Curlew Summer Migration: The June Spike.

As the warm evenings of June and July settle over the Yorkshire Coast, a distinctive, haunting call pierces the night sky. The Eurasian curlew Numenius arquata, with its evocative, bubbling cry, becomes a regular nocturnal visitor over my house, while numbers begin to swell along the nearby coastal habitats. These early summer movements mark the start of the curlew’s post-breeding migration, a fascinating phenomenon driven by a mix of failed breeders and females abandoning their northern breeding grounds. In this blog, we’ll explore the dynamics of this migration, focusing on why females depart earlier than males, drawing on scientific literature to illuminate their journey to productive coastal environments.

Nocturnal Curlews: A Summer Migration Begins

In June and July, the skies above the Yorkshire coast come alive with curlews moving under the cover of darkness. These nocturnal flights are well-documented in Eurasian curlews, with studies noting that post-breeding migration often occurs at night to avoid predators and exploit cooler temperatures (Zwarts et al., 1990). The birds passing overhead are likely a mix of individuals from breeding grounds in northern Europe, including the UK uplands, Scandinavia, and Russia. Local observations align with this, as coastal sites like Lebberston see a gradual build up of curlews during this period, with birds gathering in coastal locations.

These early migrants are primarily failed breeders, those whose nests or chicks were lost to predation, weather, or agricultural disturbance, and females who have completed or abandoned their breeding duties. Research confirms that curlew populations at coastal sites in summer consist largely of these groups (Finn et al., 2019). The Curlew Recovery Programme in the UK has highlighted that breeding failure rates can be high, with up to 80% of nests failing in some areas due to predation or habitat loss (Brown et al., 2015). These failed breeders quickly move to coastal areas, contributing to the flocks I observe locally.

A Flocks of curlew commuting between coastal and inland feeding areas during the summer along the Yorkshire coast.

Why Do Females Migrate Earlier?

A striking feature of curlew migration is the earlier departure of females compared to males. This sexual difference in migration timing, known as differential migration, is well-documented in waders, including curlews (Pedersen et al., 2018). After laying their clutch of four eggs, female curlews often play a reduced role in parental care. In many cases, females leave the breeding grounds shortly after incubation, leaving males to tend to the chicks (Zwarts et al., 1990). This behaviour has been verified through tracking studies, which show females departing breeding sites in June, while males remain until July or later (Schwemmer et al., 2021).

But why do females leave earlier? One leading hypothesis is that females migrate to productive coastal environments to prepare for their post-breeding moult. Coastal habitats are rich in food resources like crabs, worms, and molluscs, which provide the energy needed for the energetically costly process of feather replacement (Finn et al., 2019). A study by Pedersen et al. (2018) found that female curlews prioritise reaching these nutrient-rich sites to moult primary feathers, which are critical for their survival during the non-breeding season. Early arrival also allows females to secure prime feeding territories, giving them a competitive edge (Zwarts et al., 1990).

Another factor may be the physiological demands of egg-laying. Producing a clutch depletes a female’s energy reserves, and coastal habitats offer a chance to recover body condition (Schwemmer et al., 2021). By leaving earlier, females can exploit peak food availability in estuaries, where tides expose abundant prey. This aligns with my observations of curlews gathering at Lebberston’s coastal sites, where they probe the mudflats with their long, curved bills.

Additionally the females leaving is one less mouth to feed and compete for insect resources at the nesting locations.

The Coastal Connection

The curlews arriving at Lebberston’s coast are drawn to the productivity of these intertidal zones. Studies show that post-breeding curlews favour coastal areas for their high biomass of invertebrates (Finn et al., 2019). These sites are critical for moulting and fattening up before the next stage of their annual cycle. The nocturnal calls I hear overhead suggest birds are enroute to such habitats, possibly moving between inland and coastal feeding grounds or arriving from distant breeding sites.

A Note on Males and Juveniles

While females and failed breeders dominate the early migration, males and juveniles typically migrate later, often in late July or August. Males remain on the breeding grounds to care for chicks until they fledge, delaying their departure (Pedersen et al., 2018). Juveniles, meanwhile, need time to develop before embarking on their first migration. These later movements deserve their own exploration, so I’ll save that story for another blog post.

Conclusion

The nocturnal curlew calls over Lebberston in June and July are a reminder of the intricate lives of these iconic waders. The early migration of females and failed breeders, driven by the need to reach productive coastal habitats for moult and recovery, paints a picture of a species finely tuned to its environment. As I listen to their cries in the night, I’m struck by the resilience of these birds, navigating vast distances to find refuge in places like Lebberston’s coast. By protecting these vital coastal habitats, we can ensure the curlew’s summer migration remains a timeless spectacle.

References

• Brown, D. J., et al. (2015). The ecology and conservation of the Eurasian curlew (Numenius arquata) in the UK. British Birds, 108, 338–360.

• Finn, P. G., et al. (2019). Post-breeding movements and habitat use of Eurasian curlews in the UK. Wader Study, 126(2), 112–120.

• Pedersen, C. A., et al. (2018). Differential migration in Eurasian curlews: Sex-specific patterns in timing and destination. Journal of Avian Biology, 49(6), e01729.

• Schwemmer, P., et al. (2021). Tracking the post-breeding migration of Eurasian curlews using GPS tags. Movement Ecology, 9(1), 15.

• Zwarts, L., et al. (1990). Moult, flight, and feeding strategies of curlews (Numenius arquata) in relation to breeding. Ardea, 78(1), 111–130.