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Use of Anthropogenic Nest Substrates by Crested Caracaras
James F. Dwyer, and Jeffrey P. Dalla Rosa

Southeastern Naturalist, Volume 14, Issue 1 (2015): N10–N15

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2015 Southeastern Naturalist Notes Vol. 14, No. 1 N10 J.F. Dwyer, and J.P. Dalla Rosa Use of Anthropogenic Nest Substrates by Crested Caracaras James F. Dwyer1,*, and Jeffrey P. Dalla Rosa2 Abstract - Caracara cheriway (Crested Caracara) typically nest in Sabal palmetto (Cabbage Palms) in Florida and isolated thorny shrubs (e.g., Celtis pallida [Granjeno]) in Texas. The species has not previously been reported to use anthropogenic nest substrates. We found Crested Caracara nests in an electrical substation, on a radio tower, and on a billboard near Clewiston, FL, and in an electrical substation and on a lattice electrical-transmission tower near Houston, TX. Our observations of nesting on anthropogenic substrates may support 3 distinctly different inferences. First, the behavior could be ongoing but not previously reported. Second, because individual Crested Caracaras in breeding plumage persist for years as floaters, novel use of anthropogenic nest substrates may indicate adult Caracaras seeking any possible nesting opportunity, even if nest success is low. Third, Crested Caracaras may be modifying their breeding behavior to capitalize on high-quality resources in areas that lack traditional nest substrates. Comparison of productivity between nests on anthropogenic and natural substrates would resolve the latter 2 competing hypotheses. Because management focuses primarily on nest sites, novel nesting-behavior could have important management implications. Future research should quantify productivity on anthropogenic substrates and document whether individuals produced at these sites tend to return to anthropogenic substrates to breed. Introduction. Caracara cheriway Jacquin (Crested Caracara; hereafter Caracara) are unique among falcons in that Caracaras are the only Falconidae to construct nests (Morrison and Dwyer 2012). Other falcons nest directly on a substrate, such as a cliff or cave, or occupy an existing nest built by another species (Ferguson-Lees and Christie 2001). Natural nest substrates used by Caracaras tend to be consistent within regions but vary among regions. In Florida, Caracaras nest primarily in Sabal palmetto (Walter) Lodd. ex Schult. & Schult. f. (Cabbage Palm; Morrison 2007, Smith and Scholer 2013) with scattered individual records in other trees (Dickinson and Arnold 1996, Morrison and Dwyer 2012). In Texas, Caracaras nest primarily in Celtis pallida (Klotzsch) Liebm. (Granjeno; Actkinson et al. 2007) with rare records in other thorny shrubs and trees (Morrison and Dwyer 2012). In Baja California, Mexico, Caracaras nest primarily in Pachycereus pringlei (S.Watson) Britton & Rose (Cardon Cactus); records of nests in other cacti are rare (Rivera-Rodríguez and Rodríguez-Estrella 1998). Caracaras have not been reported using anthropogenic nest substrates, although it is possible the behavior occurred historically but was not observed or reported. The closely related Caracara plancus (Miller) (Southern Caracara) nests on anthropogenic substrates in Argentina (Seipke 2012; one nest), and Argentinian Patagonia (16 of 35 nests documented from 2010 through 2013; M. Saggese, Western University of Health Sciences, Pomona, CA, unpubl. data). Two Phalcoboenus megalopterus Meyen (Mountain Caracara) nests have been reported on concrete electrical-transmission poles (White and Boyce 1986), and 3 Milvango chimachima Vieillot (Yellow-headed Caracara) nests were reported in open-topped boxes affixed to buildings (Johansson et al. 1999). Other than these isolated observations, caracara species appear to nest consistently in trees (e.g., Daptrius ater Vieillot [Black Caracara; Whittaker 1996], Milvago chimango Vieillot [Chimango Caracara; Morrison and Phillips 2000], Southern Caracara [Goldstein 2000], Ibycter americanus (Boddaert) [Red- 1EDM International, Inc., Fort Collins, CO 80525. 2CenterPoint Energy Houston Electric, Houston, TX 77017. *Corresponding author - Manuscript Editor: Karl Miller Notes of the Southeastern Naturalist, Issue 14/1, 2015 N11 2015 Southeastern Naturalist Notes Vol. 14, No. 1 J.F. Dwyer, and J.P. Dalla Rosa throated Caracara; McCann et al. 2010]), or on rock ledges or the ground (e.g., Phalcoboenus australis (Gmelin) [Striated Caracaras]; Raimilla et al. 2014). Here we report 3 Caracara nests on anthropogenic structures in Florida and 2 Caracara nests on anthropogenic structures in Texas. We do not know if this behavior is new or simply not previously reported, but because Caracaras are threatened in Florida, changes in nesting behavior have important management implications. Methods. We observed Caracara nests on anthropogenic structures in Florida while conducting field-work to investigate the ecology of non-breeding and breeding Crested Caracaras in Florida (Dwyer 2010). This work involved seeking and tracking Caracaras throughout the species’ range in Florida (Dwyer et al. 2012a, b). We observed nests on anthropogenic structures in Texas while developing and implementing an avian protection plan (APP) for CenterPoint Energy Houston Electric. APPs are designed to identify and mitigate avian electrocution and collision concerns on overhead power systems (APLIC 2006). We determined these structures to be Caracara nests based on our observations of adult Caracaras attending nests and caring for young. Results. On 7 February 2007, we found a nest on an A-frame lattice electrical-transmission tower in a Florida Power and Light (Miami, FL) substation located 6 km south of Clewiston, FL. The nest was 15 m above the ground, and contained 2 nestlings (Fig. 1). The substation was surrounded by private property; thus, we were unable to access it again to determine whether the nest fledged young or was used again in ot her years. Figure 1. Anthropogenic nest substrates used by Crested Caracaras in Florida, 2007–2009. (A) Location of nest on A-frame in substation. (B) Close view of 2 nestlings in substation nest. (C) Location of nest on billboard-support structure; note 1 adult in the nest and another adult on billboard. (D) Location of nest on radio tower; note adult with prey item in foreground. 2015 Southeastern Naturalist Notes Vol. 14, No. 1 N12 J.F. Dwyer, and J.P. Dalla Rosa On 4 March 2009, we found a nest on a radio tower 14 km south of Clewiston, FL. The nest was on a maintenance walkway 20 m above the ground. We observed the nest through 2010 and noted adult Caracaras in the nest in incubating posture suggesting that a clutch was produced, but we were unable to access the tower to count eggs or nestlings. On 17 March 2009, we found a nest on a billboard-support structure 6 km west of Lakeport, FL. The nest was 10 m above the ground and was used annually through 2012 (J.F. Dwyer, pers. observ; J. Smith, University of Nebraska-Lincoln, Lincoln, NE, pers. comm.). We observed adult Caracaras annually, but we do not know if this nest produced young. Each of these nests occurred in landscapes dominated by sugar-cane agriculture. On 20 March 2014, we found a nest containing 3 eggs on a CenterPoint Energy Houston Electric (Houston, TX) 138-kV lattice electrical transmission tower, near La Porte, TX (Fig. 2). The tower supported a power line crossing a 0.75-km-wide ship channel with the nearest land 0.15–0.25 km southwest, depending on the tide. The nest was 12 m above the water; it produced 2 eggs and at least 1 fledgling. This structure had historically supported Ardeidae species (heron and egret nests; Dalla Rosa, unpubl. data), but those species were absent in 2014. We found a nest containing 2 eggs on a 138-kV A-frame lattice electrical transmission tower on 23 March 2014 near Woodgate, TX. The nest was in a 0.5-km2 NRG Energy (Houston, TX) power-generation facility that served suburban areas. The nest was 21 m above ground and contained 2 eggs. We identified it during an investigation of a highvoltage flashover that interrupted electrical service. The location of the nest relative to Figure 2. Anthropogenic nest substrates used by Crested Caracaras in Texas, 2014. (A) Location of nest on A-frame in substation. (B) Location of nest on lattice tower. (C) Eggs in nest on lattice tower. (D) Nestlings in nest on lattice tower. N13 2015 Southeastern Naturalist Notes Vol. 14, No. 1 J.F. Dwyer, and J.P. Dalla Rosa high-voltage equipment posed potential electrocution and fire risks to the Caracaras. Consequently, under a permit issued by the US Fish and Wildlife Service (USFWS), and with guidance from the USFWS Migratory Bird Office, the nest was removed. Following the care standards of the National Wildlife Rehabilitators Association (Miller 2000), the 2 eggs were transferred to the Wildlife Center of Texas (Houston, TX), incubated, and the single resulting chick was raised. During the time we collected these data in Florida, we studied factors affecting detection of Caracara nests (Dwyer et al. 2012b); we found and monitored 49 nests on natural substrates (Dwyer 2010). Thus, 6% of the nests we found in Florida were on anthropogenic structures. We do not have similar data for Texas because the nests on which we report here were not found as part of a species-specific nest-monitoring pro gram. Discussion. Our observations of Caracaras nesting on anthropogenic substrates may support 3 competing hypotheses. First, Caracaras may have been nesting on anthropogenic substrates for some time but the nests were not noticed or reported. This suggestion seems unlikely, particularly in Florida where Caracara nesting has been studied in detail since the early 1990s (e.g., Dwyer et al. 2012b, Morrison 2007, Morrison and Dwyer 2012, Morrison and Humphrey 2001, Morrison et al. 1997, Smith and Scholer 2013). Anthropogenic nesting substrates have not been described previously for this species, though there are reports of the birds using atypical natural substrates (Morrison et al. 1997). Studies of Caracara breeding ecology in Texas (Actkinson et al. 2007) and Baja California (Rivera-Rodríguez and Rodríguez-Estrella 1998) are fewer, but also do not mention nesting on anthropogenic substrates, even when Caracaras’ relationships to human activities are focal topics (e.g., Rodríguez-Estrella 1996). Thus, we believe the nesting behavior reported here is novel. A second hypothesis is that use of alternate, anthropogenic nest substrates may reflect individuals accepting any possible nesting opportunity, even if the probability of nest success is low. Individual adult Caracaras in breeding plumage can persist for years as floaters in non-breeding groups, presumably seeking but unable to secure a breeding territory (Dwyer 2010; Dwyer et al. 2012a, b). The Caracara population in Florida is thought to be limited by the availability of nesting territories (reviewed in Dwyer 2010). If this hypothesis is correct, comparison of nest productivity on natural versus anthropogenic substrates should indicate lower nest success on anthropogenic structures than on natural substrates. A third competing hypothesis is that Caracaras are finding high-quality resources in areas that lack traditional nest substrates, and are modifying their behavior to capitalize on these resources. Given the species’ ability to use different nest substrates between regions, and that Caracaras are apparently breeding successfully in natural substrates in urban areas in Florida (J.F. Dwyer and J.L. Morrison, unpubl. data), this hypothesis seems most likely. If correct, comparison of nest productivity on natural versus anthropogenic substrates should indicate greater nest success on anthropogenic structures than on natural substrates. Thus, nest productivity comparisons may provide simultaneous tests of 2 mutually exclusive competing hypotheses. Simultaneous comparison of productivity in natural areas to productivity of Caracara nests in urban areas may provide additional insight into the competing hypotheses. Caracara nests are typically constructed 4–8 m above ground level (Actkinson et al. 2007, Morrison and Dwyer 2012). The nests reported here were much higher (mean = 15.6 m, SE = 2.2). Young Caracaras typically fledge prior to being well-flighted (Morrison and Dwyer 2012). Fledglings glide to the ground below the nest and spend 2–3 days sheltering under low vegetation near the nest substrate before they are able to complete sustained flights (Morrison and Dwyer 2012). Because we did not monitor nest success, we do not 2015 Southeastern Naturalist Notes Vol. 14, No. 1 N14 J.F. Dwyer, and J.P. Dalla Rosa know whether higher nests impacted this behavior or whether nesting above open water might have impacted fledgling survival at the nest in the ship channel. If productivity at various nest substrates is compared, careful quantification of nest heights would be warranted. We do not know if Caracaras fledged from anthropogenic substrates tend to return to anthropogenic substrates to breed, as Falco peregrinus Tunstall (Peregrine Falcon; Tordoff et al. 1998) and Accipiter cooperii Bonaparte (Cooper’s Hawk; Mannan et al. 2007) do in a process known as habitat imprinting (Temple 1977) or habitat-preference induction (Stamps 2001). If imprinting occurs, and if nesting in anthropogenic structures produces at least as many young per nest as nesting on traditional substrates does, then we would expect nesting on anthropogenic substrates to become more common in the Florida and Texas populations. This behavior will have important management implications because current management strategies for Caracaras in Florida focus almost exclusively on nest sites (reviewed in Dwyer 2010). Constrictive pressures exerted by habitat loss and fragmentation in Florida (Morrison and Dwyer 2012) may decrease if Caracaras can breed successfully in anthropogenic nest substrates or in urban areas. In future research comparing productivity at anthropogenic versus natural nest substrates, Caracaras produced in anthropogenic or urban nests should receive unique leg bands to facilitate evaluation of imprinting. Acknowledgments. CenterPoint Energy Houston Electric, EDM International, Inc., and NRG Energy supported this study in Texas. B. Sacra and B. Hughes found and monitored nests. S. Schmalz and M. Pickell cared for nestling Caracaras until they could be released. J. Fraser and J. Morrison advised the Ph.D. Dissertation research at Virginia Tech that informed and facilitated documentation of anthropogenic nest substrates in Florida. D. Eccleston, R. Harness, K. Miller, J. Smith, and 3 anonymous reviewers provided helpful comments on drafts of this manuscript. 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