When mates behave differently

Iceland’s 40,000 Oystercatchers are an interesting mix of resident birds and migrants, providing an ideal system in which to study the costs and benefits of the two options, and to try to work out what influences whether an individual becomes a ‘resident’ or a ‘migrant’. I’ve added the inverted commas because many residents migrate within Iceland in spring and autumn; it’s just that they don’t undertake long-distance flights across the Atlantic.

In their paper in Ecology & Evolution, Verónica Méndez and colleagues from the South Iceland Research Centre (University of Iceland), the University of Aveiro (Portugal) and the University of East Anglia (UK) investigate the timing and success of breeding attempts by resident, migratory and mixed (resident/migratory) pairs of Icelandic Oystercatchers.

Iceland’s Oystercatchers

As outlined in Mission Impossible: counting Iceland’s wintering Oystercatchers, about 30% of Icelandic Oystercatchers never leave the country, coping with cold temperatures, short December and January days and a restricted diet. In the winter months they can be found in the tidal zone of a few estuaries, mostly in the warmer west.

The majority of Iceland’s Oystercatchers fly 1000 km or more across the Atlantic, to Ireland, the UK and the coastal fringe of western Europe. Here, many colour-ringed birds have been spotted by birdwatchers, who play a vital part in migration studies. The blog Which Icelandic Oystercatchers cross the Atlantic? revealed that males and females were equally likely to migrate, while there appeared to be no assortative mating in spring (residents don’t preferentially choose resident partners, for instance).

It would be easy to envisage circumstances in which resident birds might be at an advantage, at the start of the spring breeding season, having not had to cross the Atlantic and thus being ready if an early nesting opportunity opens up. On the other hand, in a cold spring or after a particularly harsh winter, perhaps they could be in poorer condition than newly arrived migrants, and this may potentially delay breeding. What actually happens?

Fieldwork

Both resident and migrant Oystercatchers breed throughout lowland Iceland. Within breeding pairs, it is estimated that about 20% of pairs are resident, 46% are migrant and 34% are mixed. These are long-lived birds that generally maintain the same partners between years, despite the fact that individual males and females may spend seven months of the year up to 3000 km apart. Parents tend to be equally involved in incubation duties, territorial defence and chick rearing, although males tend to remain with their youngsters longer than do females.

Between 2015 and 2018, Verónica Méndez and her colleagues monitored the breeding attempts of Oystercatchers in southern Iceland, continuing a study of marked individuals that started in 2013. Adults were caught on the nest and sexed by later analysis of feather samples. With the help of a network of volunteer observers, the winter locations of 186 (out of 537) marked birds had been established when the paper was first written. Using these known outcomes and with additional information from stable isotope analysis, it was possible to assign the remaining 351 birds as ‘residents’ or ‘migrants’. Amazingly, 73 of these 351 birds have been seen since the isotope data were analysed and all of the assumptions on winter locations were found to have been correct.

Early nesting attempts may be hampered by spring snowfall

The first migrant Oystercatchers arrive in Iceland in February but no nesting has been recorded before mid-April. Searches for colour-ringed birds and nests were conducted every 2-3 days and then nests were followed through to hatching or failure. Second (and third) nesting attempts were also monitored. Oystercatchers remain in the vicinity of the nest after hatching their chicks and then feed them throughout the growing period. Chicks were metal-ringed just after hatching and individually marked with colour-rings when around two weeks old. Families were monitored every 3-4 days until all chicks were fledged or lost, allowing productivity (number of chicks fledged per pair) and fledging success (number of chicks fledged in nests where at least one egg hatched) to be recorded.

Who breeds when?

Verónica and her colleagues were able to estimate laying dates for 138 pairs with known migratory behaviour (56 migrant, 50 mixed and 32 resident pairs) in one or more seasons during 2015-2018, providing a total of 228 observations.

The top graph shows that, on average, 2015 was a much later breeding year than the other three. This was a colder spring; the sort of colder conditions that an older Oystercatcher may well have encountered frequently in its youth! (The longevity record for BTO-ringed Oystercatcher is 41 years – see Waders are long-lived birds – and the trend for there to be more frequent warmer springs is discussed in this Black-tailed Godwit blog).

The lower graph shows a breakdown of the data into the three categories – Resident (black dots), Mixed (grey) and Migrant (white). There is no difference between the egg-laying dates for residents across the four years. However, in the 2015 breeding season, in cases where either member of the pair is a migrant, there was an average nesting delay of over a week. An analysis in the paper shows that it does not matter which member of a mixed pair was the migrant, the delay in 2015 was the same.

Reproductive performance

Unusually amongst waders, adult Oystercatchers feed their chicks

As expected, Oystercatcher pairs that made earlier nesting attempts were more likely to lay a replacement clutch after nest loss, had higher productivity and higher fledging success. This is in line with the modelling paper described in Time to nest again. Early-nesters tended to have bigger clutches too. Any differences between the performance of residents, mixed pairs and migrants could be accounted for just by the timing of nest initiation.

In the papers’ Discussion, the authors suggest that, in the three warmer years, earlier nesting of pairs that included at least one migrant was sufficient to slightly enhance nest success but not overall productivity, above that achieved by pairs with residents. The migratory behaviour of the male within a pair appeared to have a stronger effect on fledging success than the migratory behaviour of the female, suggesting that males may play a more important role than females at the chick stage. This is interesting in the context of previously-published research by Verónica and her colleagues, as described in The Dad Effect blog.

What does this all mean?

In other studies, described in the Discussion, residents in systems where some individuals migrate have been found to have advantages over migrants, because they can get on with breeding earlier. This was not the case for Icelandic Oystercatchers, potentially because migrants can arrive in good condition in all but the coldest of years.

Hatching brood of three

In the cold year of 2015, Oystercatcher pairs nested an average of between a week and 12 days later than in other years. This delayed nesting occurred in migrant and mixed pairs but not in resident pairs, suggesting that the effect of the severe weather may have been greater on migrants than residents. Cold spring conditions in Iceland tend to be part of a wider pattern of cold weather across northwest Europe. The authors suggest that wintering conditions might influence the body condition required to reproduce and that these conditions may be more variable for migrants.

Only one cold year occurred during this study, so the authors don’t know whether pairs with migrants consistently breed later in colder years. Given that cold springs are increasingly rare in Iceland, 2015 may turn out to have been one of the few remaining opportunities to reveal the dynamic nature of links between weather, migratory behaviour and breeding phenology at these latitudes.

One potential explanation of the difference in the timing of nesting is the effect of habitat. The Icelandic team has found that there is a strong tendency for migrants to breed inland, whereas residents tend to breed along the coast. During the cold spring of 2015, inland habitats were not available as early as in the following years (everything was frozen), mostly delaying the breeding attempts of migrant and mixed pairs, rather than residents pairs.

Long-term studies

Verónica Méndez with one of the marked birds

The take-home message of the paper by Verónica Méndez and her colleagues is that it pays to nest early, which is not unexpected. Perhaps it is surprising that, in the cold spring of 2015, mixed pairs still bred at the same time as pairs of migrants, suggesting that residents waited for their migrant partners. Perhaps, the benefits of nesting with the same partner are very strong, or finding an alternative mate is difficult or both?

The study suggests that the links between individual migratory behaviour and reproductive success can vary over time and, to a much lesser extent, with mate migratory behaviour. Understanding these effects of pair phenology on breeding success may help researchers to understand the potential impacts of changing environmental conditions on migratory species. Such variation is very difficult to capture unless long-term funding is available. Four years may seem like a long time to observe the same Oystercatchers but, for birds that may easily live twenty years, this is nothing!

The full paper can be found here:

Effects of pair migratory behaviour on breeding phenology and success in a partially migratory shorebird population. Méndez V., Alves J.A., Gill, J.A., Þórisson, B., Carneiro, C., Pálsdóttir, A.E., Vignisson, S.R. and Gunnarsson, T.G. Ecology & Evolution

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WaderTales blogs are written by Graham Appleton (@GrahamFAppleton) to celebrate waders and wader research. Many of the articles are based on published papers, with the aim of making shorebird science available to a broader audience.