Both C (2023) Benefits of a warming climate? On the effect of winter weather on the survival of Stonechat Saxicola rubicola, Chiffchaff Phylloscopus collybita and Blackcap Sylvia atricapilla. LIMOSA 96 (4): 158-173.

Short-distance migrants often show variation in migratory behaviour: some individuals may stay at the breeding site, whereas others migrate. It is expected that when winters get warmer, the advantage of a resident strategy may increase. In order to better understand the effect of winter temperature on this variation in wintering strategy, I analysed two sources of information on the size of the breeding and wintering population of three common breeding bird species in the Netherlands: Common Chiffchaff, European Stonechat and Eurasian Blackcap. The majority of all three species migrates to the Mediterranean, with an estimated winter population relative to the postbreeding numbers ranging from a mere 0.01% in Blackcap to 0.91% in Stonechats. I used both systematically collected data on the winter (point transect counts) and breeding numbers for the Netherlands by Sovon Centre for Field Ornithology, and data that were non-systematically collected through the web portal Waarneming.nl. In the systematic counts, for Chiffchaff and Stonechat the index of the winter population was lower when the previous winter was colder, suggesting that in those colder winters more individuals disappear and possibly selection against the resident strategy leads to lower numbers of individuals that are winter resident. Because systematic winter counts are only performed in December, I used non-systematic counts to see whether winter numbers decline more strongly during the winter. To overcome double counts in these non-systematic data, I used the number of one-kilometre squares with observations for periods of 15 day-periods during the winter, corrected for changes in observation effort. First I show that these non-systematic data are strongly correlated with the systematic mid-winter data for Chiffchaff and Stonechat. Next, I show that the decline during the winter is stronger during colder winters for Chiffchaff and Stonechat, but not for Blackcap. Furthermore, only for Stonechat there is evidence that more individuals start wintering when autumn temperature are higher, suggesting some flexibility in migration strategy. Because these analyses do not proof that individuals that are wintering in an area are also breeding locally, I performed observations on wintering Stonechats during three winters in the Westerbroekstemadepolder in the province of Groningen. Based on distinct individual plumage features of males, I established that clearly identifiable individuals were yearround residents. In this area, most Stonechats wintered as heterosexual pairs, males were more common during winter than females, and females more often disappeared during the winter than males. In this area, a much larger fraction of the breeding population seemed to be resident than estimates for the Netherlands as a whole: locally 55% of males and 38% of females was estimated to winter, whereas for the Netherlands the estimate is ca. 1%.

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