van den Bremer L., Piersma T. & van Turnhout C.A.M. (2024) Does the provisioning of artificial nest cups and nesting structures help House Martins Delichon urbicum? ARDEA 112 (1): 63-72

To verify the common assumption that the provisioning of new nesting structures and/or artificial nest cups helps breeding populations of House Martins Delichon urbicum, we examined (1) the use of new self-standing nest site constructions (offering multiple artificial nest cups) erected to replace lost nesting sites or as additions, and (2) the extent to which House Martins breeding on buildings use artificial nests rather than self-built nest cups. We contrasted breeding performances, measured during two country-wide citizen science projects in The Netherlands, on two soil types, clay and sand, with clay being the expected preferred building material for nest cups over sand. The likelihood that artificial nest site constructions erected as replacements for lost nest sites were occupied was on average almost four times higher (occupancy rate 66%) than added structures (17%). Soil type had no significant effect on the occupancy rate. Confirming that clay is a better nest-building substrate than sand, self-built nest cups on sandy soils were more likely to collapse during a breeding season than nest cups built in areas with clay soils. Artificial nests had higher rates of occupation on sandy soils and in colonies where fewer self-built nest cups from previous seasons remained. On clay soils, self-built nests showed higher nesting success than artificial nests, with the reverse on sandy soils. The probability of second breeding attempts was higher in artificial nests than in self-built nests, but surprisingly this was only so on clay soils, with a strongly negative effect of first laying date. We conclude that, especially on sandy soils, the provisioning of artificial nest cups helps House Martins by enabling breeding and/or by improving breeding success. Where old breeding sites disappear, local populations can be maintained by providing ready-made structures instead. Although House Martin populations can thus be helped with nest sites and artificial nests, a comprehensive evaluation of limitations on the population warrants scrutiny of other factors, such as food, i.e. the supply of aerial insects.