Scientific paper by researcher selected to the cover of journal “Ecology and Evolution”
Published in 8/4/2019
It is common knowledge that during Easter shops get flooded with chocolate eggs which make the delights of young and older alike. But what few will know is that the traditional game of “egg hunting” in early spring is due in large part to the existence of the very first wader nests of the year!
In northern Europe, particularly in England, but also in Scotland and the Netherlands, egg collection of wader eggs in Easter was once at such scale that it originated a parliamentary act for the protection of a wader species, forbidding the sale of Vanellus vanellus (Lapwing) eggs from 14 March to 11 August (The Lapwing Act 1928). Although many birds have advanced their laying dates and wild egg collection is increasingly less frequent, many species are currently declining due to other factors. For example, when they are unable to match the advances in the phenology of the resources they depend upon, such as the insects that compose the diet of many of these species.
In fact, the warming trend felt globally in recent years seems to be the origin of changing phenology in several groups of species, be it the date of first flowering of plants, the hatching of insects, or the migratory movements of bird species. But, the changing of the timing of these events may also influence demographic processes and ultimately affect population growth. This was the issue that an international team of researchers, lead by Dr. José Alves (Dep. Biology and CESAM – University of Aveiro) decided to explore. In order to do so they selected a population of a migratory wader species that breeds in the northernmost limit of its distribution area, Iceland, where the effects of global warming are very apparent due to “arctic amplification”.
The researchers first had to “play egg hunting” to find the nests of the black-tailed godwit (Limosa limosa islandica), in Iceland, during several years in order to assess the variation in phenology of laying in relation to local temperature. This study initiated in 2001 (a particularly cold breeding season in Iceland) started to yield results ten years later, in 2011 (another cold season), and carried on until 2013, encompassing a period in which the average air temperature fluctuated between 9 and 11 degrees Celsius during the breeding season of this bird. This information allowed establishing a relationship between temperature and laying dates: the warmer it is, the earlier these birds are able to lay their clutches. As this species conceals its nests in the vegetation, these birds must wait for the local plants to growth following snowmelt and defrosting of soils. In warmer seasons, soil defrosts and plant growth occur at an higher rate, allowing godwtis to safely lay their eggs earlier (successfully hiding those from potential predators).
But this was only the first step in the study. It was important to understand if such local responses could have an effect at the population level. In order to do so, the chicks from this species were located every two days by radio-tracking their parents (which guard them around the clock), to establish how much time is required for them to become independent and fledge. By tracking chicks born in years during which air temperatures differed, it was possible to establish that in warmer years, chicks fledge about 15 days earlier than chicks born in colder years (considering the total period since egg laying to fledging). With the help of an extensive network of collaborators across Europe that report colour-ringed Icelandic godwits across its range (Iceland-Iberian Peninsula), it was possible to carry on tracking these chicks, now juveniles, until the end of their first migration, and this allowed demonstrating that chicks born earlier enjoy an higher recruitment probability.
Hence, in warmer years, the breeding phenology of this bird advances and chicks from those years experience a higher probability of increasing the number of individuals on the population. Once average temperatures in Iceland have increased since 1845, and warm springs have been more frequent in recent decades, the research team was able to link the population growth experience by this population and its expansion into new areas in Iceland and in West Europe (during the non-breeding season), to the warming recorded on its natal island c. At the start of the XX century, this population was restricted to a breeding area in the SW cost of Iceland and it is estimated to haven been composed by 5000 individuals. The warming trend in temperatures occurring since, allowed the birds in these original locations to produce offspring that gradually colonized almost all coastal areas of the country. Their number increased to 50.000 individuals, with areas colonized earlier producing more individuals as local temperatures increased.
This study goes beyond descriptive patterns of individual responses to climate change, establishing a link between those and the potential effects in the demography and distribution of species. These effects are extremely important, as they can help to improve our predicting capacity to what may occur to biodiversity in a climate change scenario. In the case of the Iceland godwit, global warming seems to be promoting its expansion, which could be expected for a population that occurs at the northern part of its distribution range, and is likely to be limited by the low temperatures at such latitudes. However, this is not the case for many other species of migratory birds that often are unable to advance their phenology at a similar rate of the insects they depend upon for themselves and to raise their young.
We should therefore promote current efforts for the reduction of carbon emissions in an attempt to slow down global warming which is already very apparent. More than helping biodiversity in its attempts to respond to such changes, we will surely avoid that many chocolate eggs melt in their hiding places while we search for them in ever warming Easters.
The original article in open access available here.