Journal of Tropical Ecology (2006) 22:385–395. Copyright © 2006 Cambridge University Press doi:10.1017/S0266467406003191 Printed in the United Kingdom Seasonal shift in the foraging niche of a tropical avian resident: resource competition at work? Julie A. Jedlicka∗1, Russell Greenberg†, Ivette Perfecto‡, Stacy M. Philpott†1 and Thomas V. Dietsch‡# ∗ Department of Ecology and Evolutionary Biology, University of Michigan, 830 N. University Ave., Ann Arbor, MI 48104, USA † Smithsonian Migratory Bird Center, National Zoological Park, 3001 Connecticut Ave NW, Washington DC 20008, USA ‡ School of Natural Resources and the Environment, University of Michigan, 430 E. University, Ann Arbor, MI 48109, USA # Center for Tropical Research, University of California, La Kretz Hall, Suite 300, Box 951496, Los Angeles, CA 90095-1496, USA (Accepted 23 December 2005) Abstract: This study examined the foraging behaviour of a resident bird species, the rufous-capped warbler (RCWA, Basileuterus rufifrons), in a shaded-coffee farm in Chiapas, Mexico. Unlike many resident species that use shaded-coffee agroecosystems seasonally, RCWAs do not move to other habitats when migrants are present. RCWA foraging was compared when migrant birds were present (dry season) and absent (wet season). It was hypothesized that RCWAs would exhibit a seasonal foraging niche shift because of resource competition with migrants. Observations from both the canopy and coffee understorey show that RCWAs foraged almost equally in both vegetative layers during the wet season although they were more successful foraging in the canopy. In the dry season, migrants foraged primarily in the canopy and RCWAs shifted so that 80% of RCWA foraging manoeuvres were in the understorey. At that time RCWAs foraged less successfully in both vegetative layers. Avian predation in the dry season was found to reduce densities of arthropods by 47–79% in the canopy, as opposed to 4–5% in the understorey. In the canopy, availability of large (>5 mm in length) arthropods decreased by 58% from the wet to dry season. Such resource reductions could have caused the RCWA foraging niche shift yet other alternative or additional hypotheses are discussed. Shifts in foraging niche may be a widespread mechanism for some small insectivorous residents to avoid seasonal competition with abundant migrant species. Resumen: Este estudio examino´ el comportamiento de forrajeo de un ave residente, Basileuterus rufifrons (RCWA), en una finca de cafe´ con sombra en Chiapas, Mexico.´ A diferencia de muchas aves residentes que usan los agroecosistemas de cafe´ con sombra solamante durante una estacion,´ RCWAs no se van a otros habitats´ cuando los aves migrantes estan´ presente. El forrajeo de RCWA fue comparado cuando las aves migrantes eran presente (la epoca´ seca) y ausente (la epoca´ de lluvia). La hipotesis´ fue que los RCWA exhibir´ıan un cambio de forrajeo con los cambios de estaciones a causa de la competencia de recursos con los migrantes. Observaciones en el docel y el sotobosque en un cafetal muestran que durante la epoca´ de lluvia, los RCWAs forrajean igualmente en los dos niveles de vegetacion,´ pero tienen mas´ exito´ forrajeando en el docel. Durante la epoca´ seca, los migrantes forrajean principalmente en el docel y los RCWAs se mueven al sotobosque donde efectuan´ 80% de las maniobras de forrajeo. Durante ese tiempo, los RCWAs tuvieron menos exito´ forrajeando tanto en el docel como en el sotobosque. Durante la epoca´ seca el numero´ de artropodos´ bajo´ entre 47–79% en el docel y entre 4–5% en el sotobosque a causa de la depredacion´ de las aves. En el docel, la disponibilidad de artropodos´ grandes (>5 mm en longitud) bajo´ en un 58% de la epoca´ de lluvia a la epoca´ seca. Tales reducciones de recursos podr´ıan causar el cambio de lugar a forrajeo observado en RCWA, pero otras explicaciones e hipotesis´ son discutidas. Pueden ser que este cambio de forrajeo sea comun´ en algunas aves residentes pequenas˜ que comen artropodos´ para evitar la competencia con los migrantes durante la epoca´ seca. Key Words: avian insectivory, birds, breeding demand, Chiapas, Mexico, coffee agroecosystem, direct interference, foraging, indirect competition, migrant–resident interaction, resource competition, rufous-capped warbler, sexual segregation, shaded coffee INTRODUCTION 1 Corresponding author. Current address: Environmental Studies Department, University of California, 1156 High Street, Santa Cruz, Each year millions of insectivorous birds arrive in tropical CA 95064, USA. Email: [email protected] habitats during what is often the resource-poor time 386 JULIE A. JEDLICKA ET AL. of year. Not surprisingly, avian ecologists have focused should be reduced where migrants primarily forage and considerable effort on understanding the mechanisms by (2) resident birds should forage less successfully at those which migratory and resident populations are able to locations once migrants are present. One would expect coexist (Greenberg 1995, Johnson et al. 2005, Rappole & that with time after arrival, migrants would also forage Warner 1980, Ricklefs 1989). The interaction of the two less successfully due to resource depletion, but testing this avifaunas may be connected to the population dynamics prediction is outside the realm of this study. of organisms at lower trophic levels in tropical areas. A Shade-grown coffee farms in the Americas usually large annual influx of insectivores with no detectable support a few resident bird species and a large number competitive effects would suggest that predator–prey of insectivorous neotropical migrant species that arrive populations are not tightly coupled. Studies examining in high densities around the beginning of the wet season the foraging behaviour of overwintering migrant birds in (Calvo & Blake 1998, Greenberg et al. 1994, 1997a,b; tropical habitats are common (see Greenberg 1986 for Johnson 2000, Johnson & Sherry 2001, Perfecto et al. a review), yet surprisingly few have investigated how 2003, Petit et al. 1993, Robbins et al. 1992, Vannini foraging behaviour of tropical residents is affected by 1994, Willis 1966, Wunderle 1999, Wunderle & Latta migrant presence. Previous comparative studies showed 1996, Wunderle & Waide 1993). Because shaded-coffee that many resident birds were specialists and required farms support such large densities of migrants and more specific microhabitat conditions than did migrant because the vegetation is oftentimes heavily pruned, species (for review see Greenberg 1986, Herrera 1978, eliminating much of the vegetative complexity found Leisler 1990, Remsen & Parker 1983, Stotz et al. in a tropical forest, it is a comparatively easier system 1996). Accordingly, most migrants have been thought to look for shifts in foraging niche. This study was to avoid resource competition while in the tropics by conducted on a shaded-coffee farm where the rufous- foraging for resources underused by resident species capped warbler (RCWA, Basileuterus rufifrons) was the and underrepresented in their foraging microhabitats numerically dominant foliage-gleaning resident species. (Herrera 1978, Lack 1986, for review see Leisler 1990, Additionally the RCWA was chosen for this study because 1992; O’Connor 1981). However data from many studies it spends a considerable amount of time foraging in the fail to support these hypotheses (Johnson & Sherry coffee understorey. Unlike many resident species that 2001, Lack 1986, Post 1978, Strong 2000) and these use shaded-coffee agroecosystems seasonally, RCWAs hypotheses are now recognized as overly simplistic maintain territories year-round and do not move to (Greenberg pers. comm.). other habitats when migrants are present (Dietsch The four studies that examine how tropical residents 2003). They breed during April–July when migrants forage before and after migrant arrival found that while are largely absent (Stiles & Skutch 1989). By observing a majority of resident species did not appear to change both foraging behaviour and location of RCWA in the their foraging behaviour upon migrant arrival, a few wet and dry season, this study tests the hypothesis that generalist resident species exhibited seasonal shifts in rufous-capped warblers exhibit a seasonal foraging niche foraging niche (Chipley 1976, Rabøl 1987, 1993; Waide shift. Also, by coupling these data with measurements 1981). In these cases, certain resident bird species were of arthropod abundance and migrant presence, this found to forage higher in the vegetative strata (in study tests the hypothesis that the seasonal niche more favourable or preferred locations) when migrants shift is caused by resource competition from migrants. were absent than when they were present. Contrary to Alternative hypotheses for this seasonal niche shift, such conventional wisdom that resident birds out-compete as climatic and seasonal effects, breeding demand, sexual migrants, these resident resource generalists may shift segregation, predator avoidance and direct interference to under-exploited niches to avoid resource competition hypotheses are also examined in the discussion. when migrants are very abundant. The resource competition hypothesis is commonly offered as a possible explanation for this behavioural shift METHODS (Chipley 1976, for review see Leisler 1990, 1992; Rabøl 1987, Waide 1981). When migrants arrive they Overview of study site consume large numbers of arthropods in the canopy, and the resource competition hypothesis holds that this This research was conducted in Finca