Tweede broedgeval van de Zeearend Haliaeetus albicilla in Nederland

In the winter of 2004/05, an adult male and 2nd/3rd year female (ringed as a chick in northern Germany in 2003) had settled in the Oostvaardersplassen, and attempted breeding for the first time in 2006 (successfully raising a chick). The same pair (female individually colour-ringed, male fully adult and behaving like the bird in 2006) nested again in 2007, using the same nest. In the intervening winter, a webcam had been positioned near the nest. The real life ongoings at the nest were relayed to an internet site, covering the entire day from dawn to dusk (www.staatsbosbheer. nl). We stored the video footage for the first 15 minutes of each hour of daylight on a hard disk, to be analysed at a later date. We consider these data as representative of the behaviour throughout the day and throughout the nesting cycle. Between 2 February (start of filming) and 8 July 2007 (fledging date), we recorded 427 hours systematically distributed across the entire breeding cycle, especially from egg laying onwards (Fig. 1). The observations below were made within this time interval. New nesting material was transported to the nest between 13 February and 5 June, involving 71 trips. This encompasses the pre-incubation period, the incubation period and the nestling stage. Male and female were both involved in nest maintenance, bringing sticks for the nest rim and grasses, mosses and other plant material for the nest cup (Table 1). At the end of the 2006 breeding season, when short of one year old, the nest diameter measured 1.7x 1.6 m, and nest depth was 2.1 m. On 4 October 2007, after the second brood had been raised, the diameter had been slightly expanded to 1.8x1.7 m and the depth had increased to 2.65 m. Copulations were recorded 6 times, peaking just before egg laying and none after egg laying (Fig. 2). The earliest copulation, on 23 February, may have been without cloacal contact (duration 7 seconds only). The average copulation took 12 seconds (between mounting and dismounting), ranging between 7 and 14 seconds. Four out of six copulations took place before 10h00, with others at 14h00 and 18h00. The – presumably single – egg was laid on 9 March. Male and female took about an equal share in incubation (Fig. 3), and together covered the egg for almost 100% of the time. The egg was turned on average 1.42x per hour by the female (70 times in 2943 minutes of observation), and 1.62x per hour by the male (82x on 3028 minutes) (Fig. 4). Egg turning on average took 2.4 minutes (range 1-7 minutes); the frequency of egg turning slightly increased towards the end of the incubation period (in both sexes). Whilst on the nest the female was seen biting a stick regularly, especially prior to egg laying; the male was never involved in such an activity. Calling on the nest was common prior to egg laying, both in male and female. During incubation, calling frequency declined, and was then mostly restricted to the female. Her calling frequency increased again near hatching date, to steadily drop off afterwards till calling ceased completely after the chick had reached an age of 5-6 weeks (Fig. 5). During its first weeks of life the chick was covered for extended periods of time. When 30-40 days old, sleeping still took about half of the time during daylight (Fig. 6); this declined steadily to about 10 minutes per hour in the ten days before fledging. Preening occurred throughout the nestling period, but never took more than 15% of the daylight hours. When 38 days old, the chick started wing practicing. The amount of time spent wing flapping increased steadily over time, and peaked in the 10 days prior to fledging (taking about 7 minutes per hour, often jumping across the nest and up into the air for several meters). From hatching onwards (15 April, between I4h00 and 14h 15), the female’s share in nest attendance increased. Up to an age of some 40 days old, the chick was attended for 70% or more of the time, then left mostly alone till fledging (on 8 July). The chick was measured, weighed and ringed on 24 May, when 39 days old (Table 2). According to body mass, tarsus width (following Helander et al. 2007) and bill depth, the chick was sexed as a female, which was verified with a DNA-sample. Up till about 40 days old, the chick was fed by its parents. From then on it increasingly started to tear prey items apart by itself (Fig. 7). This shift in behaviour was accompanied by a steep decline in nest attendance by the parents. In the three weeks before fledging, prey was simply dropped on the nest and left to be handled by the now full-sized nestling. Removal of prey remains by the parents was not witnessed, but must have occurred given the number of prey brought to the nest, and prey remains found on the nest during two nest visits. Few prey were brought to the nest during the incubation period (Fig. 8). As the sexes shared incubation equally, presumably feeding was done away from the nest. Prey delivery rate peaked just after hatching, then remained more steady at 2-3 prey items per day. Across the nesting cycle, prey delivery rate averaged 0.19 prey /hour, much higher for the male (0.16/hr) than for the female (0.04/hr). The higher proportion for the male was partly linked to its tendency to feed on the nest (31% of the cases); this was rarely seen in the female (17% of the cases), who fed away from the nest. Prey choice was rather straightforward, consisting mostly of Greylag Goose, ducks. Coot, Carp and Musk Rat (Table 3). Prey choice as revealed from webcam observations did not differ significantly from that based on prey remains collected on the nest during and after the nestling stage (lumping prey in categories: birds, fish, mammals). Greylag Goose are mostly captured as goslings, incidentally also adults when moulting. Gosling remains on the nest varied in age from 10-22 days old (with estimated masses of 330-1000 g; Fig. 8), but it is likely that both younger and older goslings were taken as well (as witnessed via the webcam). Greylag Goose is a common breeding bird in the Oostvaardersplassen, with 412 nesting pairs in the marshes in 2007. Breeding is rather synchronised; the majority of goslings hatches in the first two weeks of April. Adult geese take the goslings to grasslands bordering the marsh, where most feeding takes place. Hatching date of the eaglet, 15 April, coincided with peak numbers of goslings available. On top of that, each year some 20.000 Greylag Geese stay in the marshes to moult; unable to fly for some weeks, they may also be an easy catch for White-tailed Eagles. Agonistic interactions with other birds were frequent, notably with Marsh Harriers Circus aeruginosus (nesting nearby). Buzzards Buteo buteo (ditto). A White Stork Ciconia ciconia shortly visited the nest when both parenst were absent; the chick was not attacked. A pair of Ravens Corvus corax frequently visited the trees surrounding the nesting tree during the incubation period, without being chased by the eagles.