Effect of radio transmitters on return rates of Swainson's Warblers

Journal฀of฀Field฀Ornithology J. Field Ornithol. 80(2):206–211, 2009 DOI: 10.1111/j.1557-9263.2009.00210.x Effect of radio transmitters on return rates of Swainson’s Warblers Nicholas M. Anich,1 Thomas J. Benson,2 and James C. Bednarz Department of Biological Sciences, Arkansas State University, Jonesboro, Arkansas 72467, USA Received 1 July 2008; accepted 22 October 2008 ABSTRACT. Although radio telemetry can provide useful information, the possible negative effects of transmitters, including biased results and detrimental effects on the birds, must also be considered. Several investigators have examined the effects of transmitters on larger birds, but few have examined their possible long-term effects, as assessed by return rates, on small passerines. We examined the possible negative effects of transmitters on a small (15.5 g) passerine. We used glue to attach radio transmitters to 40 male Swainson’s Warblers (Limnothlypis swainsonii) at two study sites in Arkansas in 2005 and 2006. To assess possible effects of transmitters on survival, we compared the following-year return rates of radio-tagged birds to birds that were captured and color banded, but did not receive transmitters. Least squared mean return rates for all birds ranged between 0.46 and 0.57. We found no significant difference in the following-year return rate of radio-tagged and nonradio-tagged birds. In addition, we found no significant differences in return rates between study sites or years, and no significant interactions between any combination of radio transmitter (radio tagged or not), site, and year. Our results indicate that attachment of transmitters did not affect return rates of male Swainson’s Warblers and that the glue-on technique is a good option for short-term studies of small passerines. RESUMEN. Efecto de radiotransmisores en las tasas de retorno de individuos de Limnothlypis swainsonii Aunque la radiotelemetrı́a puede proveer de información de utilidad, el posible efecto negativo de estos, incluyendo sesgo en la información y efecto detrimental, también debe tomarse en consideración. Varios investigadores han examinado los efectos de radiotransmisores en aves grandes, pero pocos han examinado la posibilidad de efectos a largo alcance, evaluando las tasas de retorno de individuos, en paserinos pequeños. Examinamos el posible efecto negativo de los radiotransmisores en aves pequeñas (15.5 g). Durante el 2005 y el 2006 en investigaciones realizadas en dos sitios de estudio en Arkansas, usamos pegamento para adherir radiotransmisores a 40 machos de la Reinita de Swainson (Limnothlypis swainsonii). Para determinar el posible efecto de los radiotransmisores en la sobrevivencia, comparamos la tasa de retorno de individuos a los cuales se les colocaron transmisores con individuos capturados y marcados con anillos de colores. Las tasas promedio de retorno para todas las aves variaron entre 0.46 y 0.57. No encontramos diferencia significativa en la tasa de retorno, durante el primer año, entre ambos grupos estudiados. Además, no encontramos diferencia significativa en la tasa de retorno entre individuos en diferentes años o de diferentes localidades y ninguna interacción significativa en ninguna combinación de aves con transmisores, localidad del estudio o año de trabajo. Nuestros resultados indican que el uso de radiotransmisores no altera la tasa de retorno en el ave estudiada, y que la técnica de utilizar pegamento para anclar los radiotransmisores es una buena opción para estudios de corto alcance con paseriformes pequeños. Key words: Limnothlypis swainsonii, return rate, survival, Swainson’s Warbler, telemetry, transmitter attachment, transmitter effect Radio telemetry can provide valuable information about ranging patterns, habitat use, behavior, physiology, and demography (Millspaugh and Marzluff 2001). However, evaluating the possible negative effects of transmitters is also important, particularly for species 1 Corresponding author. Current address: 13210 Rose Garden Lane, Durham, NC 27707, USA. Email: nicholas.m.anich@gmail.com 2 Current address: Illinois Natural History Survey, 1816 South Oak Street, Champaign, IL 61820, USA.  C 2009 of conservation concern. Although some studies indicate that transmitters have few or no apparent effects (Neudorf and Pitcher 1997, Powell et al. 1998, Johnson et al. 2001), others have revealed negative effects, including behavioral changes (Vaughn and Morgan 1992, Pietz et al. 1993), temporary increases in levels of stress hormones (Suedkamp Wells et al. 2003), reduced breeding success (Croll et al. 1996, Ackerman et al. 2004), as well as injuries and probable mortality (Paton et al. 1991, Dougill et al. 2000). Withey et al. (2001) reviewed 96 papers published between 1972 and 2000 that focused on C 2009 Association of Field Ornithologists The Author(s). Journal compilation  206 Vol. 80, No. 2 Effect of Transmitters on Swainson’s Warblers effects of transmitters on animals and only four involved passerines. We searched ornithological and wildlife journals published from 2000 to 2008 in North America and found five additional studies of the effects of transmitters on passerines. Most of these studies focused on possible short-term effects and some of these effects have been negative, including loss of mass by Common Yellowthroats (Geothlypis trichas) with harness-attached dummy transmitters (8.6% of body mass; Sykes et al. 1990) and short-term increases in stress responses in Dickcissels (Spiza americana; Suedkamp Wells et al. 2003). Transmitters likely caused some mortality of juvenile Louisiana Waterthrushes (Seiurus motacilla; Mattsson et al. 2006) and, due to entanglement of transmitter antennas, of Palilas (Loxioides bailleui; Dougill et al. 2000). Other investigators have reported no apparent short-term effects of transmitter attachment on songbirds (Neudorf and Pitcher 1997, Powell et al. 2000, Woolnough et al. 2004, Davis et al. 2008). Clearly, the effects of transmitter attachment may vary among species, as well as with method of transmitter attachment and transmitter weight, but, at least for some species of songbirds, transmitter attachment appears to have little or no short-term effect. However, little is known about the possible long-term effects of transmitter attachment. By focusing on short-term effects, possible longer-term impacts, for example, on annual survival, may be missed. To our knowledge, only one study has focused on the possible longterm effects of transmitters on passerines. In a study of Wood Thrushes (Hylocichla mustelina), Powell et al. (1998) found that harness-mounted transmitters (4% body mass) had no impact on following-year return rates. However, the number of recaptured birds was relatively small (13 of 110 radio-tagged birds and 12 of 173 banded-only birds). Thus, additional study of the possible long-term effects of transmitter attachment on small passerines is needed. Swainson’s Warblers (Limnothlypis swainsonii) are ground foragers that inhabit dense understory vegetation in forests (Brown and Dickson 1994), and have larger home ranges than most warblers (mean = 9.38 ha, range = 1.55– 30.75 ha, N = 37; Anich 2008). Although these large home ranges could impose additional energetic costs for Swainson’s Warblers with 207 radio transmitters, we attached transmitters to Swainson’s Warblers and observed no apparent negative effects during the tracking period. However, migratory birds can be affected by sublethal negative impacts that carry over to the next season (Norris and Taylor 2006, Norris and Marra 2007). Thus, transmitters may have had negative effects that carried over during migration and the wintering period, resulting in reduced survivorship. To examine this possibility, we compared return rates of radio-tagged Swainson’s Warblers to those of Swainson’s Warblers that were captured and color banded, but not radio tagged. METHODS Study sites. We radio-tracked Swainson’s Warblers at two sites that differed in vegetation and topography: St. Francis National Forest (SFNF) and the Alligator Lake area of White River National Wildlife Refuge (WRNWR; Bednarz et al. 2005, Anich 2008). SFNF is a 9150-ha forest where Swainson’s Warblers are found in bottomland hardwood forest habitat and in valleys in upland deciduous forest. The understory at this site was dominated by shrubs and vines, and giant cane (Arundinaria gigantea) occurred in only a few areas. WRNWR is among the largest tracts of contiguous bottomland hardwood forest remaining in the Mississippi Alluvial Valley (approximately 65,000 ha; Gardiner and Oliver 2005). The study area near Alligator Lake consisted entirely of bottomland hardwood forest, with patchily distributed understory vegetation. In some locations, cane, shrub thickets, and vines created a nearly impenetrable patch of understory; in other locations, the understory was sparse. Telemetry methods. In 2005 and 2006, we captured and fitted male Swainson’s Warblers with transmitters to examine home-range characteristics and patterns of habitat use. We used a compact-disc player to broadcast Swainson’s Warbler songs and call notes to attract and capture males in mist nets. Birds were banded with an aluminum U.S. Geological Survey band and a unique combination of three color bands. We used 0.42-g radio transmitters (LB-2N, Holohil Systems, Carp, Ontario, Canada) that we waterproofed with rubberized plastic coating, and attached a small piece of chiffon to the 208 N. M. Anich et al. underside using cyanoacrylate glue (Sykes et al. 1990). The transmitter antenna was a single stainless steel wire 0.18 mm in diameter and 14 cm long. Total weight of the transmitter, rubberizer, chiffon, and glue was approximately 0.51 g, or 2.9–3.6% of the body mass of the birds we tracked (mean = 15.5 g, SE = 0.1, range = 14.3–17.4 g, N = 35). We clipped a small patch of feathers off the interscapular region of the bird, cleaned the area with acetone, and attached the transmitter to the feather stubble using cyanoacrylate glue (Johnson et al. 1991). To ensure the glue had dried, we held the transmitter gently in place on the bird for 5 min before release. We tracked radio-tagged birds by homing to radio signals using H-antennas (RA-14 and RA2A, Telonics, Inc., Mesa, Arizona) and receivers (R-1000, Communications Specialists Inc., Orange, California). We tracked birds until we either obtained 50 locations for each bird or the transmitter fell off the bird or failed. We tracked birds between 27 April and 3 July 2005 and 2006. Each bird was tracked for 5–13 d (mean = 8.5 d), between 05:00 and 18:00 CST, and locations were determined at intervals ≥20 min. We also attached transmitters to two females, but excluded them from our analysis due to unknown site fidelity and reduced detectability. Surveying for returning birds. Both study sites were surveyed for Swainson’s Warblers in 2006 and 2007, the years following radio tracking. Occupied sites were visited at least three times during the breeding season to check for occupancy, and many were visited weekly. Because our study was part of a larger project (Benson 2008) that involved a mark–resight– recapture component, nest searching, and the broadcast of Swainson’s Warbler songs to detect birds, we believe that most birds returning to the areas surveyed were detected. Statistical methods. We used generalized linear mixed models to investigate effects of transmitters (transmitter or no transmitter), site (SFNF and WRNWR), year (2005–2006 and 2006–2007), and interactions among these variables on return rate (PROC GLIMMIX, SAS Institute 2003; Littell et al. 2006). The mixed model allowed us to perform a logistic regression while accounting for nonindependence caused by some birds being present at the site for both time periods. J. Field Ornithol. RESULTS We detected 55 male Swainson’s Warblers in 2005 (16 at SFNF and 39 at WRNWR) and 71 in 2006 (29 at SFNF and 42 at WRNWR), and attached radio transmitters to 40 males (7 at SFNF in 2005, 10 at WRNWR in 2005, 11 at SFNF in 2006, and 12 at WRNWR in 2006). We obtained 1851 locations of 37 radio-tagged birds. Because of transmitter failure, we did not obtain enough locations to estimate home-range sizes for three birds. However, we included these males in our analysis of return rates. Least squared mean return rates ranged between 0.46 and 0.57 (Fig. 1). We found no significant difference in the following-year return rate of radio-tracked birds and nonradio-tracked birds (Table 1). We also found no differences in return rates between either study sites (SFNF and WRNWR) or years (2005–2006, 2006– 2007), and there were no significant interactions among radio transmitter, site, and year. Observed return rates were 57.5% (23 of 40) for birds with transmitters, 50% (43 of 86) for birds without transmitters, 46.7% (21 of 45 birds) at SFNF, 55.6% (45 of 81 birds) at WRNWR, 52.7% (29 of 55 birds) between 2005 and 2006, and 52.1% (37 of 71 birds) between 2006 and 2007. DISCUSSION We found that transmitter attachment and subsequent radio tracking had no effect on the return rates of Swainson’s Warblers. Similarly, Cardinal (2005) found that return rates of radiotracked Willow Flycatchers (65%; Empidonax traillii) were similar to those of nonradioed birds (53–69%). Powell et al. (1998) reported no difference between radio-tagged and nonradiotagged Wood Thrushes in the number recaptured in following years. However, Kirtland’s Warblers (Dendroica kirtlandii) with transmitters returned to breeding grounds the next year at a significantly lower rate than birds without transmitters (Samuel and Fuller 1996, P. Sykes, pers. comm.). The use of relatively heavy transmitters (about 8% of warbler body mass) may have contributed to the decreased survival of Kirtland’s Warblers (Sykes et al. 1990, P. Sykes, pers. comm.). In contrast, the Willow Flycatchers (Cardinal 2005) and Wood Thrushes (Powell et al. 1998) carried transmitters that were Vol. 80, No. 2 Effect of Transmitters on Swainson’s Warblers 209 Fig. 1. Return rates of Swainson’s Warblers to their breeding grounds in eastern Arkansas. Least squared means and 95% confidence intervals from a generalized linear mixed (logistic regression) model show no effect of radio transmitters (0.51-g transmitter package was attached or not), study site (St. Francis National Forest [SFNF] and White River National Wildlife Refuge [WRNWR]), or year (2005–2006 and 2006– 2007). Numbers above bars indicate the number of birds that could have potentially returned and had their color-bands re-sighted. approximately 4% body mass. Clearly, additional study is needed before any conclusions can be drawn concerning possible negative impacts of transmitters on small songbirds. For studies that involve the use of transmitters on songbirds, we recommend that investigators attempt to relocate birds the following year to determine Table 1. Effects from generalized linear mixed (logistic regression) models used to predict the return of Swainson’s Warblers to two breeding sites in eastern Arkansas. Effect df F P Transmittera 1,29 0.6 0.44 Siteb 1,30 0.9 0.35 1,29 0.0 0.95 Yearc Transmitter × site 1,28 0.1 0.77 Transmitter × year 1,27 0.4 0.52 Site × year 1,28 1.5 0.24 Transmitter × site × year 1,24 0.3 0.62 a 0.51-g transmitter package was attached and bird was radio tracked, or no transmitter was attached. b St. Francis National Forest or White River National Wildlife Refuge. c 2005–2006 or 2006–2007. if carry-over effects of transmitters might affect survival. We assumed that detectability was similar for radioed and nonradioed Swainson’s Warblers, and assuming uniformly high detectability allowed us to infer that return rates were a good index of survival. However, if detectability differed, several factors could introduce bias (Martin et al. 1995). For example, we may have been better able to detect radio-tracked birds the year following instrumentation because their territories were delineated more accurately and most Swainson’s Warblers exhibit breeding site fidelity and have relatively stable territories. However, we searched most suitable habitat, with the use of audio playbacks if needed, and believe we encountered most birds present during the breeding season. A bias toward lower detectability of radioed birds was also possible if transmitter attachment caused birds to become more wary of humans or shift locations. However, based on our observations, this seems unlikely. Some birds with transmitters had been studied for 2 yr prior to radio tagging, and their territories were spot mapped. After attaching transmitters, they continued using the same 210 N. M. Anich et al. areas and continued to sing near capture sites, suggesting that capture and transmitter attachment did not alter space use. In general, our results indicate that detectability of breeding males was high. Analysis of a larger sample of male Swainson’s Warblers (N = 191) revealed only three instances where marked individuals were not detected one year, but were subsequently detected in following years. The estimated resight– recapture probability was 0.88, with no evidence of a difference in resight–recapture probability between years or locations (Benson 2008). We did not observe any obvious effects of transmitters on warbler behavior, including flying, singing, foraging, and parental care. Two females with transmitters built nests, incubated eggs, and brooded young, and one of these females fledged young from a first nest and started a second nest later in the season. In addition, a male was recaptured 1 mo after transmitter attachment and we noted only a small bare patch on the back where some feathers attached to the transmitter had been lost. There were no signs of abrasion or other skin damage. On one occasion, a male entangled the transmitter antenna around debris on the ground and he was observed flapping on the ground, apparently unable to fly. When approached, the bird flapped more vigorously and flew off, leaving the transmitter and a few scapular feathers behind. Audio playback (about 1 h later) confirmed that the bird was alive and apparently unharmed, and he returned to the same territory the following year. Other researchers studying small passerines have also reported occasional antenna entanglement (Allen and Sweeney 1989, Pitts 1995, Logue 2007). Dougill et al. (2000) reported a bulbous-tipped, near-vertically hanging limp antenna wire caused entanglement for multiple Palilas and recommended avoiding limp, bulbous-tipped antennas, and keeping antenna length ≤16 cm. Although the antennas on our transmitters were stiff and not bulbous tipped, we suspect a shorter wire would be less likely to cause entanglement. Our study design was focused on obtaining 50 locations in a relatively short period of time. Thus, we did not need radios to remain attached to birds for >2 weeks and the glue-on technique we used met the needs of our study. Mong and Sandercock (2007) suggested that attaching transmitters with glue may have fewer J. Field Ornithol. potential negative effects on birds than harness attachment. Although results may vary among species, based on our success with this technique and the absence of any apparent adverse impacts on male Swainson’s Warblers, we advocate the use of the glue-on method whenever possible. ACKNOWLEDGMENTS Funding for this research was provided by the Arkansas Game and Fish Commission and U.S. Fish and Wildlife Service (USFWS) through a State Wildlife Grant. Additional funds were received from a cost-share program with USFWS and Arkansas State University. We thank C. Rideout for continued support of this research, and L. Barnhill for helping to secure initial funding. R. Hines, C. Hunter, S. Reagan, and J. Everitts provided invaluable assistance. B. Mattsson, T. Risch, G. Ritchison, P. Spaeth, and two anonymous reviewers offered comments that improved this manuscript. We are grateful to K. Ballantyne, J. Brown, E. Huskinson, K. Jones, B. Paterson, C. Roa, J. Sardell, and W. Edwards for their hard work and enthusiasm in the field. LITERATURE CITED ACKERMAN, J. T., J. ADAMS, J. Y. TAKEKAWA, H. R. CARTER, D. L. WHITWORTH, S. H. NEWMAN, R. T. GOLIGHTLY, AND D. L. ORTHMEYER. 2004. Effects of radiotransmitters on the reproductive performance of Cassin’s Auklets. 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