Interaction of climate change with effects of conspecific and heterospecific density on reproduction
Authors: Møller Anders Pape , Balbontín Javier , Dhondt André A. , Adriaensen Frank , Artemyev Alexandr , Bańbura Jerzy , Barba Emilio , Biard Clotilde , Blondel Jacques , Bouvier Jean‐Charles , Camprodon Jordi , Cecere Francesco , Charter Motti , Cichoń Mariusz , Cusimano Camillo , Dubiec Anna , Doligez Blandine , Eens Marcel , Eeva Tapio , Ferns Peter N. , Forsman Jukka T. , Goldshtein Aya , Goodenough Anne E. , Gosler Andrew G. , Gustafsson Lars , Harnist Iga , Hartley Ian R. , Heeb Philipp , Hinsley Shelley A. , Jacob Staffan , Järvinen Antero , Juškaitis Rimvydas , Korpimäki Erkki , Krams Indrikis , Laaksonen Toni , Leclercq Bernard , Lehikoinen Esa , Loukola Olli , Mainwaring Mark C. , Mänd Raivo , Massa Bruno , Matthysen Erik , Mazgajski Tomasz D. , Merino Santiago , Mitrus Cezary , Mönkkönen Mikko , Nager Ruedi G. , Nilsson Jan‐Åke , Nilsson Sven G. , Norte Ana C. , von Numers Mikael , Orell Markku , Pimentel Carla S. , Pinxten Rianne , Priedniece Ilze , Remeš Vladimir , Richner Heinz , Robles Hugo , Rytkönen Seppo , Senar Juan Carlos , Seppänen Janne T. , da Silva Luís P. , Slagsvold Tore , Solonen Tapio , Sorace Alberto , Stenning Martyn J. , Török János , Tryjanowski Piotr , van Noordwijk Arie J. , Walankiewicz Wiesław , Lambrechts Marcel M. 
Published: August 2020

We studied the relationship between temperature and the coexistence of great tit Parus major and blue tit Cyanistes caeruleus, breeding in 75 study plots across Europe and North Africa. We expected an advance in laying date and a reduction in clutch size during warmer springs as a general response to climate warming and a delay in laying date and a reduction in clutch size during warmer winters due to density‐dependent effects. As expected, as spring temperature increases laying date advances and as winter temperature increases clutch size is reduced in both species. Density of great tit affected the relationship between winter temperature and laying date in great and blue tit. Specifically, as density of great tit increased and temperature in winter increased both species started to reproduce later. Density of blue tit affected the relationship between spring temperature and blue and great tit laying date. Thus, both species start to reproduce earlier with increasing spring temperature as density of blue tit increases, which was not an expected outcome, since we expected that increasing spring temperature should advance laying date, while increasing density should delay it cancelling each other out. Climate warming and its interaction with density affects clutch size of great tits but not of blue tits. As predicted, great tit clutch size is reduced more with density of blue tits as temperature in winter increases. The relationship between spring temperature and density on clutch size of great tits depends on whether the increase is in density of great tit or blue tit. Therefore, an increase in temperature negatively affected the coexistence of blue and great tits differently in both species. Thus, blue tit clutch size was unaffected by the interaction effect of density with temperature, while great tit clutch size was affected in multiple ways by these interactions terms.