The Auk 124(4):1359–1372, 2007 © The American Ornithologists’ Union, 2007. Printed in USA. MODELING POPULATION GROWTH OF THE OVENBIRD (SEIURUS AUROCAPILLA) IN THE SOUTHERN APPALACHIANS Adrei L. Podolsky, Theodore R. Simons,1 and Jaime A. Collazo U.S. Geological Survey, North Carolina Cooperative Fish and Wildlife Research Unit, Department of Zoology, North Carolina State University, Raleigh, North Carolina 27695, USA Abstract.—Studies of source–sink dynamics are oft en prompted by concerns about negative population trends. Estimates of population trajectories are usually based on assumptions about survival rates and empirical measures of fecundity. Most models ignore the infl uence of the rates of renesting and multiple brooding. We used the Ovenbird (Seiurus aurocapilla) as a model Neotropical migratory songbird species to investigate the relative eff ects of annual female survival and components of annual fecundity on population growth rates. We applied productivity data from a three- year fi eld study and data from Hann (1937) to several models of annual fecundity to examine the sensitivity of lambda to variations in annual female survival and the likelihood of renesting and double-brooding. Our simulations illustrate the impor- tance of incorporating estimates of annual survival and rates of additional breeding att empts in songbird population models because population growth rates are quite sensitive to variations in these parameters. Lambda is especially sensitive to survival estimates and changes with them at the same order of magnitude. Whenever feasible, female survival and probabilities of additional breeding att empts should be estimated by direct methods. The indirect methods used in our study (annual female survival estimated from the age ratio of breeding females, and rates of renesting and double- brooding determined from the timing of reproduction) probably underestimated these parameters. Received 27 September 2005, accepted 21 December 2006. Key words: annual fecundity, annual survival, double-brooding, Ovenbird, popu- lation growth models, renesting, Seiurus aurocapilla. Modelado del Crecimiento Poblacional de Seiurus aurocapilla en el Sur de los Apalaches Resmen.—Frecuentemente, los estudios de la dinámica de fuentes y sumideros son motivados por preocupaciones relacionadas con tendencias poblacionales negativas. Las estimaciones de las trayectorias poblacionales usualmente están basadas en suposiciones acerca de las tasas de supervivencia y en mediciones empíricas de la fecundidad. La mayoría de los modelos ignoran la infl uencia de las tasas de renidifi cación y de las nidadas múltiples. Utilizamos a Seiurus aurocapilla como un modelo de una especie de ave canora migratoria Neotropical para investigar los efectos relativos de la supervivencia anual de las hembras y de componentes de la fecundidad anual sobre las tasas de crecimiento poblacional. Aplicamos datos de productividad de un estudio de campo de tres años y datos obtenidos de Hann (1937) a varios modelos de fecundidad anual para examinar la sensibilidad de lambda ante variaciones en la supervivencia anual de las hembras y en la probabilidad de renidifi car y de tener nidadas dobles. Nuestras simulaciones ilustran la importancia de incorporar estimados de la supervivencia anual y de las tasas a las que tienen lugar intentos adicionales de reproducción en los modelos de poblaciones de aves canoras, pues las tasas de crecimiento poblacional son bastante 1Address correspondence to this author. E-mail: [email protected] 1359 1360 Podolsky, Simons, and Collazo [Auk, Vol. 124 sensibles a variaciones en esos parámetros. Lambda es especialmente sensible a los estimados de supervivencia, y cambia con éstos en el mismo orden de magnitud. Siempre que sea posible, la supervivencia de las hembras y las probabilidades de intentos de reproducción adicionales deben ser estimadas mediante métodos directos. Los métodos indirectos empleados en nuestro estudio (la supervivencia anual de las hembras fue estimada a partir del cociente de edades de las hembras reproductivas y las tasas de renidifi cación y de nidadas dobles a partir del momento de reproducción) probablemente subestimaron esos párametros. Polation declines observed in Neotropical att empts (Pease and Grzybowski 1995, Powell et migratory landbirds in eastern North America al. 1999, Grzybowski and Pease 2005), the rela- are att ributed primarily to habitat fragmentation, tionship between clutch size and annual fecun- higher rates of predation, and brood parasitism dity (Flaspohler et al. 2001; Farnsworth and (Wilcove 1985, Robbins et al. 1989, Terborgh Simons 2001, 2005), and annual survival rates of 1992, Donovan et al. 1997, Askins 2000). These females (Temple and Cary 1988, Faaborg et al. fi ndings have stimulated many studies of bird 1998, Burke and Nol 2000, Simons et al. 2000). reproductive success and source–sink dynam- We examined the importance of annual ics in fragmented versus contiguous habitats female survival and rates of renesting and (Villard et al. 1992, Faaborg et al. 1995, Manolis double-brooding in models of songbird popu- et al. 2000a, Flaspohler et al. 2001, Murphy 2001, lation growth. We used the Ovenbird (Seiurus Podolsky 2003). Of 356 studies of avian repro- aurocapilla) because it is a common model ductive success published from 1984 to 1997 for songbird source–sink relationships. The in nine major European and North American Ovenbird is generally considered a single- peer-reviewed journals, 54% did not distinguish brooded species (Van Horn and Donovan between nest success and productivity, less than 1994), though there is a single report of three half considered renesting and second broods in cases of double-brooding (Zach and Falls multibrooded species, and only 10% properly 1976). In contrast to most published studies of estimated annual fecundity (Thompson et al. Ovenbird demographics (Gibbs and Faaborg 2001). Recent studies have shown that renesting 1990, Donovan et al. 1995b, King et al. 1996, aft er a nest failure and double-brooding, oft en Burke and Nol 1998, Porneluzi and Faaborg ignored by population-growth models, may 1999, Flaspohler et al. 2001, Manolis et al. 2002, account for ≤40% of annual fecundity in birds Matt sson and Niemi 2006), our research was (Murray 1991, 1992; Martin 1995; Schroeder 1997; conducted near the southern extent of the spe- Farnsworth and Simons 2001). Verhulst et al. cies’ range, where a longer breeding season may (1997) developed a model predicting the trade- provide greater opportunities for double-brood- off s between successive reproductive att empts. ing. Our objectives were to develop alternative Podolsky (2003) and Nagy and Holmes (2004, models of Ovenbird annual fecundity in Great 2005) found that failing to account for the con- Smoky Mountains National Park based on our tribution of renesting and double-brooding in fi eld estimates of nesting success and brood studies of avian demo g raphy can result in seri- size, and both observed and published esti- ous underestimates of annual fecundity, which mates of female survival, and rates of renesting could bias estimates of population growth rate, and double-brooding. We also wanted to assess source–sink dynamics, and population viability. how assumptions about these parameters infl u- Although model building in population ecol- ence estimated population growth rates. ogy always involves trade-off s among general- ity, realism, and precision (Levins 1966), limited Methods demographic data oft en impose a number of simplifying assumptions on source–sink mod- Modeling Aroach els of forest passerines. These include assump- tions about model parameters that are oft en Study area.—Great Smoky Mountains National diffi cult to estimate in the fi eld, such as disper- Park, established in 1934, is located along the sal (Nichols et al. 1981), the number of breeding North Carolina–Tennessee border. Our seven October 2007] Modeling Ovenbird Population Growth 1361 study sites, cumulatively covering >700 ha, et al. (1995b) and Burke and Nol (2000) assumed were located between Gatlinburg, Tennessee one renesting aft er failure, and Flaspohler et al. (N35°42’52”, W83°30’41”), and Waterville, North (2001) considered a 5–10% possibility of double- Carolina (N35°47’02”, W83°06’44”), within the brooding. Hann’s (1937) benchmark three- Gatlinburg, Mount Le Conte, Jones Cove, Mount year study of a marked Ovenbird population Guyote, Hartford, Waterville, Cove Creek Gap, reported up to fi ve unsuccessful consecutive and Luft e Knob U.S. Geological Survey quad- breeding att empts, but no reliable evidence of rangles. The sites support large continuous tracts double-brooding. We re-examined Hann’s data of mixed deciduous forest 75–100 years old, at and found clear evidence of no more than three elevations of 400–1,100 m. renesting att empts following an unsuccessful Annual fecundity and population growth fi rst nest (Table 1). Thus, estimates of lambda rate.—We defi ne annual fecundity (F) as the will vary according to assumptions about pr number of juvenile females produced annu- and pd. Below, we consider six basic scenarios ally per breeding female (Ricklefs 1973). In the grouped in two alternative models. simplest case, assuming 100% pairing success of Single-renesting–double-brooding model.—A mod- females, equal fl edgling sex ratio, and mono- ifi cation of Pulliam’s (1988) model to incorporate cyclic reproduction with no renesting aft er a renesting and double-brooding can be expressed as nest failure, annual fecundity can be computed from empirical estimates of the average fl edged λ = PA + PJ ½ [ps B + ps (1 – ps) pr B + ps pd ps B + brood size (B) and nesting success (ps) sensu ps pd ps (1 – ps) pr B] Mayfi eld (1975) as: F = ½ B ps. Hypothetically, = PA + PJ ½ B ps [1 + pr – ps pr + ps pd + females could also undertake several con- ps pd (1 – ps) pr] (1) secutive breeding att empts by renesting aft er previously failed nests and double-brooding This single-renesting–double-brooding (SRDB) aft er successful nests. We developed models of model (Fig. 1A) assumes that there are successful Ovenbird annual fecundity to explore how vari- (ps) and unsuccessful (1 – ps) fi rst nests.