This paper reports on the ringing recoveries of oiled, rehabilitated and released Guillemots from rehabilitation centres in The Netherlands. The question posed is: what are the chances of rehabilitated oiled Guillemots to survive in the wild after their release? Birds from the British eastcoast have been shown to be the main source of birds wintering in Dutch waters (Harris & Wanless 1997; Harris et al. 1997). Mortality patterns and survival were therefore compared to a control group of un-oiled Guillemots ringed at the Isle of May. A total of at least 1844 Guillemots have been ringed and released in The Netherlands, 1723 of which between 1979 and 1995 (Figure I). From the total sample, 119 recoveries (6.5%, n-1844) are registred by the Dutch ringing centre (Vogeltrekstation Arnhem in Heteren), including two sightings of Dutch ringed Guillemots in the Isle of May colony (see Harris & Wanless, this issue, for details). Most released birds, however, were reported from the Dutch (75.6%) and German (10.9%) North Sea coasts (Figure 2) and shortly after release. Of the 108 recoveries from the sample from between 1979 and 1995, the median recovery lime was 12 days after release (mean ± S.E. = 164 ± 22 days, range 0-2973 days: Table I). 69.4% of the recoveries were within a month after release, 91.6% within a year (Table 2). The Isle of May control sample comprises 2176 'full-grown ’ Guillemots, ringed during 1979-95, including approximately 21.8% non-breeding birds. A total of 57 of these birds were recovered (2.6%), the median recovery time was 671 days after release (mean ± S.E. = 1376 ± 38 days; range 14-6118 days). Only one bird (1.8%) was recovered within one month, and 19.3% were found dead within a year (Table 2). The recovery rate of the control birds (2.6%), their estimated annual survival (85%), and the annual ringing totals were used to calculate an expected number of recoveries for each year since ringing for the Dutch releases (Figure 3). The null-hypothesis that the two groups show similar behaviour is clearly rejected. The main difference between the expected and observed recoveries of birds ringed in The Netherlands, was the reporting of most of the recoveries in the first year after release (91.6% observed, 6.9% expected; Figure 3a). When this first year mortality is excluded, the recovery rate matched that of the control group. Hence, if the released birds survive the first year, they should be considered 'successful' releases. These figures suggest that approximately 78% of the released birds died within a year after release (in fact most died within one month), 22% survived. The proportion of birds recovered immediately slowly declined over time, suggesting continuing improvements in treatment in rehabilitation centres in The Netherlands (Table 4). A similar result was reported from Plymouth, UK (Partridge 1997). The paper reports on all known 'long- time’ survivors (16 birds, Table 3) and on the poor survival of 'rehabilitated' birds that were first released in cages at the Netherlands Institute for Sea Research, before releasing them in the wild. These cages were certainly suitable for survival of healthy birds, as other Guillemots successfully bred for several years in the cages. These observations, and the high immediate mortality of released birds into the North Sea show that many birds are released in a condition that is too poor for survival. Unfortunately, the successful releases cannot be explained in terms of parameters such as: age, oiling rate, duration of captivity, mass at arrival or mass at release, or within- or between centres variations in treatment. The rehabilitation of oiled Guillemots in The Netherlands has yielded more than incidental success, since about one fifth of the released, ringed birds survived. However, considering that some 1.5 million Guillemots winter in the North Sea (Skov et al. 1995), with 15% found in the Dutch sector (250 000; Camphuysen & Leopold 1994), and that in some years in the 1980s 15 000-20 000 Guillemots washed ashore oiled and dead (Camphuysen 1989), the contribution of the rehabilitation programme to the conservation of seabirds is very small (hundreds of birds over the years).

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Nederlandse Zeevogelgroep

Kees (C.J.) Camphuysen, Piet Duiven, Mike P. Harris, & Mardik F. Leopold. (1997). Terugmeldingen van in Nederland geringde Zeekoeten Uria aalge: de overleving van gerehabiliteerde olieslachtoffers. Sula, 11(3), 157–174.