Early return to breeding areas is widely acknowledged to be ‘a good thing’ but why? Some people suggest that early migrants can choose the best territories, others argue that early chicks have a disproportionately high chance of fledging but there are other explanations too. In their paper in Ecology & Evolution, Catriona Morrison and her colleagues ask how much of the advantage of being an early migrant could be associated with having an option to nest again, if the first attempt fails.
Setting the scene
In a previous WaderTales blog, about Icelandic Black-tailed Godwits, there is clear evidence that the species is increasing in number and spreading into new breeding areas. In another blog you can read that the expansion is linked to warmer spring conditions, which allow earlier nesting. How might this change in nesting phenology influence overall productivity and contribute to the population growth in Black-tailed Godwits, and do the same processes work for other species?
In their 2019 paper, Catriona Morrison and her colleagues from the Universities of East Anglia (UK), Iceland and Aveiro (Portugal) used a simulation model to ask whether the greater time available for laying replacement clutches can create a pattern of increased productivity among early-arriving migrants, without the need to think about territory choice or local resource availability. They suggest that early arrival can lead to greater breeding success simply because early birds have more time available to nest again, following nest loss. Within the model they explore the effect on breeding success of varying several important relationships:
Whether early clutches are more likely to hatch than later ones (seasonal variation in nest survival rates) – such a trend could be created by predation patterns, resource availability and opportunities to conceal nests.
- Whether one or more replacement clutches is possible within the time available during the breeding season (number of re-nesting attempts).
- Whether late chicks are less likely to survive and become breeding adults than earlier ones (seasonal variation in recruitment rates) – which would make re-nesting attempts less valuable.
The models developed for the paper showed that, when the chance of losing a breeding attempt does not change during the course of the breeding season, species experiencing intermediate nest survival rates will benefit most from re-nesting. This makes sense; a species that has a very high chance of hatching its chicks will not need to re-nest and one that has a negligible success rate is not going to do much better if it lays more than one clutch.
Nest success may not be constant over the course of a season. Late pairs may find it harder to distract predators if they don’t have the support of other breeding birds, with a consequent drop in success over the summer. Alternatively, species that nests in clumps of grass, such as Snipe, might find it easier to hide their nests later in the season, thereby increasing nesting success over time.
Picking out just a few of the scenarios that are covered in more detail in the paper:
- When nest survival rates are constant and replacement clutches are possible, early arrival increases the probability of achieving a successful nesting attempt. These benefits of early arrival can be substantial enough to persist even when late-hatched chicks (from replacement nests) are less likely to survive and recruit into adulthood.
- If there is a seasonal decline in nest survival, late-arriving individuals will have far fewer successful nesting attempts in their lifetime than early-arrivers. In this case, laying replacement clutches only slightly increases the number of successful nesting attempts and the subsequent number of recruits.
- If there is a seasonal increase in nest survival, early-arriving individuals will tend to lose their first clutches but these individuals have time to re-nest, and are likely to fledge the subsequent attempt. Late-arriving individuals arriving will be more likely to have a successful first attempt and hence the number of successful nesting attempts varies little with arrival date.
The main take-home message of the paper is that, in almost all of the circumstances considered, early arrival can lead to higher breeding success, simply because of the greater time available to lay replacement clutches.
What does this mean for waders?
Repeat nesting is a common strategy in waders; a female Oystercatcher, for instance, can quite quickly lay a second clutch if the first clutch is lost. Strategies exist that can lead to a female having more than one successful brood in a season, as seen when a female Dotterel leaves a male to incubate a clutch of eggs and moves on to another male. In most circumstances, however, a pair of waders has time to raise one brood of chicks in a season, by succeeding with the first attempt or taking opportunities to lay replacement clutches if time and resources allow.
It is obvious that, if nesting success is very high, there will be little need to lay a second clutch and if success is really low, little will be achieved by laying more clutches. Waders tend to have intermediate nest-success; most are ground-nesters, making them vulnerable to a wide range of mammalian and avian predators of eggs and chicks. The scenarios modelled in the paper are particularly (but not exclusively) appropriate to breeding waders
The modelling used in this paper shows that having the time to try again is likely to increase the probability of annual success, as long as the breeding season is long enough. We know that pairs of Ringed Plovers breeding at temperate latitudes have time for several breeding attempts but pairs at high latitudes may have little chance for a second attempt, especially if nest failure occurs late in the incubation period. One way of increasing the time available to breed is to arrive earlier and the benefits of early arrival may be particularly strong for birds that occupy areas where there is a lengthening potential breeding season, something that can be made possible through climate change and warmer springs.
It is not uncommon for a breeding wader to live for five years, ten years – or even longer for larger species (WaderTales blog). During its lifetime, an individual may experience breeding seasons with differing levels of predator activity or other causes of nest loss, such as flooding or trampling, might occur. Although an individual might migrate at the same time each spring, the number of nesting attempts it will be able to fit in during any particular year will depend upon factors such as weather, prey availability and predation pressure.
Nest survival rates in wader populations can show seasonal declines (e.g. Sandercock 1999 – Semipalmated Sandpipers), increases (e.g. Reneerekens et al. 2016 – Sanderling) or little seasonal variation in survival (e.g. Sandercock 1999 – Western Sandpiper), but in all cases there is variability between years. All of these seasonal patterns of survival change were modelled in the Morrison et al paper. In almost every situation, a wader will have a higher chance of successfully rearing youngsters if it (and its mate) are on an early spring migration schedule.
Turning up early on breeding grounds in spring can potentially lead to higher reproductive success, solely as a result of the greater time available for laying a replacement clutch. Using modelling, Catriona Morrison and colleagues show that this early-arrival-benefit can be conferred even when later nesting attempts are less likely to produce successful recruits.
Advances in the timing of spring migration are occurring in many species and these findings highlight the potential role of replacement nests as a driver of population increase in those areas where repeat nesting becomes increasingly possible. Professional ornithologists and citizen scientists who study nesting birds (not just waders) are encouraged to do so for the whole season, especially by following marked individuals. Birds that wear geolocators, which can record incubation patterns for nesting attempts that would otherwise remain undetected, may be particularly helpful when trying to discover just how likely birds are to re-nest and with what success.
Only part of the story
In the long run, the success of an individual bird can be measured by the number of offspring it has in its lifetime and even by the number of its genes that are present in future generations. The number of chicks that fledge each year is only part of the story, therefore. How many of these youngsters recruit to the breeding population? Do they end up breeding in areas where they will have high breeding success? Will their progeny live for a long time and hence have many opportunities to produce their own chicks? Long-term wader studies might reveal some of these answers – eventually.
The paper was published in Ecology & Evolution.
Why do earlier-arriving migratory birds have better breeding success? Catriona A. Morrison, José A. Alves, Tómas G. Gunnarsson, Böðvar Þórisson and Jennifer A. Gill.
The paper is freely available to view.
Graham (@grahamfappleton) has studied waders for over 40 years and is currently involved in wader research in the UK and in Iceland. He was Director of Communications at The British Trust for Ornithology until 2013 and is now a freelance writer and broadcaster.
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