Sekse- en leeftijdsgebonden duur van de nabroedtijd van Boomvalken Falco subbuteo

Bijlsma R.G. 2024. Post-fledging period of Hobbies Falco subbuteo: age- and sex-specific timing of departure from breeding sites. De Takkeling 32: 147-162. Daily or frequently visited nests of Hobbies on the Veluwe in the central Netherlands in the 1970s and 1980s (n=22) were used to determine the timing and duration of post-fledging periods of young, and departure dates of adults (midpoints used when not daily visited). These data were compared to daily visits to single breeding sites in 1992 and 2023 in Drenthe in the northern Netherlands. Mean fledging date for 24 pairs was 7 August (SD = 7.3, range 19 July-20 August), mean departure date of juveniles 15 September (SD = 5.4, range 28 August-24 September). Post-fledging duration (time between first fledging and last departure) averaged 37.9 ± 5.4 days, but with a substantial range of 22-53 days. Post-fledging duration declined with fledging date, lasting >50 days for nestlings that fledged in the second half of July to some 40 days for fledging dates in the first half of August and some 30 days for nestlings that fledged mid-August. Departure from the breeding sites by juveniles was often preceded by that of the adults, i.e. 18x 1-6 days earlier and 6x on the same date as the juveniles. Whenever sex-specific departure dates could be pinpointed, adult females usually departed ahead of males (14x, compared to 6x synchronous departure of male and female); the reverse – male ahead of female – was not observed. Departure from the breeding sites coincided with the main migration period of Hobbies, the latter based on 293 autumn migrants observed during systematic counts of migration in 1972-2023 (11,162 observation hours). Migration was recorded between 31 July and 1 November (with 7 September as median date of passage; 75% had passed on 17 September, 90% on 26 September). Of age-identified Hobbies, first-years were the first to start migration (median date of passage = 17 August, SD = 10.6, range 31 July-5 September, n = 12), followed by adults (median date 30 August, SD = 16.6, range 15 July-23 October, n = 113). Migration of juveniles was recorded between 1 September and 1 November, with a median date of passage of 21 September (SD = 13.9, n = 53). Spring migration took place between 20 March and 31 May, with mean date of passage on 27 ± 16.0 April (n = 137), and 50%, 75% and 90% dates of passge on 27 April, 9 May and 15 May. Timing of spring migration reflected arrival dates on breeding sites, showing that the pre-breeding period of Hobbies on the breeding grounds covers nearly 1.5 months. Over the period of 1972-2020, mean start of laying in The Netherlands averaged 9 June (n = 758), without any change during the warming spring temperatures since the 1990s. This is in sharp contrast to another long-distance migrant, the Honey Buzzard Pernis apivorus, which gradually advanced mean start of laying by 9 days between 1974 and 2014 (Schreven et al. 2023) in parallel with warming spring temperatures. Spring arrival of Hobbies on breeding sites was (and still is) in April, onset of laying invariably peaked in the first half of June (with outliers in May and up to early July) and fledging in the first half of August. Breeding phenology of main prey species has (Starling Sturnus vulgaris), or has not (Barn Swallow Hirudo rustica) advanced in past decades, whereas the advancement of Odonata relevant as prey to Hobbies (mainly Libellula, Orthethrum, Anax and Aeshna species) was partly nullified by delays caused by higher summer temperatures. The post-fledging period of Hobbies coincided with peak flight dates of Aeshna species and maximum abundance of Barn Swallows, whereas departure from the breeding sites in the second half of September coincided with the sudden decline in Barn Swallow numbers at roosts (synchronous onset of migration). Post-fledging date and duration of Hobbies have been inflexible in the past 50 years, contrasting with advancements in the phenology of main prey spcies (birds and dragonflies).