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Nesting when the tides are turning

posted May 28, 2019, 2:01 AM by Clemens Kuepper   [ updated May 28, 2019, 2:07 AM ]
Time flies. Literally. It’s been nearly seven years since I wrote the last blog post on this website! That’s a long pause, even by my standards. A lot of things have happened since in the plover world. From our Ceuta population, we have published a number of studies on how the basic ecology and something fundamental such as the male-female ratio of adult plovers shapes the parental care behaviour of different plovers. We also figured that emancipated females that disperse during the breeding season contribute to high gene flow across the species, a behaviour that ultimately may help to slow down speciation. And we were able to document how severely the Ceuta population has been declining during a very short period putting it at the brink of extinction.

Yet, reading through my last blog entries from 2012, it is very fitting to continue with a very basic but important problem: how do breeding plovers deal with the changing tides?  Plovers are habitat specialists that often nest in intertidal habitat. This has many advantages, as in these open areas approaching predators can be spotted early, so that the nesting plovers can leave their nests and stay safe somewhere until the predator is gone. From their safe spot they can then watch how their expertly camouflaged eggs are overlooked by hungry predators - more often than not. But living in intertidal habitats brings up a new set of differently challenges. Probably the biggest one is the periodically changing water levels that shift the shoreline throughout the lunar month. From egg- laying to nesting each Snowy Plover nest will experience 2 to 3 spring tides that could flood the precious nests and kill their eggs. Unlike other salt marsh specialists, plover embryos cannot endure nest flooding that last longer than a few hours. And once the eggs are laid the plovers cannot really change the nest location, so this is an important decision. On the one hand, they need to choose the nesting spot wisely otherwise the nest is flooded. On the other hand, they want to nest close to the water as water means food especially for their chicks once they have hatched. How do they deal then with the risk of nest flooding? I was very lucky to recruit Silvia Plaschke, a bright student from University of Graz, back in 2016 to work with me on this for her Master project. 

The observations of 2012 suggested they should take the flooding risk seriously as back then a lot of nests failed due to flooding. In fact an entire cohort was wiped out in this year, not a single offspring fledged. But then 2012 seemed an exceptional year for our local Snowy Plovers, we had not witnessed such a devastating flooding event in any of the seasons before. A first question then is, how big is the threat of flooding? To investigate this we gathered nest fate and nest initiation patterns from ten breeding seasons and compared the differences between flooded and not flooded nests. First, we found that actually very few nests are flooded by the tides. From about 750 nests monitored, only 6% were flooded over the years. However, we found that in two of the ten seasons nest flooding was actually the second most important reason for nest failures. We also discovered that nest flooding ware much more likely to happen late in the season. Nearly all flooded nests were initiated in the second half of the season. Tropical storms in our population preferably occur at the end of the season. At this time the tides are already getting higher again and the two factors probably contribute to the higher rate of nest flooding then. One wonders, how the climate change will impact the timing of the storms and then how the plovers will deal with this.    

Finally, we found that the timing within a spring tide cycle was important. This required the use of circular statistics to model the periodicity of the spring tide cycle. A simple linear model cannot deal with this variation. I’m glad that our colleague Martin Bulla took over this task. The models showed that nests initiated around neap tides had a 50% higher risk to be flooded than nests initiated around spring tides. From this we concluded that timing of nest initiation was really important. One reason why spring tide nests do better than neap tide nests could be that the plovers experience how far the highest tides during the month reach into the salt flat and can comfortably pick a safe spot. By contrast, Snowy Plovers that initiate their nests during neap tide do not have this information. 

Following this we asked whether Snowy Plovers then do the sensible thing and initiate their nests during spring tide. The answer for this is a confident ‘possibly’. Indeed, we did find that more nests were initiated during spring tides than during neap tide but not overwhelmingly more. Plovers might also take other measures to ensure that the nests are not flooded. For example, they might instead nest on an elevated spot. However, many of these alternative mechanisms have disadvantages because nesting on an elevated spot in an otherwise flat area means also that their nests will be more conspicuous to predators. In our paper we therefore argue that actually adaptive timing could be a cheap mechanism to deal with the risk of tidal flooding effectively.

Strikingly, when one looks at the maximum high tides over the years, the plovers have picked exactly the time when the high tides are the lowest. Of course, only then much of their nesting habitat becomes available but it also helps to reduce the risk of flooding. This seems to be common also in other coastal bird populations from temperate zone but there during the rest of the year the conditions for rearing offspring are not great. In Mexico, I sometimes wondered why the plovers do not simply start earlier as temperatures are warm. Our study suggests that the tides could hold the keys for understanding why plovers breed when they breed at our site too.