How long does a godwit wait around to see if last year’s mate will turn up?
Colour-ringing enabled Tómas Gunnarsson to follow the lives of pairs of Black-tailed Godwits nesting near his parents’ home in Iceland. In this world, that is ruled by timing and opportunity, the pairings, divorces and re-pairings could form the plot for a TV soap-opera. The studies turned into a fascinating Nature paper that was written up in The Telegraph newspaper. The two main characters were christened Gretar and Sigga by the journalist but they’re more commonly known as RY-RO and RO-RO.
A tale of two godwits
2002: Colour-ringed Black-tailed Godwits RY-RO (red yellow – red orange) and RO-RO bred successfully in Laugaras, in the inland part of Iceland’s Southern Lowlands. Come the autumn, they left Iceland. The female (RO-RO) probably spent the winter in Portugal, although she was only seen there in later years, and the male (RY-RO) opted for the somewhat colder conditions of eastern England.
2003: Next spring, RO-RO arrived on territory on 6 May, before her mate. She cannot have known whether he was late or dead when she made the decision to move in with a new male, who was later colour-ringed as OR-OO. When RY-RO arrived back a week later, on 13 May, he had to find himself a new female (GG-YO), who had been paired to a different male in 2002.
2004: Come the spring of 2004, RO-RO and RY-RO arrived at the same time and got back together.
This is only one story but it seems to illustrate that there are good reasons to nest with a partner that is well known to you. This could help to illustrate why individual godwits are generally very good at timing their arrival back on territory, to synchronise with their partners, as revealed in this Nature paper, published in 2005.
Gunnarsson, T.G., Gill, J.A., Sigurbjörnsson, Þ. & Sutherland W.J. (2004) Arrival synchrony in migratory birds. Nature, 431, 646-646. DOI: 10.1038/431646a
In the paper the authors described the return of pairs of Icelandic Black-tailed Godwits to Laugaras in the spring of 2003. Godwits generally arrive in Iceland over a one-month period, between mid-April and the middle of May.
On average, previously paired males and females in the study arrived within 3.1 days of one another, despite the fact that males and females from the same pair had spent the winter on average about 1000 km apart and that there is no evidence that any pairs had met at passage sites prior to crossing the Atlantic. Arrival synchrony seems to be related to mate retention, as the only divorces occurred in two of the three pairs that arrived more than eight days apart.
Synchrony in timing of arrival on the breeding grounds may be important for retaining a mate from the previous year and avoiding a costly divorce – but how it is achieved is a mystery.
Tómas Gunnarsson and his father, Gunnar Tómasson, have been studying the timings of spring arrival in south Iceland of a range of species since 1988. In a 2011 paper in Bird Study they estimated that the timing of arrival of the first black-tailed godwit moved earlier by about 5.5 days per decade over that period. Here’s a link to the paper.
As this advance in spring timing of migration was already happening when Tómas was making observations of the paired birds in Laugaras in 2003, we were all interested to see whether the schedules of marked birds would advance in similar ways. Interestingly, we have been able to show that the timing of arrival of individual Black-tailed Godwits is actually not changing at all. There is year-to-year variation in the dates on which individuals arrive, but no trend. Instead it is new recruits into the population that are driving the earlier migration. There’s a blog about this here.
Whilst there are processes in play that mean new recruits are migrating earlier than their predecessors, there must also be reasons why time-fidelity is important for individual birds. Perhaps synchrony increases the probability that individuals will be able to nest with the same mate in subsequent years? This is hinted at by the fact that godwits have been observed to re-pair with previous partners if opportunities present themselves.
Potential benefits of re-pairing with the same mate
For Black-tailed Godwits, not enough is known about the benefits of retaining the same mate. Given that divorce events are rare, it would be hard to measure any consequences for productivity – even if the nests were easy to find and youngsters easy to track – neither of which is the case. For the moment, all that is available is evidence of divorce and the possibility that females will not wait for males that are late.
Black-tailed Godwits are long-lived birds, with breeding territories in which resources are generally predictably distributed, and a pair is likely to be familiar with local predator densities and distributions. Whilst one member of the pair is incubating the eggs, the other spends a lot of time looking out for potential predators, and this mutual protection may well confer benefits for the adults and the eggs. Perhaps knowing the behaviour of one’s partner is important during the incubation period?
The complexities of incubating eggs
If the daily routines associated with parental change-overs at the nest become established over time, might this be an important driver towards fidelity? Fast forward to a paper on shorebird incubation patterns, published in Nature in 2016 by Martin Bulla et al, which might provide some clues:
Unexpected diversity in socially synchronised rhythms of shorebirds Nature 540,109–113 (01 December 2016) doi:10.1038/nature20563
This paper is the result of a collaboration between Martin Bulla and 75 of his wader biologist colleagues, all happy to share data on nest incubation patterns which Martin then analysed. This resulted in an amazing data-set of 729 nests from 91 populations of 32 shorebird species, from which Martin was able to report remarkable within- and between-species differences in nest incubation rhythms.
This study suggests that energetic demands are not an important ecological driver of incubation bout length, but instead that pairs have developed idiosyncratic incubation patterns, possibly as an anti-predation strategy. Effectively, risk of predation, rather than risk of starvation, may have a key role in determining some of the variation in incubation rhythms. This means that species that hide their nests (and themselves) incubate for longer and change places less frequently.
Ringed Plovers, for instance, walk away from their eggs when a potential predator approaches and change places on the nest frequently. A male or female Redshank, on the other hand, will sit tight and brood for about six hours before exchanging with its partner. While partner A is hunkered down on the nest, partner B leaves the area, so as not to draw any attention to the pre-packaged protein that partner A is sitting on. If B is only going to return when A is ready for a surreptitious change-over then the activities of the two need to be well synchronised.
As the authors point out in the paper, although the context for this comparative study was diversity in biparental incubation, it is possible that diverse behavioural rhythms may also arise in other social settings (for example, in the context of mating interactions or vigilance behaviour during group foraging). These are other circumstances in which it may well be beneficial to know one’s partner.
What does this mean for RY-RO and RO-RO?
Perhaps fidelity and synchronicity are really important to Black-tailed Godwits? If only their nests were easier to find and nest success was easier to measure! For the moment, all that we know is that pairs of Icelandic Black-tailed Godwits are remarkably synchronous in their arrival times on breeding territory, and something important must have driven the evolution of such a finely tuned migratory strategy.
WaderTales blogs are written by Graham Appleton, to celebrate waders and wader research. Many of the articles are based on previously published papers, with the aim of making wader science available to a broader audience.