Using a long-term data set on 66 single-tagged or repeatedly tagged common swifts and 56 non-logger birds from the same breeding colony in Germany, we detected no differences between logger and non-logger birds in different traits regarding survival and life-history traits over eight years. The return rate of single-tagged and repeatedly tagged logger birds did not differ from the return rate of non-logger birds, and the return rates were similar to return rates of other bird species of similar body weight or even less [17, 19, 20].
Logger birds tend to arrive earlier than non-logger birds at the breeding site in spring, following the general pattern with larger swifts and those heavier in body weight arrive earlier at the breeding site . The timeframe between arrival and clutch initiation did also not differ between logger and non-logger birds. Thus, logger birds fitted to the pattern that “early birds” started egg laying earlier (Fig. 2F). This is important, because timing of breeding is crucial for the reproductive success of a complete breeding season [51, 52]. The fact, that logger birds did not delay clutch initiation is also important in respect to another aspect. Due to technical reasons, it was necessary to recapture the logger bird between arrival and clutch initiation to retrieve the logger to download the data. Catching a bird during this time, however, might have been a major negative impact on the breeding success due to a delayed clutch initiation [53, 54] or even nest desertion in this sensitive bird, but this was not the case in our study.
Our analysis showed that logger birds, regardless of sex, were as successful in reproduction as non-logger birds in our breeding colony. We found no differences in date of clutch initiation, in number of eggs, number of chicks nor in number of fledglings. Thus, we detected no “carry-over” effects neither on a between-individual level nor on a within-individual level, i.e. investing the same individuals when they are tagged during the previous non-breeding season vs. when they are not tagged.
Our repeatability analysis showed that logger birds were highly consistent in arrival date and moderate consistent in date of clutch initiation. We detected this individual consistency in arrival date in another data set of repeatedly observed 29 males and 29 females of our long-term study of this breeding colony as well (Wellbrock et al in preparation). Thus, logger birds exhibit similar patterns like non-logger birds. Further, high consistency in arrival at the breeding site was also found in other studies with migratory bird species [33, 55]. Therefore, we assume that common swifts have their individual timing for arriving at the breeding site and starting egg laying following their internal clock . It would be interesting to check, whether the timing of arrival and egg laying exist already with the first breeding attempt or will be developed during years of breeding experience .
Since we have indications that common swifts do have their own timing of life [repeatability analysis,13], we suggest that future studies should look more into traits on within-individual level rather than into between-individual differences. In another study on common swifts of this breeding colony, we could receive migration routes and overwintering sites of three males over two successive overwintering periods . We found that all three males used different migration routes and overwintering areas, but each male used the same routes and regions in two successive wintering periods. This emphasizes the difference between individuals and the consistency within individuals in one trait in this species
Although we find no negative effect of single or repeated tagging in swifts, tagging remains an important issue. The current loggers for small birds do not allow real-time monitoring. Thus, we can only examine logger birds that actually returned to the breeding site, but we have no information about the “non-returnees” and could only speculate whether they are dead or breed at another breeding site. It is possible that negative effects are masked by the fact that the returnees were in the better physical condition and could compensate for possible negative effects during non-breeding period and arrived at the breeding site. However, we found no difference in the body weight nor in wing length between returnees and non-returnees when they were tagged in year x.
Another study on common swifts and pallid swifts Apus pallidus revealed a reduced apparent survival on logger birds comparing to non-logger birds as their control group . It seems that the weight of the logger did not influence the survival but the logger characteristics. When the logger was equipped with a light stalk, the apparent survival was lower indicating that the logger set up might have a major impact on the return rate . However, since the return rate in our study did not differ from the control group (non-logger birds) and sample size in each logger-type group would be small, we did not take a closer look on the logger characteristics affecting the return rate. Nevertheless, the study  makes a significant point that it could be far more important how a logger is built and shaped rather than just focusing on weight, as aerodynamics matters a lot in birds, especially in long-distance migratory species .
To better understand individual based decisions regarding life-history traits, we need more studies using repeatedly tracked birds [13, 14]. Although we found no differences in return rate and parameters determine breeding success in repeatedly tagged swifts in our breeding colony, further long-term studies are essential to evaluate effects of such repeatedly used techniques to get more knowledge on possible impacts on migration behaviour and reproductive success in long-distance migratory birds in general.