The Condor 113(2):400–411 The Cooper Ornithological Society 2011
Influence of condition and habitat use on survival of post-fledging songbirds A ndrew C. Vitz1 and A manda D. Rodewald School of Environment and Natural Resources, The Ohio State University, Columbus, OH 43202 Abstract. Habitat quality of a bird’s breeding grounds has been typically evaluated by investigating patterns in nesting success, whereas events that follow fledging have been largely ignored. One especially overlooked aspect of breeding-habitat quality is how habitat affects the survival of young birds after they leave the nest, a period when mortality is notoriously high. We studied survival of fledglings of two mature-forest species, the Ovenbird (Seiurus aurocapilla) and Worm-eating Warbler (Helmitheros vermivorum), to identify intrinsic (e.g., age, condition) and extrinsic (e.g., habitat structure) factors that influence survival. From 2004 to 2007, we radio-tagged 51 Ovenbird and 60 Worm-eating Warbler fledglings in southeast Ohio. We recorded the birds’ locations daily and compared vegetation structure at the fledglings’ and paired random locations. Using known-fate models in program MARK, we calculated post-fledging survival to be 65% for the Ovenbirds (51 days after fledging) and 67% for the Worm-eating Warblers (31 days after fledging). Fledglings’ condition at the time of radio tagging was positively related to survival after fledging, implying carryover effects from the nestling period. Fledglings of both species used dense vegetation with 40–60% more woody stems in the understory than at random locations. Moreover, use of dense vegetation actually promoted survival. Although riparian thickets and tree-fall gaps within some forests may provide abundant habitat for fledglings, other forests may lack the structural attributes that promote fledglings’ survival. Our findings highlight the importance of both breeding and post-fledging requirements being considered in avian conservation plans. Key words: fledgling, habitat use, juvenile survival, Ovenbird, post-fledging, predation, Worm-eating Warbler.
Influencia de la Condición y el Uso del Hábitat sobre la Supervivencia Posterior al Emplumamiento de Aves Canoras Resumen. La calidad del hábitat en las áreas reproductivas ha sido típicamente estudiada investigando los patrones de éxito de anidación, mientras que los eventos que siguen al emplumamiento han sido generalmente ignorados. Un aspecto que ha sido especialmente ignorado sobre la calidad del hábitat reproductivo es como el hábitat afecta la supervivencia de las aves jóvenes después de que dejan el nido, un periodo durante el cual la mortalidad es notoriamente alta. Estudiamos la supervivencia de aves recientemente emplumadas de dos especies típicas de bosques maduros, Seiurus aurocapilla y Helmitheros vermivorum, para identificar los factores intrínsecos (e.g., edad, condición) y extrínsecos (e.g. estructura del hábitat) que influencian la supervivencia. Desde 2004 a 2007 marcamos 51 individuos recién emplumados de S. aurocapilla y 60 de H. vermivorum con radiotransmisores en el sur de Ohio. Registramos la localización de las aves diariamente y comparamos la estructura de la vegetación en el punto de captura de los individuos y en localidades pareadas localizadas al azar. Utilizando los modelos de destino conocido en MARK, calculamos la supervivencia durante el periodo posterior al emplumamiento, que fue de 65% para S. aurocapilla (51 días después del emplumamiento) y de 67% para H. vermivorum (31 días después del emplumamiento). La condición de los volantones en el momento en que se adhirió el transmisor se correlacionó positivamente con la supervivencia después del emplumamiento, implicando efectos influenciados por el periodo en el que los polluelos estuvieron en el nido. Los volantones de las dos especies usaron sitios de vegetación densa con un 40–60 % más de plantas leñosas en el sotobosque que en los sitios localizados al azar. Además, el uso de esa vegetación densa aumentó la supervivencia. A pesar de que sectores de vegetación ribereña densa y espacios abiertos producidos por la caída de árboles adentro de algunos bosques pueden proveer de hábitat abundante para los volantones, otros tipos de bosque pueden no tener los atributos estructurales necesarios para promover la supervivencia de los volantones. Nuestros resultados resaltan la importancia de considerar los requerimientos reproductivos y posteriores al emplumamiento en los planes de conservación de aves.
Manuscript received 3 February 2010; accepted 14 October 2010. 1 Current address: Powdermill Avian Research Center, Carnegie Museum of Natural History, 1847 State Route 381, Rector, PA 15677. E-mail:
[email protected] The Condor, Vol. 113, Number 2, pages 400–411. ISSN 0010-5422, electronic ISSN 1938-5422. 2011 by The Cooper Ornithological Society. All rights reserved. Please direct all requests for permission to photocopy or reproduce article content through the University of California Press’s Rights and Permissions website, http://www.ucpressjournals.com/ reprintInfo.asp. DOI: 10.1525/cond.2011.100023
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INTRODUCTION Avian conservation has traditionally focused on science and management related to nesting ecology (Robinson et al. 1995) and more recently winter ecology (Johnson et al. 2006). Our understanding of other stages of the avian life cycle remains surprisingly incomplete, and this knowledge gap may be most acute for the post-fledging period (Anders et al. 1997). For migratory birds, the post-fledging period generally spans 2–3 months beginning with fledging from the nest and extending until the initiation of fall migration. The first several weeks following fledging appear to be the most critical in terms of survival, a result of fledglings having limited mobility, being conspicuous to predators (i.e., begging calls), and depending on parents for food (Anders et al. 1997). After fledglings gain independence, mortality remains elevated because of inexperience in foraging and evading predators (Lack 1954). Recent studies have documented high rates of mortality of fledgling songbirds, which often exceed 50% (Sullivan 1989, Anders et al. 1997) and profoundly affect population demography (Robinson et al. 2004), making the period after fledging a critical stage in the avian life cycle. Despite evidence that the post-fledging period plays an important role in avian conservation (Anders et al. 1997), during this period, the values of basic ecological metrics (i.e., survival, habitat use) are completely unknown for most species. Consequently, in the absence of empirical data, annual survival of juvenile passerines is frequently estimated at 0.31 (Anders and Marshall 2005, Yackel Adams et al. 2006) or half of adults’ survival (Donovan et al. 1995). Yet recent studies suggest that fledglings’ survival varies dramatically by species and region (Anders and Marshall 2005, Vitz 2008), and survival during only the first few weeks after fledging can be lower than conventional estimates for juveniles’ annual survival (Yackel Adams et al. 2006). Traditionally, use of breeding and post-fledging habitat was assumed to be similar, but empirical studies show this is not necessarily the case (Anders et al. 1997, Vega Rivera et al. 1998). In fact, habitat needs might be expected to change through the annual cycle, given that breeding birds must meet numerous requirements (i.e., nest-site selection, mate acquisition, courtship, nestling provisioning), whereas nonbreeding birds, including fledglings, should be focused primarily on survival. Differences in habitat use between the breeding and postfledging periods are well documented for the Ruffed Grouse (Bonasa umbellus), which frequently moves its broods into areas of dense vegetation, including young aspen stands (Thompson and Dessecker 1997) and alder thickets (Rusch et al. 2000). Similarly, there is strong evidence that Wood Thrushes (Hylocichla mustelina) move from their breeding areas in mature forest into habitats with dense vegetation during the post-fledging period (Anders et al. 1997, Vega Rivera et al. 1998, Powell et al. 2000). The extent to which most other forest-breeding migratory birds follow this pattern remains unknown.
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Despite evidence for a habitat shift and high mortality during the post-fledging period, few studies have investigated relationships between fledgling survival and habitat selection (King et al. 2006). This knowledge gap is particularly noticeable in the face of reports that numerous forest-breeding songbirds (juveniles and adults) are frequently captured in early successional habitats (e.g., regenerating clearcuts) during the post-fledging period (Pagen et al. 2000, Marshall et al. 2003, Vitz and Rodewald 2006, but see Mitchell et al. 2009). In fact, some of the highest capture rates in early-successional habitats have been of species generally regarded as forest-interior specialists (Vitz and Rodewald 2006). Nevertheless, questions remain about how fledgling songbirds are affected by earlysuccessional habitats, especially given that neither habitat use (Garshelis 2000) nor animal densities (Van Horne 1983) necessarily indicate high-quality habitat. Our poor understanding of post-fledging ecology results primarily from the difficulty associated with collecting data on birds during a phase when they are furtive and do not broadcast their presence through song. Fortunately, recent technological advances have allowed for the miniaturization of radio-transmitters, thereby permitting researchers to track birds’ movements, habitat use, and survival. Despite increased interest and an unprecedented number of studies on post-fledging ecology over the last 10–15 years, most research in eastern North American forests has focused on a single species, the Wood Thrush (Anders et al. 1997, Vega Rivera et al. 1998, Powell et al. 2000). In this study, we examined fledgling survival of two other mature-forest species, the Ovenbird (Seiurus aurocapilla) and Worm-eating Warbler (Helmitheros vermivorum), and the extent to which survival was affected by intrinsic (e.g., age, condition) and extrinsic (e.g., habitat structure) factors. We predicted that the probability of survival during the post-fledging period would be greater for birds (1) using dense understory vegetation, (2) in better condition at the time of fledging, (3) from nests with smaller broods, (4) from nests not parasitized by the Brown-headed Cowbird (Molothrus ater), (5) fledged earlier in the season, and (6) in closer proximity to regenerating clearcuts (i.e., access to early-successional habitat). METHODS Study sites
From 2004 to 2007, we studied fledgling ecology within the Ohio Hills physiographic province in southeast Ohio (Athens and Vinton counties). The region is characterized by rolling forested hills (approximately 70% forest cover) perforated by regenerating clearcuts; nonforest land (i.e., small towns and agriculture) occurs in some valleys. Trees common within these forests include the yellow-poplar (Liriodendron tulipifera), white (Quercus alba) and red (Q. rubra) oaks, red (Acer rubrum) and sugar (A. saccharum) maples, hickory
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(Carya spp.), and white ash (Fraxinus americana). Potential predators common at study sites include the eastern chipmunk (Tamias striatus), gray squirrel (Sciurus carolinensis), southern flying squirrel (Glaucomys volans), weasels (Mustela spp.), striped skunk (Mephitis mephitis), Virginia opossum (Didelphis virginiana), raccoon (Procyon lotor), red fox (Vulpes vulpes), Blue Jay (Cyanocitta cristata), Broad-winged (Buteo platypterus) and Cooper’s (Accipiter cooperii) Hawks, and black rat snake (Elaphe obsoleta). We radio-tagged Ovenbirds and Worm-eating Warblers at seven sites within Zaleski State Forest, Lake Hope State Park, and MeadWestvaco forest lands. We randomly selected study sites from all identified potential sites after taking into account criteria for the study, (i.e., deciduous forest, proximity to a clearcut, spatial separation among sites). Each study plot was 16 ha in size, separated by ≥4 km, and composed of mature forest (75–110 years old). We located four sites adjacent to a recent clearcut (≤10 years old, 800 m from regenerating forest clearcut ≤15 yr previously). We radio-tagged similar numbers of fledglings at sites adjacent to a clearcut and at sites surrounded by mature forest. Study organisms
We focused on the Ovenbird and Worm-eating Warbler because both are considered forest-interior species (Van Horn and Donovan 1994, Hanners and Patton 1998) and use regenerating clearcuts frequently during the post-fledging period (Vitz and Rodewald 2006). Using behavioral cues from adults, we located Ovenbird and Worm-eating Warbler nests and monitored them every 3–5 days, increasing to every 1 or 2 days as fledging approached. On the day of fledging, we randomly selected one nestling from each nest (or located near the nest if the nestlings had fledged earlier that day) and fitted it with a radio-transmitter with a figure-8 leg-loop harness made of a cotton/nylon blend (Rappole and Tipton 1991). Because members of the same family group are not independent, we radio-tagged a single nestling per nest (Garton et al. 2001). We considered fledglings within the same study site independent because they frequently moved out of the site, and radiotagged individuals were not found moving together (Vitz and Rodewald 2010). We weighed, measured the tarsus of, and banded each radio-tagged bird with a U.S. Geological Survey aluminum band. We fitted Ovenbirds with transmitters that were designed to function for 6 weeks and weighed approximately 5.9% of their fledging and 4.6% of their adult mass (0.90 g, BD-2, Holohil Systems, Ltd.). Radios used on Wormeating Warblers were designed to last 4 weeks and weighed 4.8% and 4.2% of their fledging and adult mass, respectively (0.55 g, BD-2N, Holohil Systems, Ltd.). Behavioral observations of radio-tagged birds suggested that transmitters did not influence fledglings’ behavior or survival, as tagged and nontagged individuals within the same brood behaved similarly
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and generally remained motionless as they waited to be fed (A. Vitz, pers. obs.). Radio-tracking
We located each radio-tagged individual daily by the homing method, approaching the bird on foot until we sighted it (White and Garrott 1990). We tracked birds with ATS R2000 receivers (Advanced Telemetry Systems, Inc.) and threeelement folding Yagi antennas. The transmitters’ range was generally 300-900 m but varied with the terrain. The number of locations per individual bird ranged from 1 to 51 for the Ovenbird (mean = 27.3, se = 2.07) and 2 to 31 for the Wormeating Warbler (mean = 15.95, se = 1.11). Once we found a bird, we identified it as alive or dead and recorded its location with a WAAS-enabled GPS unit (Magellan Meridian). We recorded an individual as dead not only if a carcass was located but if we located the transmitter next to feather remains, within feces, with tooth or beak impressions, or tracked it to a predator. If we could not locate an individual for several days and did not expect the battery to fail, we flew an Ohio Division of Natural Resources airplane (Partenavia P68) wired for radio telemetry over the study region to search for “lost” individuals. Because of time constraints, we measured vegetation characteristics of the microhabitat at bird locations and paired random locations every other day. Random locations were located 50 m from actual bird locations in a random direction. We used a separation of 50 m between actual and random points because the forest contained numerous canopy gaps, riparian areas, and edge habitat that vary the understory’s characteristics on a fine scale (Vitz pers. obs.), rendering this distance far enough to represent a potentially different microhabitat but close enough for fledglings to reach it easily (Vitz and Rodewald 2010). Further support for a 50-m distance was a lack of strong correlation between vegetation metrics from the two samples (correlation coefficients 38 cm dbh) trees and visually estimated percent canopy cover. We believe our values for canopy cover reflect biological differences accurately because observers received extensive training in estimating canopy cover. Furthermore, we were most interested in relative differences between birds’ and random locations rather than absolute measures of canopy cover. For each individual, we averaged data from all environmental variables, considering the individual (rather than the daily location) the appropriate replicate (Garton et al. 2001). Because we also were interested in differences between nesting and post-fledging habitat, we recorded the same vegetation measurements at each nest where a nestling had been radio-tagged.
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Statistical analyses
For both analyses, we square-root-transformed habitat variables to meet the assumptions of normality. We used a discriminant function analysis to examine differences in vegetation characteristics (number of woody stems, canopy cover, and number of small, medium, and large trees) between fledgling and random locations, as well as between fledgling and nest locations. We did not measure vegetation at random locations in 2004 so that year dropped the birds’ locations from the analysis involving random locations. Because we recorded no habitat data on fledglings that died during the first 24 hr, we removed these individuals from both analyses. We tested for annual differences in each habitat variable (separately for birds’ and random locations). Because we found no evidence for annual differences (P > 0.05, Proc Glm, SAS Institute 1990), we did not include year as a factor in this analysis. We estimated daily and cumulative survival for both species with known-fate models in program MARK (White and Burnham 1999). Though similar to the Kaplan–Meier product-limit estimator, known-fate models are preferable because they allow for the inclusion of covariates. We examined whether survival varied within or between years and used biologically meaningful covariates to build a priori models that we evaluated and ranked in an AIC framework. Covariates used in the models included season of fledging as early (0 reflects a mass greater than expected for a certain body size (i.e., good condition), whereas a residual
