We used an experimental approach to determine the effects of PIT tagging on rhinoceros auklet breeding outcomes. Our approach will permit the development of a broader scale rhinoceros auklet automatic detection network on Middleton Island to record nest attendance and adult survival in future years.
Our results suggest that the effects of PIT tagging on breeding success are negligible. Specifically, a power analysis shows that given our effect sizes, we would need 2 000 000 individuals to detect a statistically-significant (P < 0.05) difference between control and PIT tagged, 1858 individuals to detect a difference between incubation and chick-rearing and 424 individuals to detect a difference between single and double tagged. In short, the effects would only be measurable statistically with enormous sample sizes.
Parental commitment towards breeding increases as the breeding season progresses. Thus, we expected that PIT tagging during incubation would cause higher abandonment than during chick rearing. This effect has been documented on rhinoceros auklets (Sun et al. 2020) as well as other burrowing seabird species (Carey 2009), although Kelly et al. (2015) found no impact of disturbance during incubation on breeding success of Atlantic puffins. However, we did not detect a difference in abandonment between breeding stages. This result most likely reflects the high effort we exerted in reducing disturbance as much as possible. Artificial nest boxes were particularly useful for that purpose, as the birds were easily accessible. The handling time was only three to five minutes long. We did not observe rhinoceros auklets displaying signs of high stress (e.g., breathing heavily, trying to escape) during the marking process. In addition, and unlike most monitoring tools, PIT tags are extremely small and lightweight. The PIT tags we used weighted ≈ 0.1g which represents around 0.02% of the birds’ mass.
Our results are encouraging as both tagging and handling have well documented negative effect on seabird reproductive success (Rodway et al. 1996; Whidden et al. 2007), including reduced colony attendance (Söhle et al. 2000), chick growth rate (Ackerman et al. 2010; Villard et al. 2011) and higher nest abandonment (Sun et al. 2020). Considering our results, we can make recommendations for future PIT tagging experiments on rhinoceros auklets, and likely other burrow-nesting birds. Individuals should be marked during the last days of incubation or the very first days of chick rearing when they are accessible during daytime. Tagging individuals at night requires many visits to the same burrows multiple times, which can disturb individuals in the colony. However, this should be considered with precaution as handling seabirds during incubation goes against most recommendations (e.g. Elliott et al. 2010; Elliott 2016; Sun et al. 2020).
As neither the number of adults tagged, nor the position of the tag seem to have any effect on reproductive success, we believe both adults of every burrow should be tagged in the neck. Having both adults tagged is an opportunity to study nest attendance at a finer scale, while neck implants are better fitted for the detection technology we used and reduce risks of tag loss.
Indeed, the PIT tag need to be in a particular orientation when going through the antenna to maximize detection probability. Depending on the size of the access tunnel or individual walking physiology, the bird’s leg might not have the right orientation. PIT tags oriented perpendicularly to the magnetic field induced by the antenna would have a low detection probability, which could end up in data loss. This could be the reason why individuals with neck implants are detected a higher number of times each night. This could also be attributed to a difference in individual behavior but our data don't allow us to determine this. During the next season, several burrows will be monitored constantly with infrared cameras which will help us understand that phenomenon better. Lastly, detecting the PIT tag with a handheld detector through the opening atop the nest box would be much simpler when implanted in the back than attached on the leg.
Studies report various loss rates for subcutaneously implanted pit tags. The size of the bird and the injection method seem to be important factors (Bonter and Bridge 2011). However, the tag loss rate was 0% on our sample (n = 38). In addition, for diving species, subcutaneous tagging might help decrease the added drag, as internal implantation of tags generally causes reduced impacts (Wilson et al. 1986; White et al. 2013; Evans et al. 2020). For example, positioning tags on the leg or the tail can cause instability (Vandenabeele et al. 2014; Elliott 2016). A study on crested auklets (Aethia cristatella) showed that a tarsus mounted tracking device (1% of the bird body mass) changed the behavior and at sea survival of marked individuals (Robinson and Jones 2014). Similarly, increasing buoyancy or mass via a leg-mounted tag altered the dive behaviour of thick-billed murres (Uria lomvia; Elliott et al. 2007), and attaching a leg-mounted tag for a year increased stress (corticosterone) in the same species (Elliott et al. 2012). The mass of the 3D-printed leg band and PIT tag we used in our study was around 1g (≈ 0.2% of the body mass), yet it could still have undesirable effects that we can avoid with subcutaneous implants.
However, this statement has to be taken cautiously as all tags should be considered to have an impact even if they are hard to detect (Elliott 2016). The impact of such small devices might be particularly hard to detect as their mass are negligible. They could still have some long-term negative impact on survival (some marked individuals could suffer lesions from the tag migrating in the body) or may affect reproduction during subsequent years. For this reason, it is important to carry this study during several years before concluding on potential PIT tagging effects. Nonetheless, PIT tagging reduces disturbance on birds compared to manual burrow checks and GPS tagging due to decreased human presence on colonies. Thus, combining PIT tagging and automatic detection devices is a valuable method in studying sensitive seabird species during their breeding season.
Several detectors revealed individuals visiting alternate burrows in our study. This behavior has been observed in Leach’s storm-petrel (Zangmeister et al. 2009) but not in the rhinoceros auklet. This phenomenon is unlikely to be due to a bird seeking for extra pair copulation, as data were collected late in the breeding season. One possibility is that those extra burrow visits could be attempts at kleptoparasitism (Senzaki et al. 2014). Monitoring intra-specific kleptoparasitic behaviors could be a way to monitor ecosystem health via food shortages. Indeed, this behavior is most likely to happen when food availability is scarce (Beintema 1997; Ashbrook et al. 2008).
This experiment also revealed that parents could come back to a burrow even after the chick disappeared. During productivity checks, we found several burrows without a chick that had one before. We considered the absence of a chick as proof of nest abandonment by the parents, yet no corpse was found in the burrow. In two cases, the parents came back to the nest after we concluded the burrow was abandoned. In one case, both parents had a nest attendance pattern similar to successfully breeding birds even if the chick was considered dead two weeks previously. Clearly, auklets continue visit their burrows even after chicks are dead.
These observations highlight the potential of PIT tagging to bring more knowledge on certain species. There is growing evidence that, even if monitoring seabird species gives us access to helpful data in understanding their biology and the state of their environment, monitoring is done at the expense of their fitness. This problem is concerning, especially during long studies that could have negative long-term carryover effects on populations. In extreme cases, researchers could be missing important factors that would be otherwise demonstrated in an undisturbed population. PIT tagging could be used extensively in sensitive seabird species and in the light of our results and previous ones, should be considered for rhinoceros auklets, other puffins and other burrow-nesting species.
In conclusion, PIT tagging rhinoceros auklets nesting in artificial burrows is a promising way to monitor their behavior, breeding outcomes and long-term survival while having the least possible disturbance. This technique could be applied to other sensitive seabird species with a similar biology like tufted puffins (Fratercula cirrhata). By replacing usual nest monitoring with automatic PIT tag recording, we reduce disturbance and probability of burrow collapse due to less frequent visits.