SPIERENBURG P, ALTENBURG J, BOUDEWIJN T, BIL W, DIJK O, DROP J, HAGEMAN G, PEKEL S, SCHOLTEN J, VERKADE H & BILL W (2023) The private life of the Barn Swallow Hirundo rustica under the magnifying glass. LIMOSA 96 (3): 119-129.

The habit of breeding semicolonially in farm sheds makes the Barn Swallow a suitable species for studies on breeding success and adult survival. Despite extensive monitoring in the past, relatively little is known about the proportion of non-breeders, and to what extent individuals have second broods (and whether these second broods involve the same individuals). These parameters can only be derived through individually marked birds and careful brood monitoring. Therefore, four ringing groups from different parts of the Netherlands started a joint colour ringing project in 2019 to determine individual lifetime reproductive success.
In this article, we present the results for 2020, focusing on the proportion of single and double brooded birds within the local populations. The results show that the majority of the birds breeds at least once. Only 5% of the females could not be assigned to a local breeding attempt, which suggests that within-season breeding dispersal is rare in females. For males the proportion of non-breeders was 16%. These males might, however, have a nest elsewhere or still generate offspring through extra pair copulations, which cannot be assessed through observations of provisioning behaviour. Of the individuals taking part in the breeding process 62% undertook more than one breeding attempt. The overall average breeding success was 6.0 young per female and 6.3 per male, varying between 0 and 13 young. Older birds (> second calendar year) were more frequently involved in second, or incidentally, third broods and produced in general more offspring. During the study a number of improvements were identified and executed. (1) Elaborate photo documentation of colour-ringed birds within a breeding site, which provides more accurate information on pair formation than focused observations of nests. (2) Gathering more biometric data in order to identify potential fitness indicators. (3) Focused attention on documenting cases of polygamy. (4) DNA-collection (feathers) of nestlings and adults to address the levels of extra-pair parenting, which currently limits the ability to determine individual reproductive success.

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