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?


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

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.

Chick squeaks

During the first few weeks of life, wader chicks rely upon their parents to take them to good feeding spots and to look out for potential predators. These youngsters will need to come when called and to freeze or hide when a crow flies over or a fox is on the prowl. A wader chick communicates with its parents too, so that they know where it is and can be alert to distress, but are there more subtle messages being communicated and why do male and female chicks produce discernibly different calls?

Kristal Kostoglou has been listening to the chicks of Red-capped Plovers and Southern Masked Lapwings in Australia and analysing recordings of their calls. Her research is presented in a 2022 paper in Ibis.

Handling waders

Broken-wing distraction display by parent Red-capped Plover

Anyone who studies breeding waders will be aware that parents get very anxious if a chick that is being ringed and measured starts making alarm calls – but how many people have recorded those calls? As Kristal Kostoglou says in the abstract of the paper she has written with her coauthors “We opportunistically recorded calls of chicks while they were in the hand and analysed the calls to determine whether call structure is related to sex or body mass (a proxy for age)”. The results are fascinating.

At the start of the paper, there’s a useful summary of previous studies of the calls made by chicks. It is believed that developmental changes in chick vocalisations can allow parents to assess chick age and/or condition, with evidence that calls get deeper (frequency drops) as chicks get larger. There have been a couple of previous wader studies, relating to Pied Avocet and Eurasian Stone-curlew. In the Ibis paper, Kristal Kostoglou investigated potential relationships of call traits to body mass and sex in chicks of two shorebird species; Red-capped Plover and Southern Masked Lapwing.

The two study populations

The calls of Red-capped Plover chicks were recorded in the Cheetham Wetlands (Victoria, Australia) and the Southern Masked Lapwings were studied on Phillip Island (Victoria, Australia). Parents brood and defend their chicks which fledge at about 35 days (plovers) and 45 days (lapwings) respectively. Nests were found during incubation and most chicks were ringed just after hatching. Each chick was measured & weighed, a recording was made, and a small amount of blood was taken, to establish sex.

Recording the sounds made by a Red-capped Plover chick, as it is weighed and measured

Sound recordings were made using a portable digital recorder and an omnidirectional microphone. The 26 plover chicks made between 1 and 248 calls each and the 95 lapwing chicks produced between 1 and 336 calls. Data were collected from 9 female plovers and 17 males, between from the day of hatching and approximately 4 weeks of age. For lapwings the equivalent figures were 46 females, 49 males and up to 5 weeks of age.

Six call traits were analysed: call duration, the time between calls, entropy, minimum dominant frequency (kHz); dominant frequency range (the difference between a call’s minimum and maximum dominant frequency); and frequency modulation. Please see the Ibis paper for more details and technical information about the analyses. This related paper may also be of interest: Anatomy of avian distress calls: structure, variation, and complexity in two species of shorebird.


For Red-capped Plovers:

  • Males were more vocal – time between calls was shorter for males than for females.
  • Heavier (and hence older) chicks called more frequently.
  • As mass increased, the dominant frequency range of calls decreased (calls became less shrill).

For Southern Masked Lapwings:

  • For both sexes, dominant frequency range decreased with increasing body mass (calls became less shrill with age).
  • The decline in dominant frequency range was greater in males, resulting in a lower dominant frequency range than for females. This meant that the difference between the calls of the two sexes became more discernible over time.
  • Frequency modulation was lower for males than for females.
  • As body mass increased, frequency modulation and entropy of lapwing calls decreased.

In the Discussion, the authors consider how a heavier bill and changes to the structure of the upper vocal tract might be linked to the results. It has been suggested that chicks modify sound output to utter more adult-like calls, as they get older.

Red-capped Plover chick

A faster repetition rate of distress calling, as observed in Red-capped Plovers, might encourage parents to provide more defensive support for male chicks, which could contribute to the higher survival of male over female chicks, as reported for several plover species.

For Southern Masked Lapwings, there appeared to be sex-linked differences in calls from hatching, with these differences getting more marked with age. This could mean that either the voice anatomy develops differently or that vocal control is different in male and female chicks.

Conservation implications

Southern Masked Lapwing chicks, at the point of hatching

Both Red-capped Plovers and Southern Masked Lapwings are considered to be of ‘least concern’, according to the IUCN/BirdLife conservation criteria. Many other waders, around the globe, are under various levels of threat, with predator pressure being a significant cause of decline for some species. The ability of adults and chicks to stay together through to the fledging of the chicks could be particularly important, in these cases, with communication being key to success.

Kristal Kostoglou’s study analysed distress calls, which she and others suggest may be under the influence of natural selection. She points out that some non-distress vocalisations, such as contact calls, might communicate further information about the caller’s sex. It is also possible that distress calls may serve to communicate with siblings or with other chicks, not just with parents. In the paper, the authors suggest that future studies could investigate associations between shorebird chick calls and sex, using the full repertoire of chick calls and across species whose adult call repertoires and characteristics vary between sexes. There were few recaptures of chicks in this study and the authors suggests that it would be interesting to observe how calls of individually-marked chicks change over time.

A passing thought

Head-started Black-tailed Godwit chicks have no contact with their parents

One issue that might be considered by others is how chick calls develop if there are no parents in attendance, as we see if chicks are head-started. There is a growing movement to support populations of threatened wader species by removing first clutches of eggs, incubating them and then rearing the chicks in captivity. In their pens, they have no contact with adults and communicate only with each other.

It is clear that head-starting has worked for Spoon-billed Sandpipers, a species that was heading for extinction as discussed here, and the early signs are good for England’s breeding Black-tailed Godwits but do these hand-reared individuals miss out in some ways? Potentially the ‘language’ developed within the family may be important when the chicks are themselves parents? That’s not going to be easy to test! However, perhaps it might be possible to see if there are any differences in the development of calls between hand-reared and parent-reared chicks of the same species?

Communication between parents and chicks helps to keep the family together

In summary

The study published in Ibis provides the first evidence for charadriid chicks of (a) a sex difference in call structure and rate and (b) gradual growth-related changes in call structure and rate, across chicks. The detailed write-up will hopefully be useful in further studies of shorebird vocalisations during growth, which may help further to explain the development and functional significance of all that squeaking!

Vocal traits of shorebird chicks are related to body mass and sex. Kristal N. Kostoglou, Edward H. Miller, Michael A. Weston and David R. Wilson. Ibis. https://doi.org/10.1111/ibi.13055

Red-capped Plover chick seeks to hide in the cracked mud, using the shadow to break up its outline

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.