Jason D. Riddle

The Importance of Habitat Shape and Landscape Context to Northern Bobwhite Populations

Abstract Northern bobwhite (Colinus virginianus) populations have declined nationally for at least the past 4 decades. Field borders have been promoted as an important component of conservation plans to reverse this decline. Field border characteristics, such as shape and the landscapes in which the borders are established, have the potential to influence their effectiveness for recovering northern bobwhite populations. We established narrow linear (approx. 3-m-wide) and nonlinear field borders on farms in agriculture-dominated and forest-dominated landscapes in the Coastal Plain of North Carolina, USA, after collecting pretreatment data on summer bobwhite abundance. After establishment of field borders, summer bobwhite abundance nearly doubled on farms in agriculture-dominated landscapes and increased approximately 57% on farms with nonlinear field borders. Summer bobwhite abundance did not increase on farms with linear field borders in forest-dominated landscapes. Nonlinear and narrow linear field borders can be used to increase bobwhite numbers on farms in landscapes dominated by agriculture. Less flexibility exists in forest-dominated landscapes, where we found only nonlinear field borders resulted in an increase.

INFLUENCE OF PATCH SIZE AND SHAPE ON OCCUPANCY BY SHRUBLAND BIRDS

Abstract. Populations of many shrubland bird species are declining in the eastern United States. Efforts to restore shrubland and early-successional forest may help to ameliorate these declines. However, uncertainty remains about how the size and shape of habitat patches and the surrounding habitat matrix affect patch occupancy by shrubland passerines. Our objectives were to determine if shrubland birds avoid small or irregularly shaped habitat patches and to identify minimum area requirements for area-sensitive species. Additionally, we sought to determine if the proportion of mature forest cover in the landscape influences patch occupancy. We surveyed 35 individual habitat patches in 2007 and 43 in 2008 for the presence of eight shrubland birds. Then, we modeled the probability of five of these species occupying an individual patch relative to patch area, patch shape, and percent forest cover within 1 km of the patch. We documented evidence of area sensitivity for the Yellow-breasted Chat (Icteria virens) and Prairie Warbler (Setophaga discolor) and estimated their minimum area requirements at 2.3 and 1.1 ha, respectively. The Blue Grosbeak (Passerina caerulea) also was area-sensitive in irregularly shaped patches. Predicted patch-occupancy probability was >0.9 in patches ≥5.5 ha for all area-sensitive species. Patch shape alone and proportion of forest cover were not important predictors of occupancy for these shrubland birds. Restored shrubland and early-successional forest in agricultural landscapes can provide habitat for many shrubland birds, but patches should be >5 ha to maximize shrubland bird diversity.

EFFECTS OF PRIOR DETECTIONS ON ESTIMATES OF DETECTION PROBABILITY, ABUNDANCE, AND OCCUPANCY

ABSTRACT. Survey methods that account for detection probability often require repeated detections of individual birds or repeated visits to a site to conduct counts or collect presence-absence data. Initial encounters with individual species or individuals of a species could influence detection probabilities for subsequent encounters. For example, observers may be more likely to redetect a species or individual once they are aware of the presence of that species or individual at a particular site. Not accounting for these effects could result in biased estimators of detection probability, abundance, and occupancy. We tested for effects of prior detections in three data sets that differed dramatically by species, geographic location, and method of counting birds. We found strong support (AIC weights from 83% to 100%) for models that allowed for the effects of prior detections. These models produced estimates of detection probability, abundance, and occupancy that differed substantially from those produced by models that ignored the effects of prior detections. We discuss the consequences of the effects of prior detections on estimation for several sampling methods and provide recommendations for avoiding these effects through survey design or by modeling them when they cannot be avoided.

The importance of agriculture-dominated landscapes and lack of field border effect for early-succession songbird nest success.

In recent decades, many early-succession songbird species have experienced severe and widespread declines, which often are related to habitat destruction. Field borders create additional or enhance existing early-succession habitat on farmland. However, field border shape and the landscape context surrounding farms may influence the effectiveness of field borders in contributing to the stabilization or increase of early-succession bird populations. We examined the influence of linear and nonlinear field borders on farms in landscapes dominated by either agriculture or forests on nest success and Brown-headed Cowbird (Molothrus ater) brood parasitism of Indigo Bunting (Passerina cyanea) and Blue Grosbeak (Passerina caerulea) nests combined. Field border establishment did not affect nest survival probability and brood parasitism frequency of Indigo Bunting and Blue Grosbeak nests. Indigo Bunting/Blue Grosbeak nest success probability was more than twice as high in agriculture-dominated landscapes (39%) than in forested landscapes (17%), and brood parasitism frequency was high (33%) but did not differ between landscapes. Edges in agriculture-dominated landscapes can be higher-quality habitats for early-succession birds than edges in forest-dominated landscapes, but our field border treatments did not enhance nest success for these birds on farms in either landscape. RESUME. Au cours des dernieres decennies, de nombreux passereaux de debut de succession ont subi un declin marque et generalise, souvent lie a la destruction d’habitat. Les lisieres de champs representent de nouveaux milieux de debut de succession ou s’ajoutent aux milieux de debut de succession deja existants dans les paysages agricoles. Toutefois, la configuration des lisieres et le contexte paysager aux environs des fermes peuvent avoir une influence sur l’efficacite des lisieres, en contribuant a la stabilisation ou a l’augmentation des populations d’oiseaux de debut de succession. Nous avons examine l’effet de lisieres lineaires et non lineaires sur des fermes situees dans des paysages a dominance agricole ou forestiere, sur deux parametres du Passerin indigo (Passerina cyanea) et du Guiraca bleu (Passerina caerulea) : leur succes de nidification et leur taux de parasitisme par le Vacher a tete brune (Molothrus ater). La creation de lisieres n’a pas eu d’effets sur le taux de survie des nids, ni sur le taux de parasitisme. Le taux de survie des nids de Passerin indigo et de Guiraca bleu dans les paysages agricoles (39 %) etait plus du double de celui observe dans les paysages forestiers (17 %). Le taux de parasitisme etait eleve (33 %), mais ne differait pas entre les deux types de paysages. Les lisieres presentes dans les paysages domines par l’agriculture s’averent etre des milieux de meilleure qualite pour les oiseaux de debut de succession que ne le sont les University of Wisconsin Stevens Point, North Carolina State University Avian Conservation and Ecology 5(2): 9 http://www.ace-eco.org/vol5/iss2/art9/ lisieres presentes dans les paysages domines par les forets. Les traitements que nous avons effectues sur les lisieres n’ont toutefois pas permis d’augmenter le succes de nidification de ces oiseaux dans l’un ou l’autre des deux types de paysages.

A Comparison of Methods for Estimating Northern Bobwhite Covey Detection Probabilities

Abstract We compared the time-of-detection and logistic regression methods of estimating probability of detection for northern bobwhite (Colinus virginianus) coveys. Both methods are unusual in that they allow estimation of the total probability of detection (i.e., the product of the probability that a covey is available for detection [i.e., that a covey vocalizes] and detection given availability). The logistic regression method produced an average detection probability of 0.596 (SE = 0.020) and the time-of-detection method produced a detection probability estimate of 0.540 (SE = 0.086), and the 2 estimates were not significantly different. This is the first evaluation of the time-of-detection method with empirical field data. Although the time-of-detection and logistic regression method each have advantages, both can be used under appropriate conditions to improve estimates of bobwhite abundance by allowing for the estimation of detection probabilities. Improved estimates of bobwhite abundance will allow land managers to make more informed management decisions.

An experimental evaluation of the performance of acoustic recording systems for estimating avian species richness and abundance

Comparisons between field observers and acoustic recording systems have shown great promise for sampling birds using acoustics methods. Comparisons provide information about the performance of recording systems and field observers but do not provide a robust validation of their true sampling performance—i.e., precision and accuracy relative to known population size and richness. We used a 35-speaker bird song simulation system to experimentally test the accuracy and precision of two stereo (Telinga and SS1) and one quadraphonic recording system (SRS) for estimating species richness, abundance, and total abundance (across all species) of vocalizing birds. We simulated 25 bird communities under natural field conditions by placing speakers in a wooded area at 4–119 m from the center of the survey at differing heights and orientations. We assigned recordings randomly to one of eight skilled observers. We found a significant difference among microphones in their ability to accurately estimate richness (p = 0.0...