The Dutch consortium "NoordzeeWind" operates the first offshore wind farm in Dutch North Sea waters. The park, consisting of 36 turbines on monopiles, is located NW of IJmuiden harbour, some 8 NM off the Dutch mainland coast. Named after the nearest town ashore, the park is known as "Offshore Windfarm Egmond aan Zee" (OWEZ). Erecting the 36 monopiles was done by pile driving, from a large ship using a hydro-hammer. This technique generates considerable underwater noise levels (see report OWEZ_R_251_Tc_20070327) that might be detrimental for local wildlife. In this study, possible effects on sensitive seabirds are considered. Bird species most likely to be vulnerable to underwater sound are those that forage by diving after fish of shellfish. Diving birds that may occur in relatively high densities at the OWEZ location include auks, and possibly divers and seaduck. Terns, that feed by shallow dives are considered less vulnerable and mostly occur closer to the mainland coast. Several gull species may occur in the area in high densities, but they feed at the surface only, and are considered the least vulnerable. Pile driving took place from 17 April to 28 July 2006. The potentially vulnerable divers, seaduck and auks had largely left the area by the time the pile driving started. Migration commenced early in 2006 and any birds still left in the area by mid-April would have been scared away by the shipping activities long before actual pile driving started. Further mitigation of possible effects on sensitive seabirds included a ramp-up procedure that ensured that full hamming power was only administered after a period of low-energy blows that were unlikely to cause lethal effects on any birds still present. Furthermore, an underwater pinger, aimed at scaring off marine mammals, was put into operation 3-4 hours before pile driving started. Visual observations before and during three pile driving sessions failed to detect any of the seabirds deemed sensitive to pile driving noise in the vicinity of the construction work. Birds that did fly by the construction site (mainly gulls and terns) did not show a noticeable reaction to the activities. It is therefore concluded that effects of underwater noise on seabirds, though potentially detrimental, were negligible during construction of OWEZ. This was due to fortunate timing of the work and to appropriate mitigation measures.