Ornithological Methods

INTRODUCTION

Tracking tags have become an essential tool for ornithologists, allowing them to monitor and follow individuals of various species to gain important insights into behavior and survival that would otherwise be difficult or impossible to describe (Barron et al. 2010). For example, radio-telemetry studies have dramatically changed our understanding of movement and survival during the post-fledging period (Naef-Daenzer et al. 2001, Cox et al. 2014, Jones and Ward 2022), timing and routes of migration (Robinson et al. 2010, Brunner et al. 2022, Cooper et al. 2023a, b), predator-prey interactions (DeGregorio et al. 2015), and have allowed researchers to find and study nests of secretive species (Legare and Eddleman 2001). Moreover, the recent expansion of the Motus Wildlife Tracking System (Taylor et al. 2017) and development of other automated radio-telemetry systems (e.g., Jones and Ward 2022) holds great potential for future research on avian behavior and survival. Furthermore, advances in miniaturization allows researchers to place tags on virtually any avian species (e.g., hummingbirds; Zenzal et al. 2014). However, for tracking tags to be effective tools, they must have minimal impact on behavior and survival of the organism.

Meta-analyses have shown that tracking tags can have negative impacts (Barron et al. 2010) and that negative effects are underreported in the avian literature (Hill and Elphick 2011). Researchers must therefore select tag specifications (e.g., size, mass, material, harness type) that minimize the potential for negative effects (Anich et al. 2009). Tag weight and shape generally receive the most consideration because they affect the amount of drag produced during flight, with larger, heavier tags being more energetically costly, producing more drag, and likely having a greater impact on behavior and survival (Aldridge and Brigham 1988, Obrecht et al. 1988). Consequently, institutional animal care and use committees, government regulatory agencies, and researchers often limit or select transmitters based on the relative mass of focal species, usually following a 3% or 5% of body mass rule, below which the impact of a transmitter’s weight is thought to be minimal. However, we have little empirical basis for such thresholds (summarized in Barron et al. 2010).

Tag antenna length and cases of entanglement in songbirds

Antenna length also affects birds but is generally overlooked as a tag design parameter. Antenna length contributes to the overall drag of a transmitter during flight (Obrecht et al. 1988), how a tag influences a bird’s center of gravity during flight (and thus energetics), and may influence behavior (e.g., Zenzal et al. 2014). Perhaps more importantly, antenna length influences the risk of tag entanglement with vegetation (Hill and Elphick 2011). To reduce tag mass for smaller birds, antennas are often made of thinner, more flexible materials that are prone to curl or become hook-like over time (Fig. 1; e.g., Dougill et al. 2000). Longer antennas are also more likely to touch or rub against the ground, vegetation, and pick up spider webs and other sticky substances, increasing the chances that antennas may become entangled, particularly in ground foraging or grassland species (Hill and Elphick 2011, Van Vliet and Stutchbury 2018); picking up such substances may also increase the weight and weight distribution of transmitters.

We searched the songbird tracking literature for cases when entanglements were documented alongside antenna lengths, and we also reached out to colleagues and peers for unpublished cases. Based on these published and unpublished accounts (11 studies), all but one case of songbird entanglement occurred with antennas 13 cm in length (“longer antennas”; Table 1). Notably, some antennas were two to three times longer than the focal species’ tail length, and the antenna length rivaled tags placed on birds 10 to 20 times their size (e.g., warblers [13–18 cm] vs. terns [14 cm] and geese [15 cm]; Demers et al. 2003, Rock et al. 2007). These entanglements occurred across life stages, with natural and artificial aspects of the environment (e.g., trees, barbed wire fence), and included cases where tagged nestling and incubating females became entangled in their own nests. We also note that similar antenna lengths may cause entanglements in larger bird species (e.g., Northern Bobwhite [Colinus virginianus], Mallard [Anas platyrhynchos]; Burger et al. 1995, Kirby and Sargeant 1999). Additionally, cases of antenna entanglement in songbirds appear underreported in the literature; Hill and Elphick (2011) surveyed researchers and found that amongst their studies, 27% of species incurred some issue with entanglement, with more than half of those species becoming entangled because of the antenna (lengths were not reported). Only two of these transmitter-related problems were actually published, and in several studies, researchers noted that they trimmed and used shorter antennas after the first entanglement (Hill and Elphick 2011). Lastly, many studies may tag but do not actively track birds (e.g., migratory departure, connectivity; González et al. 2020, Herbert et al. 2022), such that cases of entanglement would be under discovered. Moreover, birds that become entangled in such studies may not be able to escape their transmitter nor be rescued, further highlighting the importance of selecting an appropriate antenna length.

Despite the potential for entanglement and associated mortality (Dougill et al. 2000, Van Vliet and Stutchbury 2018), few guidelines for selecting tag antenna length are available. Similar to mass, this is largely because we have little empirical basis for determining thresholds for antenna length across species. Indeed, we are limited in what we can infer from reported cases of entanglement. For example, though cases reported in this paper hint that longer antennas may be more problematic for birds, they do not indicate rates of mortality or survival, and they might not be representative of entanglements across antenna lengths and studies using radio tags. Further complicating matters, entanglement rates by antenna length may vary depending on habitat type. Entanglements appear to be higher in birds preferring denser vegetation (Hill and Elphick 2011, Van Vliet and Stutchbury 2018), such as grassland birds and fledglings of many species (Jones et al. 2017). Shorter antennas (e.g., 6–8 cm) have therefore been recommended in those cases (e.g., Streby et al. 2015, Van Vliet and Stutchbury 2018). Perhaps most importantly, few resources exist for researchers to report cases of entanglements without fear of judgment or repercussions, even though there appears to be widespread agreement among ornithologists that such information should be made available to inform guidelines and future research (sensu Hill and Elphick 2011).

Although the solution to these cases of entanglement may at first seem simple (e.g., just use shorter antenna lengths), in reality choosing an appropriate antenna length poses a significant dilemma for researchers given the relationship between antenna length, detection distance, and detection rates. Longer antennas provide greater signal power, allowing a tag to be detected further away and with greater reliability (detection rate). Longer antennas therefore enhance the ability of researchers to find and monitor tagged birds, and for automated receiver stations to detect them. Critically, the relationship between antenna length and aspects of detection are non-linear, with each cm of an antenna length shortened likely resulting in a greater decline in detection distance and rate. Quantifying how detection distances and rates vary with antenna length under different field conditions (e.g., habitat types, topography) would therefore enable researchers and manufacturers to make more informed decisions about tracking tag design. Tests could show that detection distances and rates are similar across a range of antenna lengths (e.g., 8–20 cm) and that smaller antennas (< 10 cm) are sufficiently detected for collecting data on a variety of topics (e.g., migration, post-fledging ecology, home range use). Alternatively, tests could show a significant drop in detection range and rate with shorter antennas, highlighting a clear dilemma for researchers. Regardless of the findings, such tests could ultimately aid researchers in selecting an antenna length that balances tag detections with risks of avian entanglement and mortality.

Are tracking tag colors problematic for songbirds?

In addition to antenna length, tracking tag color is another overlooked consideration. Depending on the company and model, transmitters come in a variety of colors and may have effects on individual recognition, status signaling, mate choice, and reproductive success, as has been found in other color markers such as color bands and leg flags (Hagan and Reed 1988, Metz and Weatherhead 1991, Calvo and Furness 1992). Additionally, researchers need to consider whether tag color might compromise the camouflage of their study species. Whereas more darkly colored tags (e.g., green, brown, black) tend to blend with most plumages (e.g., juveniles), white and brighter colored tags (e.g., yellow, orange, red, blue) may be in stark contrast to the bird’s color(s), and essentially paint a target on a bird’s back (Fig. 2) for visually oriented predators (Stevens et al. 2013). We note this is likely more of an issue for nestlings, as tags can be concealed under the back feathers of adults, though this issue may still occur during periods when adults are molting. Moreover, such tag colors (white tags, in particular) may increase the likelihood of parents attacking or removing tagged nestlings from their nests, mistaking them for fecal sacs, parasites, or foreign objects (e.g., Mattsson et al. 2006, Fisher et al. 2010). Tag color could therefore lead to reduced feeding, lower body condition, early fledging, and ultimately lower survival rates, especially for incubating females and immobile nestlings or fledglings that rely on camouflage, a concern that is particularly relevant given the recent uptick in studies using white Motus nanotag transmitters (Taylor et al. 2017). To our knowledge no studies have examined associations between contrasts in transmitter color and plumage with regards to survival and behavior of parental birds. Until such studies are conducted, researchers may help to mitigate these potential issues by changing the color of their transmitters via waterproof markers, paint, or nail polish - past studies have camouflaged transmitters in birds (e.g., Severson et al. 2019) and other taxa, including those in aquatic environments (e.g., Aitken et al. 2005). Notably, researchers may need to take steps to ensure that their coloring materials do not block infrared-activation (i.e., do not color over the activation area), do not impair adhesion of harness materials to the tag (we recommend researchers glue harnesses to their tags before making color adjustments), and do not make tags conspicuous with regards to ultraviolet vision.

CONCLUSIONS

In conclusion, we emphasize that caution must be used when considering tag specifications and their potential for deleterious effects on songbirds. In addition to assessing the appropriate transmitter size and mass, we argue that selection of antenna length and the color of transmitters are also critical. Here we call on researchers to report and publish cases of entanglement and the associated antenna length used, such that more information on antenna length and survival can be compiled and eventually used to produce informed guidelines on antenna length. When data are compiled, researchers and manufacturers can make more informed decisions on tag design (e.g., manufacturers could design detachable antennas for when birds become entangled). Moreover, editors of journals and other organizations should encourage and applaud researchers for reporting and publishing such cases, such that researchers are not judged or suffer repercussions. A central database with a worldwide organization or databank (e.g., Movebank; https://www.movebank.org), as well as regional organizations (e.g., USGS Bird Banding Laboratory in North America) would allow researchers to report such cases. Further research is needed to determine (1) the effect of antenna length on rates of entanglement and survival of tagged birds; (2) what practical effect antenna length may have on detection range and rates in both automated and handheld telemetry studies; and (3) how variation in tag color affects adult and juvenile survival, including parental behavior toward tagged nestlings and their subsequent survival. Ultimately, such efforts will enable researchers and manufacturers to make more informed decisions about tracking tag design, particularly with respect to balancing the benefits of longer antennas, which provide greater power and detection distance, with risks of bird entanglement and mortality.

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