Terreingebruik en activiteitspatroon van Wespendieven Pernis apivorus op de Veluwe

In 2008, three male Honey Buzzards breeding on the Veluwe, a large (90.000 ha) woodland on glacial sands in the central Netherlands, were equipped with a GPS datalogger. The loggers were fastened on the back of the birds with a harness. The GPS tracking system was developed at the University of Amsterdam (UvA-Birdtracking system). It is a light-weight, solar-powered, high-energy and efficient bird-tracking device, with two-way wireless ZigBee (2.4Ghz) data communication to ground stations. It overcomes some of the shortcomings in the existing commercial systems. For studies on habitat use, spacing behaviour and daily activity patterns, its high frequency GPS-fixing is of particular importance. After calibration of behavioural patterns with radio telemetry (VHF-tracking), we found that a 10-minute interval between GPS-fixes yielded the best results in terms of a reliable interpretation of the behaviour in between successive fixes. Longer time intervals (15-30 minutes) could easily result in missing short foraging trips. The loggers weigh 12-18 g (depending on battery size), and measure 61x31x11 mm. The birds – In 2008, the three tracked males each initially attended a nest with two chicks; two of the males succeeded in fledging respectively one and two young, the failed nest having been depredated by – presumably – a Goshawk Accipiter gentilis in the late nestling stage. The onset of laying was calculated at respectively 16,24 and 26 May. The males (and an adult female, of which the wrong setting of the GPS-system prevented the retrieval of data) were captured on 7 and 11 July, i.e. when the age of their chicks ranged from 8-10 to 9-11 and 19-22 days old. All data retrieved refer to the nestling period. At least two of the three males returned to their breeding site in 2009, enabling the retrieval of detailed data regarding their trip to and from their African wintering quarters (Liberia and Equatorial Guinea/Gabon, respectively), and both starting to breed within a few weeks uopn arrival on the breeding grounds. Home range – Home range size of the three males, as measured by their minumum convex polygon, varied from 1295 (male 57) to 2828 (male 56) and 4257 ha (male 58). Most activities, however, were concentrated in much smaller core areas (95% and 70% kernels), i.e. respectively 1268 and 347 ha for male 56, 556 and 154 ha for male 57, and 701 and 68 ha for male 58. Habitat choice and territoriality – Woodland was clearly favoured as foraging habitat, and visited almost twice as often as expected from a random distribution across habitats. All other habitats, except road sides (wider than 2 m) and edge habitat which were used frequently, were largely avoided, i.e. heathland, grassland, arable land and built-up areas (including recreation areas, summer cottages spaced less than 50 m apart and campings in woodland). Well-spaced gardens, however, were visited on foraging trips. Within woodland, the birds showed a clear preference for clearings, open canopy forest and wide dirt roads. Dense homogeneous stands were avoided. Two neighbouring males showed hardly overlap in home range; potential intrusions were countered with low-level, high-intensity wing-clapping displays. Daily activity pattern – Foraging activities peaked from 6.00 hr through 19.00 hr Central European Winter Time, but the first movements interpreted as foraging were already discemable at 4.00 hr. Food transportations started at 5.00 hr, then progressively increased in frequency till 11.00 hr, with another – slighter – peak between 16.00 and 18.00 hr. The longest foraging distances were covered around noon, but already as early as 6.00 hr – when thermals were absent – flight distances of up 3 km were no exception. In general, flight altitudes were lower than 100 m, possibly even below canopy level. The first flight exceeding an altitude of 100 m was recorded by 6.50 hr, the 200 m boundary by 8.24 hr, 300 m by 8.58 hr, 600 m by 9.21 hr, and 700 m by 14.01 hr. Flights exceeding an altitude of 700 m were recorded up to 16.03 hr, those exceeding 600 m up to 16.38 hr, of >400 m up to 17.09 hr, of >200 m up to 17.46 and of>100 m up to 18.11 hr. Later flights were registered exclusively at canopy level or lower. Foraging – The maximum distance at which the birds foraged away from their nest varied between 3717 m for male 57 (on average 1750 m), 4337 m for male 56 (on average 1812 m), and 5359 m for male 58 (on average 1655 m). Foraging activities peaked close to the average distances. Correcting for the increasing surface area of foraging habitat at increasingly larger distances from the nest, the males intensively used the forest band within 500 m of the nest, and – less so – areas some 1500-2000 m away (shown by all three males). In general, a trend was visible of declining usage of areas pro rato their distance from the nest. Food deliveries and nest attendance – During the middle and later nestling period, the activities of male Honey Buzzards were recorded for 63 days from sunrise till sunset (with at least one GPS fix per 20 minutes). The number of prey delivered at the nest varied between 2 and 9 per day, on average 5.43 prey/day. Theoretically, some food deliveries may have been missed with the GPS-settings used. When using intervals of one fix per 10 minutes, this may have amounted to 5% at most (involving quick successive flights to and from the same nearby wasp nest). The number of food transportations by males showed some decline in the latter part of the nestling period, coinciding either with an increasing involvement of females in food deliveries (which we did not register), or an increase in the size of wasp combs delivered to the nest in the course of the season (in conjunction with the growth of hymenopteran nests). Nest attendance of males declined after the smallest chick had reached an age of 15-18 days old, about the age that the chicks can take care of their own thermoregulation. Nocturnal roosts – Male Honey Buzzards left their nocturnal roost on average 37 minutes before sunrise (N=9), and settled on their roost on average 28 minutes before sunset (N=59). The shortening of day length in the course of July and August was reflected in a concomitant timing of roosting. Nocturnal roosts were mostly away from the nest: male 58 never roosted at the nest site (N=16, nearest roost site at 82 m), male 56 seven times (N=30), and male 57 twice (N=24). The males used a wide variety of roost sites, at most 2922 mm from the nest site (male 56). Some roosts were used more than once, i.e. 6x twice, once three times, once four times, once seven times and once eleven times (by male 56, at 387 m from his nest). Repeated use of roosts did not necessarily occur on successive nights. For example, the roost used 11 times was used on 24-26 July, 29 July, 3-5 August and 7-10 August. The future – This preliminary study will continue in the next few years, preferably including females. We suspect different outcomes in relation to food abundance; in The Netherlands the relative abundance of social Hymenoptera varied by a factor of at least 40 between 1975 and 2008 (R.G. Bijlsma). Our study in 2008 coincided with a rather poor wasp abundance, particularly in the latter part of the breeding cycle of Honey Buzzards. Moreover, the mutually exclusive home range of two neighbouring males contradict earlier findings based on visual observations from tree tops and using radio tags (which showed widely overlapping home ranges), and needs validation by replication. The fact that two of the three males returned to their breeding sites in spring 2009 offers unique opportunities to register activities from arrival onwards, i.e. during the period that the most important decision of all – to breed or not to breed – is made.