Return rate and logger type
In total, we tagged 85 birds within 8 years. Because we tagged some birds repeatedly, these 85 logger birds included 66 individual swifts. In total, 50 of 85 logger birds returned to the colony during the period between 2013 and 2020. On average 0.61 ± 0.12 of logger birds and 0.60 ± 0.27 of non-logger birds returned to the breeding site in the next year (Table 2, GLMM, estimatelogger = -0.159 ± 0.600, N years of logger−birds = M years of non−logger−birds = 8, z = -0.265, P = 0.791). Looking only at birds that have been tagged the first time, 39 of 66 logger birds returned (0.59 ± 0.23, Table 2).
Table 2
Total number of breeding pairs per year and number and rate of returned logger and non-logger birds for each year.
| | logger-birds | non-logger birds |
Year (x) | Total No of breeding pairs | No. of all birds tracked in year x | No. of all tracked birds returned in year (x + 1) | No. of birds tracked for the first time in year x | No. of first tracked birds returned in year (x + 1) | return rate | No. of birds sampled for comparison in year x | No. of same birds returned in year (x + 1) | Return rate |
2012 | 44 | 10 | 8 | 10 | 8 | 0.8 | 10 | 3 | 0.3 |
2013 | 42 | 10 | 7 | 6 | 5 | 0.7 | 10 | 6 | 0.6 |
2014 | 48 | 10 | 6 | 6 | 4 | 0.6 | 10 | 4 | 0.4 |
2015 | 47 | 10 | 5 | 8 | 4 | 0.5 | 10 | 9 | 0.9 |
2016 | 53 | 10 | 7 | 5 | 5 | 0.7 | 10 | 7 | 0.7 |
2017 | 58 | 11 | 6 | 9 | 4 | 0.55 | 11 | 9 | 0.82 |
2018 | 59 | 19 | 8 | 19 | 8 | 0.42 | 19 | 17 | 0.89 |
2019 | 60 | 5 | 3 | 3 | 1 | 0.6 | 5 | 1 | 0.2 |
Sum | | 85 | 50 | 66 | 39 | 0.61 | 85 | 56 | 0.6 |
Out of the 66 logger-birds tagged first time, 16 individuals carried a logger at least for two years (12 returned in the third year ≙ 0.75). Seven of these 16 individuals were tagged in three years (5 returned in the fourth year ≙ 0.71), four individuals were tagged four times (2 returned in the fifth year ≙ 0.5) and one female was tagged five times but did not return in the sixth year.
There was neither an effect of the presence of a light stalk nor of on sex on the return rate of logger birds including repeated tagging (LMM, estimatestalk = 0.198 ± 0.492, N logger−birds = 93, z = 0.401, P = 0.688; estimatefemale = 0.552 ± 0.478, N logger−birds = 93, z = 1.155, P = 0.248).
Body weight and wing length of logger birds and non-logger-birds
We received data on body weight and wing length from 66 logger birds (31 females, 33 males, 2 unknown) during tagging in year x and from 63 non-logger birds (27 females, 34 males, 1 sex unknown). Logger-birds were significantly heavier (LMM, estimatelogger = 2.173 ± 0.652, N logger−birds = 66, M non−logger−birds = 63, df = 128.8, t = 3.334, P = 0.001) and significantly larger in wing length (LMM, estimatelogger = 1.638 ± 0.697, N logger−birds = 66, M non−logger−birds = 63, df = 128.0, t = 2.351, P = 0.020) than non-logger birds (Fig. 1A and B) in the year of logger deployment. We could not detect any significant differences in body weight and wing length between returned logger birds and those logger birds which did not return (LMM, weight: estimatereturned 0.414 ± 0.844, N returnees=3 7, M non−returnees = 29, df = 63.4, t = 0.491, P = 0.625; wing: estimatereturned 0.064 ± 0.998, N returnees= 37, M non−returnees=29, df = 65.0, t = 0.064, P = 0.949). Although we set a minimum weight and wing length for logger birds, we still covered a wide range in weight and size in the species (logger birds: range in weight: 35.9–51.8 g; range in wing length: 169.0-185.5 mm).
Arrival date
We received arrival dates of 101 individuals (50 females, 50 males, 1 unknown, including N = 38 logger and N = 63 non-logger birds) and detected a negative relation between body weight and arrival date (arrival = 155.7 − 0.6*weight, N = 101, R2adjusted = 0.073, P = 0.004) and between wing length and arrival date (arrival = 235.3–0.6*wing, N = 101, R2adjusted = 0.077, P = 0.003; Fig. 1C and D). Logger birds arrived earlier at the breeding site than non-logger birds (LMM, estimatelogger = -3.620 ± 1.382, N logger−birds = 38, M non−logger−birds = 63, df = 95.8, t = -2.619, P = 0.010; Fig. 1E), following the general pattern that heavier and larger birds arrived earlier at the breeding site than lighter and smaller birds.
Breeding parameter
The timeframe between arrival and starting egg laying (i.e. delta days) did not differ between logger and non-logger birds (LMM, estimatelogger = 2.089 ± 1.319, N logger birds = 36, M non−logger birds = 36, df = 55.3, t = 1.583, P = 0.119, Fig. 2A). When combining data of logger and non-logger birds, we found a strong positive relation between arrival date and the laying date of the first egg (eggdate = 61.3 + 0.6*arrival, N = 72, R2adjusted = 0.396, P < 0.001, Fig. 2B). We received data on breeding parameters recorded between 2013 and 2020 in 38 logger birds (22 females, 16 males) and 38 non-logger birds (14 females, 24 males). We did not detect any differences in breeding parameters between both groups. They started egg laying at the same time (LMM, estimatelogger = 0.172 ± 1.371,N logger bird = 36, M non−logger birds = 36, df = 61.2, t = 0.125, P = 0.901, Fig. 2C), had similar clutch sizes (GLMM, estimatelogger = -0.043 ± 0.147, N logger birds = 36, M non−logger birds = 38, z = -0.293, P = 0.769, Fig. 2D), a similar number of chicks (GLMM, estimatelogger = -0.026 ± 0.165, N logger birds = 34, M non−logger birds = 38, z = -0.160, P = 0.873, Fig. 2E), and a similar number of fledglings (GLMM, estimatelogger = 0.107 ± 0.227, N logger birds = 34, M non−logger birds = 38, z = 0.469, P = 0.639, Fig. 2F). Females and males, or rather their female mates (both logger and non-logger, no within-pair mates) did not differ in date of clutch initiation (LMM, estimatefemale = 0.372 ± 1.368, N = 33 females, M = 39 males, df = 57.7, t = 0.272, P = 0.787), clutch size (GLMM, estimatefemale = -0.039 ± 0.145, N = 35 females, M = 39 males, z = -0.271, P = 0.786), number of chicks (GLMM, estimatefemale = -0.129 ± 0.165, N = 34 females, M = 38 males, z = -0.785, P = 0.433) and number of fledglings (GLMM, estimatefemale = -0.086 ± 0.226, N = 34 females, M = 38 males, z = -0.382, P = 0.703)).
Arrival date and breeding parameter in same birds when tagged and when not tagged
From the 16 repeatedly tracked swifts, we received breeding data of 15 of these same individuals when they were tagged and when they were not tagged for at least one year. None of the LMMs could detect any significant effect of loggers on any of the factors mentioned (Table 3).
Table 3
Results from LMMs (“arrival”, “eggdate” and “delta”) and from GLMM (“clutch”, “chicks”, “fledge”) testing for associations between having a logger (“geo”, df = 1) and arrival date (“arrival”), date of clutch initiation (“eggdate”), timeframe between arrival and clutch initiation (“delta”), number of eggs (“clutch”), number of chicks (“chicks”) and number of fledglings (“fledge”).
Formula | N | Estimate (± SE) | t/z-value | P-value |
arrival ~ geo + (1 | ID) + (1 | Year) | 15 | 1.657 (± 1.315) | 1.260 | 0.215 |
eggdate ~ geo + (1 | ID) + (1 | Year) | 15 | 0.359 (± 1.450) | 0.248 | 0.805 |
delta ~ geo + (1 | ID) + (1 | Year) | 15 | -0.509 (± 1.822) | -0.279 | 0.781 |
clutch ~ geo + (1 | ID) + (1 | Year) | 15 | -0.028 (± 0.165) | -0.169 | 0.866 |
chicks ~ geo + (1 | ID) + (1 | Year) | 15 | 0.120 (± 0.186) | 0.645 | 0.521 |
fledge ~ geo + (1 | ID) + (1 | Year) | 15 | -0.064 (± 0.220) | -0.292 | 0.771 |
We detected no significant differences in arrival date, egg laying date, time span between arrival and laying of first egg, clutch size, number of chicks and number of fledglings in these birds when they were tagged or were not tagged.
Arrival date and laying date in repeatedly tagged logger birds on within-individual level
Out of 66 logger-birds, 16 swifts were tagged at least for two years. Of these 16 common swifts, we got 31 arrival dates of the 12 tagged and returned swifts and 30 dates of clutch initiation (“eggdate”) of 11 individuals for repeatability analysis. We detected a high within-individual consistency in arrival date (arrival: N: 12, R (± SE): 0.6 ± 0.173, P < 0.001) and moderate repeatable timing in egg laying (egg date: N: 11, R (± SE): 0.34 ± 0.201, P = 0.040).