Contrasting the flexibility of schedules in related species with different migration strategies: Black-tailed Godwits and Whimbrels breeding in Iceland

Timing of migration is critical for migratory birds and can greatly influence breeding success and survival. The timing of arrival, departure and breeding schedules of two closely related shorebird species breeding sympatrically in S Iceland are compared, the Black-tailed Godwit Limosa limosa (in 2001–2003) and Whimbrel Numenius phaeopus (1997–1999). The godwit is a short- to medium-distance migrant wintering in W Europe, whereas the Whimbrel is a long-distance migrant which winters in W Africa. Both species had a similar mean and modal laying date (around 31 May) but the nesting period of the godwits was considerably longer at both ends. Godwits arrived well before the nesting stage, most over the period 20 April to 10 May and, on average, two weeks earlier than Whimbrels which arrived in early to mid May. In some years, the earliest-arriving, individually-marked Whimbrels had left the breeding territories before the last arrived. For godwits there was no overlap in the distributions of arrival and departure dates.Godwits have been advancing their arrival dates in S Iceland by c. 0.55 days/year over the last two decades, probably as a consequence of changing climatic conditions, and possibly also because of density-dependent pressures. This earlier arrival occurs despite the fact that they do not breed until almost a month after arrival. Whimbrels on the other hand still arrive at the same time as 20 years ago and have only half the time godwits have between arrival and breeding. For both species, the period when the chicks have the highest energy demand coincides with the peak in invertebrate abundance, in late July. The difference in migration timing and trend between the species may reflect a more pressured annual schedule in the Whimbrel, a long-distance migrant, while the godwits may be able to afford greater flexibility in the timing of migration.