Phenology of migrating birds monitored at Grimsö Wildlife Research Station

24 November 2022

Ornithology has a long tradition in biological research, and there are numerous data sets on various subtopics available both via scientific publications and conservation organizations. The phenology of migrating species (e.g. arrival dates to breeding grounds) has drawn attention in the last few decades, as it describes and helps understand effects of increasing temperatures on bird migration.

Since 1985 such corresponding data have been collected at Grimsö Wildlife Research Station. The dataset at Grimsö comprises of date of the first observation within the research area each year for ca. 80 migratory species, and shows several more or less pronounced changes over the 37 years of data collected.

Common cranes (Grus grus) migrating south for winter. Photo: Rick Heeres
Common cranes (Grus grus) migrating south for winter. Photo: Rick Heeres

As expected, and seen in many studies, the long-distance migrants, that spend winter in, e.g. Africa, show very little variation in spring arrival dates between years (as seen in the graph below). In a few cases the date differs >1 week for such species, which is due to other factors than a sudden temperature change. Instead more or less the same dates/week are noted for them throughout the period. This behavior is explained in that the best timing of such long flights cannot be based on the daily weather in the wintering area, but is instead initiated mainly by day length, which in turn relates to a suitable arrival time at the breeding ground.  

Date (Julian day) of the first observation per year (since 1985) of the Common Cuckoo (Cuculus canorus) and the Barn Swallow (Hirundo rustica) within Grimsö Wildlife Research Area.
Date (Julian day) of the first observation per year (since 1985) of the Common Cuckoo (Cuculus canorus) and the Barn Swallow (Hirundo rustica) within Grimsö Wildlife Research Area.

On the other hand, several of the migratory birds in Sweden that move only short distances, i.e. southward before the winter, move gradually in response to the local weather (temperature, food availability etc) and show significantly earlier spring arrivals in recent decades compared to the first years of the survey. The mean arrival dates of, for example, the Mallard and the Common black bird were 12 and 19 days earlier, respectively, in the period 2005-2022 and 1985-2004. Moreover, some of the short-distance migratory species nowadays stay in southern and middle Sweden throughout the winter.  

Date (Julian day) of the first observation per year (since 1985) of the Mallard (Anas platyrhynchos) and the Common blackbird (Turdus merula) within Grimsö Wildlife Research Area. The seasonal movements of these species are rather related to local weather conditions than actual migrations, with an apparent trend of earlier arrivals to the breeding grounds in later years. Points along the x-axis (i.e. Julian day = 0) indicate the bird species stayed in the area till the next year.
Date (Julian day) of the first observation per year (since 1985) of the Mallard (Anas platyrhynchos) and the Common blackbird (Turdus merula) within Grimsö Wildlife Research Area. The seasonal movements of these species are rather related to local weather conditions than actual migrations, with an apparent trend of earlier arrivals to the breeding grounds in later years. Points along the x-axis (i.e. Julian day = 0) indicate the bird species stayed in the area till the next year.

Here once again this dataset highlights the importance of long term monitoring. If this survey had started for example 10 years ago, we would not yet have recognized stark changes of bird arrivals in the region/data.