to independence per breeding female) was higher in 2007. Proximity to clearcut edge did not affect nest productivity. However, fledglings from nests near regenerating sapling-dominated clearcuts (7–20 years since harvest) had higher daily survival (0.992 ± 0.005) than those from nests in interior forest (0.978 ± 0.006), which in turn had higher daily survival than fledglings from nests near shrub-dominated clearcuts (≤6 years since harvest; 0.927 ± 0.030) in 2007, with a similar but statistically non-significant trend in 2008. Our population growth models predicted growth rates that differed by 2–39% (x¯ = 25%) from simpler models in which we replaced our estimates of first-year survival with one-half adult annual survival (an estimate commonly used in songbird population growth models). We conclude that nest productivity is an inadequate measure of songbird seasonal productivity, and that results based exclusively on nest data can yield misleading conclusions about population growth and clearcut edge effects. We suggest that direct estimates of juvenile survival could provide more accurate information for the management and conservation of many animal taxa.

Mule Deer Movements in Response to Military Activity in Southeast Colorado

During January 1986-September 1988 we studied the behavioral responses of 71 radiocollared mule deer (Odocoileus hemionus) to military activity on the Pinon Canyon Maneuver Site in southeastern Colorado. Military training was initiated on the site during August 1985 and recurred about 3 times yearly for periods of one month. During a maneuver, 3/7 of the site was used for training in accordance with a rotational land use schedule. During the nonsummer seasons, female seasonal convex polygon and harmonic mean home ranges were larger in maneuver and previous-maneuver areas than nonmaneuver areas (P < 0.002). During summer, female convex polygon home ranges were larger in maneuver than nonmaneuver areas (P = 0.066). Fawn summer home ranges were larger in maneuver than previous-maneuver areas (P < 0.01). Male home range sizes differed only for 50% harmonic mean transformation annual home ranges (P = 0.056); bucks in maneuver areas had larger home ranges than in nonmaneuver areas. Female deer in maneuver areas exhibited significant home area shifts (P = 0.049) between premaneuver and maneuver periods more frequently (40.0%) than did deer in nonmaneuver (control) areas (12.5%). Mule deer in military training areas may have responded to human harassment, alteration of security cover, or destruction of the forage base. We suggest that deer may respond more intensely to unpredictable than predictable human activity.

ADVANTAGES OF CLUSTERED NESTING FOR LEAST FLYCATCHERS IN NORTH-CENTRAL MINNESOTA

Abstract Least Flycatchers (Empidonax minimus) often aggregate their territories, even though advantages of clustering are not apparent. In north-central Minnesota we investigated four potential reasons for clustered nesting in Least Flycatchers: (1) predator deterrence, (2) preferred habitat features, (3) greater food availability, and (4) competitive exclusion. In 1995 and 1996 we compared arthropod abundance, forest structure, and plant species composition inside and outside nine Least Flycatcher clusters. We also compared the response of forest birds to the presentation of a predator (Broad-winged Hawk [Buteo platypterus]), and used point counts to detect any exclusion of potential predators or competitors from the clusters. The predator deterrence hypothesis received the strongest support. In response to hawk presentations inside the clusters, 2.8 times more birds (all species combined) gave 5.8 times more alarm calls than outside, with differences due entirely to the presence of Least Flycatchers. In addition, birds responded to the hawk more quickly inside clusters. Minor differences in habitat seemed insufficient to produce such tight clustering. Neither all arthropods combined nor any of the five orders most frequently caught in sticky traps indicated greater food availability inside the clusters. Black-throated Green Warblers (Dendroica virens) were detected 2.2 to 5.4 times less frequently inside the clusters than outside, but they probably do not compete to a large degree with Least Flycatchers. The dramatic response of Least Flycatchers to the hawk presentations provides the first evidence that antipredator behavior may contribute to the clustering of their territories. Ventajas de la Nidificación en Grupo en Empidonax minimus en el Centro Norte de Minnesota Resumen. Los individuos de la especie Empidonax minimus a menudo agrupan sus territorios, a pesar de que no son aparentes las ventajas del agrupamiento. Investigamos cuatro posibles razones que podrían explicar el agrupamiento de nidos en E. minimus en el centro norte de Minnesota: (1) disuasión de depredadores, (2) uso de parches con características de hábitat preferidas, (3) mayor disponibilidad de alimento o (4) exclusión competitiva. En 1995 y 1996 comparamos la abundancia de artrópodos, estructura de hábitat y composición de especies de plantas dentro y fuera de nueve grupos de nidos de E. minimus. También comparamos la respuesta de aves de bosque ante la presentación de un depredador (Buteo platypterus) y realizamos conteos de punto para determinar exclusiones de potenciales depredadores o competidores de los agrupamientos. La hipótesis mejor apoyada fue la de disuasión de depredadores. Como respuesta a la presentación de un depredador dentro de un agrupamiento, 2.8 veces más aves (todas las especies combinadas) emitieron 5.8 veces más llamadas de alarma que fuera del agrupamiento. Las diferencias estuvieron dadas enteramente por la presencia de individuos de la especie E. minimus. Además, las aves respondieron con mayor rapidez a la presencia de un halcón dentro que fuera de los agrupamientos. La escasa diferencia entre hábitats pareció insuficiente para producir el fuerte agrupamiento observado. Ni todos los artrópodos combinados ni ninguno de los cinco órdenes que usualmente son atrapados con trampas pegajosas indicaron una mayor disponibilidad de alimento dentro del área de agrupamiento. Individuos de la especie Dendroica virens fueron 2.2 a 5.4 veces menos frecuentes dentro de los agrupamientos que fuera de ellos, aunque éstos probablemente no compiten en gran medida con E. minimus. La dramática respuesta por parte de E. minimus ante la presentación de un halcón provee la primera evidencia experimental de que el comportamiento antidepredatorio puede contribuir a la agrupación de territorios.

Red-shouldered hawk broadcast surveys : Factors affecting detection of responses and population trends

Forest-nesting raptors are often difficult to detect and monitor because they can be secretive, and their nests can be difficult to locate. Some species, however, respond to broadcasts of taped calls, and these responses may be useful both in monitoring population trends and in locating nests. We conducted broadcast surveys on roads and at active red-shouldered hawk (Buteo lineatus) nests in northcentral Minnesota to determine effects of type of call (conspecific or great horned owl [Bubo virginianus]), time of day, and phase of the breeding cycle on red-shouldered hawk response behavior and to evaluate usefulness of broadcasts as a population monitoring tool using area occupted-probability-of-detection techniques. During the breeding seasons of 1994 and 1995. we surveyed 4 10-station road transects 59 times and conducted 76 surveys at 24 active nests. Results of these surveys indicated conspecific calls broadcast prior to hatch and early in the day were the most effective method of detecting red-shouldered hawks. Probability of detection via conspecific calls averaged 0.25, and area occupied was 100%. Computer simulations using these field data indicated broadcast surveys have the potential to be used as a population monitoring tool.

Tornadic Storm Avoidance Behavior in Breeding Songbirds

Migration is a common behavior used by animals of many taxa to occupy different habitats during different periods. Migrant birds are categorized as either facultative (i.e., those that are forced to migrate by some proximal cue, often weather) or obligate (i.e., those that migrate on a regular cycle). During migration, obligate migrants can curtail or delay flights in response to inclement weather or until favorable winds prevail, and they can temporarily reorient or reverse direction when ecological or meteorological obstacles are encountered. However, it is not known whether obligate migrants undertake facultative migrations and make large-scale movements in response to proximal cues outside of their regular migration periods. Here, we present the first documentation of obligate long-distance migrant birds undertaking a facultative migration, wherein breeding golden-winged warblers (Vermivora chrysoptera) carrying light-level geolocators performed a >1,500 km 5-day circumvention of a severe tornadic storm. The birds evacuated their breeding territories >24 hr before the arrival of the storm and atmospheric variation associated with it. The probable cue, radiating >1,000 km from tornadic storms, perceived by birds and influencing bird behavior and movements, is infrasound (i.e., sound below the range of human hearing). With the predicted increase in severity and frequency of similar storms as anthropogenic climate change progresses, understanding large-scale behavioral responses of animals to such events will be an important objective of future research.

Prevalence of Antibodies against Canine Parvovirus and Canine Distemper Virus in Wild Coyotes in Southeastern Colorado

Serum from 72 wild coyotes (Canis latrans) in southeastern Colorado (USA) was collected and analyzed for prevalence of antibody to canine parvovirus (CPV) and canine distemper virus (CDV) from 1985 to 1988. The prevalence of antibodies to CPV and CDV was 71% and 57%, respectively, for the 4 yr of the study. Prevalence of antibody to CPV did not differ among years, between sexes, or with age. Prevalence of antibody to CDV did not differ among years or between sexes, but was significantly higher in adults (62%) than juveniles (33%). Prevalence of antibodies against CPV and CDV in southeastern Colorado was comparable to results reported in other serologic surveys in the western United States.

Nest-defense behavior of red-tailed hawks

From 1983 to 1986 I visited 26 Red-tailed Hawk (Buteojamaicensis) nests in east-central and southeastern Colorado. I estimated the closest distance that each defending adult approached, and recorded the number of calls that each bird gave and the number of times it dived while I was at the base of the nest tree for a 10-min period and subsequently at the nest for a 5-min period. The age and number of young in the nest, the height of the nest above the ground, and whether one or both adults were present were used as potential predictors of the intensity of nest-defense behavior. Nestling age was a significant predictor of call rate of the closest adult and nest height was a significant predictor of call rate of the farthest adult while I was at the base of the nest tree. Dive rate and closest approach were not significantly related to any measured variable. No independent variable was significantly related to nest-defense intensity while I was at the nest, however, dive rate increased and closest approach decreased when I was at the nest compared to when I was on the ground. These observations are only in part consistent with current theoretical models that predict patterns of nest-defense intensity in altricial birds.

Retirement investment theory explains patterns in songbird nest-site choice

When opposing evolutionary selection pressures act on a behavioural trait, the result is often stabilizing selection for an intermediate optimal phenotype, with deviations from the predicted optimum attributed to tracking a moving target, development of behavioural syndromes or shifts in riskiness over an individuals lifetime. We investigated nest-site choice by female golden-winged warblers, and the selection pressures acting on that choice by two fitness components, nest success and fledgling survival. We observed strong and consistent opposing selection pressures on nest-site choice for maximizing these two fitness components, and an abrupt, within-season switch in the fitness component birds prioritize via nest-site choice, dependent on the time remaining for additional nesting attempts. We found that females consistently deviated from the predicted optimal behaviour when choosing nest sites because they can make multiple attempts at one fitness component, nest success, but only one attempt at the subsequent component, fledgling survival. Our results demonstrate a unique natural strategy for balancing opposing selection pressures to maximize total fitness. This time-dependent switch from high to low risk tolerance in nest-site choice maximizes songbird fitness in the same way a well-timed switch in human investor risk tolerance can maximize ones nest egg at retirement. Our results also provide strong evidence for the adaptive nature of songbird nest-site choice, which we suggest has been elusive primarily due to a lack of consideration for fledgling survival.