An early start to spring this year meant that coal tits began laying their eggs ahead of schedule and the CREA Mont-Blanc team was out in the field in Vallorcine on a weekly basis starting on April 6th. A late April cold snap put the laying on pause and threatened the survival of chicks. Read on to learn more.
Coal tit monitoring began earlier this year than last, with the first field visit on April 6th.
Although the CREA Mont-Blanc team enjoyed a warm and sunny first few weeks of field work this year (in contrast with 2016’s cold, snowy and harsh conditions), the return of both cold and snow at the end of April took researchers and birds alike by surprise.
Study of coal tit reproduction
Thanks to the help of Sandrine, Anne B., Anne D., Hillary, Nelly, Brad and Sylvain, we were able to visit all 100+ of our nest boxes in Vallorcine (~1,300m) and Loriaz (~1,900m) every week between April 6th and June 15th.
Every year, our goal is to determine the stages of nest construction in occupied nests, the number of eggs laid, the number of chicks and their age. This year, 36% of our Vallorcine nest boxes were occupied compared with 32% at Loriaz. While the occupation rates of nest boxes has remained relatively stable in Vallorcine since 2014, the nest box occupation at Loriaz has been far more unpredictable from year to year. This year, for example, the occupation rate was more than twice what it was in 2016.
Earlier than average laying date
In Vallorcine this year, the average date of the first egg was April 18th, which is 9 days ahead of the average recorded since 2011, and 25 days earlier than the 2016 average. At Loriaz, the average date of the first egg was May 2nd, which is 2.5 days ahead of the average, and 20 days ahead of 2016. (see our Spring 2017 summary–in French)
This precocity can be explained in part by the relatively high temperatures experienced in early spring this year. As for the variation between the two sites, thanks to our temperature stations at Vallorcine and Loriaz, we know that the early season high temperatures were more significant at lower elevations than at higher ones. Because Loriaz did not experience the same above average heat, the coal tit population in that location did not start laying so early.
When we consider this impressive adaptation (which we call plasticity), we can hypothesize that coal tits are capable of adapting to temperatures in order to synchronize the development of their chicks with the peak of resource abundance (see our page on coal tit species reproduction—in French—for more information). However, this observation raises additional questions: Does the peak in resource abundance experience shift forward in response to warmer temperatures? If so, is that shift equivalent to the one observed in laying dates? (How) are survival rates of chicks affected by these temporal shifts?
We decided to examine the second of these questions a little more closely.
Vulnerability of coal tits to changes in climate
In 2017, the Vallorcine nests experienced particularly low survival rates. The above graphic illustrates average coal tit chick mortality rates in Vallorcine and Loriaz by year. The high mortality rates experienced in Vallorcine in 2017 may be explained by the cold weather and snow that returned to the area at the end of April, well after the coal tits had begun laying. As our Spring 2017 summary (in French) explains, the number of days where the temperature dropped below freezing at least once during the day was 25 in Arvieux (1,560m), or 4.2 days more than the 2008-2017 average. We can suppose that the late frosts of this spring had a negative effect on the vulnerable eggs and chicks. At a higher elevation like Loriaz, the coal tits had not laid as early, meaning that their eggs and chicks did not experience the same sudden cold snap as their lower elevation counterparts.
So, we can conclude: laying earlier might allow coal tits to maintain synchronization with their environment, but could lead to increased vulnerability to later cold snaps.
Written by : Marjorie Bison