The Blue Tit study system


Peter Santema, Emmi Schlicht, Lotte Schlicht


March 1, 2007

Blue tits – familiar garden birds to most Europeans – typically breed as a socially monogamous pair, that is, one male and one female that raise a brood together. However, about half of the nests contain one or more offspring sired by another than the social male who is caring for the brood (extra-pair paternity). Moreover, some males are mated with two or – rarely – three females (social polygyny). Our research aims to understand the mating behaviour and its evolutionary consequences. Why do some males sire extra-pair young and others not? Why do females copulate with males other than their social mate? Why do particular males and females form a social pair? Why are some males paired with more than one female? Why do some individuals breed with the same partner in different years while others ‘divorce’?

1 The “Smart-box” system

To address these and other questions, we studied a population of blue tits at the Westerholz forest in southern Germany between 2007 and 2022. Our study site contained 277 nestboxes, of which roughly half were occupied by breeding blue tits each spring. Each nestbox was equipped with RFID technology that recorded nestbox visits of PIT-tagged blue tits around the clock and all year round.

Through intensive catching, almost all individuals in the population carried a tag at any given time, and by blood sampling all caught adults and nestlings we obtained almost complete information on genetic parentage. What did we learn from using this “big brother” style monitoring system?

2 Extra-pair paternity

Our studies revealed two important predictors of why some males sire extra-pair offspring and others do not. First, juveniles (or yearlings) are substantially less successful than ≥2 year-old adults (E. Schlicht and Kempenaers 2023) , presumably because yearlings are being outcompeted by older males (Schlicht et al., in prep). Second, males that sing and are active earliest in the morning are more successful at siring extra-pair young than those that get up later (Kempenaers et al. 2010; L. Schlicht, Santema, and Kempenaers 2023). Additionally, extra-pair offspring are almost exclusively sired by males from an adjacent territory (L. Schlicht, Valcu, and Kempenaers 2014) and often by a male with whom the female had associated during winter (Beck, Farine, and Kempenaers 2020), highlighting the importance of spatial proximity and familiarity.

In contrast to males, the reproductive success of females is constrained by the number of eggs they can lay. It is therefore not obvious whether and how females benefit from copulations with males other than their social partner. Our work showed that extra-pair copulations can be a way to ensure the fertilisation of the clutch for females that are mated with an infertile male(Santema, Teltscher, and Kempenaers 2020), and to obtain extra help in raising offspring. However, an extra-pair sire only provisioned offspring if the female received no help from her social partner and the male had no nest of its own to tend (Santema and Kempenaers 2021). It is likely that multiple factors contribute to extra-pair mating behaviour of females and the jury is still out on what the main drivers are for why females seek extra-pair copulations.

3 Polygyny

A small proportion (<5%) of males that bred in our population were socially polygynous. We found that this occurred almost always because a female had lost her mate shortly before the start of breeding and settled as a secondary female with an already mated male (replacement polygyny; (E. Schlicht and Kempenaers 2021) ). The secondary female typically received little or no help with offspring provisioning and had low reproductive success. Even males appear to benefit little from polygyny, as their nests often failed and they often lost paternity (E. Schlicht and Kempenaers 2021).

4 Divorce and dispersal

We discovered that many blue tits leave the study site after breeding and come back in winter or spring (Gilsenan, Valcu, and Kempenaers 2020). We found that individual males and females are consistent in when they return, and pairs that bred together had often arrived at the breeding site around the same time and were already associated before breeding (Gilsenan, Valcu, and Kempenaers 2020; Beck, Farine, and Kempenaers 2020). When both members of a pair returned to breed the next year, only about half bred together again. The other pairs ‘divorced’, that is, both individuals mated with a new partner. Our work revealed that divorce is not a decision to no longer breed together, as the term misleadingly suggests. Instead, pair members that returned around the same time usually remated, whereas those that arrived distantly in time tended to end up with a new partner (Gilsenan, Valcu, and Kempenaers 2017). After arriving at the breeding site, it may not pay to wait for the partner of the previous year to return, given a high likelihood that he or she died. In another study area, with more variation in habitat quality, we found that when a pair divorced, the female moved to a higher quality territory and produced more offspring (M. Valcu and Kempenaers 2008).

5 Other aspects of blue tit biology

The continuous nest visit data allowed us to address many other aspects of blue tit behaviour. For instance, we found that shortly before and during the egg-laying period, blue tits regularly visit each other’s nestboxes and these visits predict extra-pair paternity (L. Schlicht, Valcu, and Kempenaers 2015). We also investigated the female’s incubation behaviour and the male’s courtship feeding during this period (Bambini, Schlicht, and Kempenaers 2019), and examined the factors underlying variation in provisioning behaviour of parents during the nestling period (Santema et al. in prep).

In some years, we also PIT-tagged nestlings to “observe” the exact moment of nest departure (fledging). We learned that nestlings sired by extra-pair males fledge earlier (L. Schlicht et al. 2012) and that young blue tits do not like to fledge alone, so they adjust their fledging time to that of their siblings (Santema et al. 2021).

Our research has examined many other aspects of blue tit biology. Blue tits lay some of the largest clutches of any passerine (up to 15 eggs in our population). Although all eggs look the same, we used proteomics to show that egg content changes with the laying order (C.-M. Valcu et al. 2019).

6 Resources

All blue tit publications, all information about the egg proteome (C.-M. Valcu et al. 2019) and the blue tit genome and transcriptome (Jakob C. Mueller et al. 2016; Jakob C. Mueller et al. 2023), and the entire database are or will soon be made publicly available.

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Bambini, Giulia, Emmi Schlicht, and Bart Kempenaers. 2019. “Patterns of Female Nest Attendance and Male Feeding Throughout the Incubation Period in Blue Tits _Cyanistes Caeruleus_.” Ibis 161 (1): 50–65.
Beck, Kristina B., Damien R. Farine, and Bart Kempenaers. 2020. “Winter Associations Predict Social and Extra-Pair Mating Patterns in a Wild Songbird.” Proceedings of the Royal Society B: Biological Sciences 287 (1921): 20192606.
Gilsenan, Carol, Mihai Valcu, and Bart Kempenaers. 2017. “Difference in Arrival Date at the Breeding Site Between Former Pair Members Predicts Divorce in Blue Tits.” Animal Behaviour 133 (November): 57–72.
———. 2020. “Timing of Arrival in the Breeding Area Is Repeatable and Affects Reproductive Success in a Non-Migratory Population of Blue Tits.” Edited by Sandra Bouwhuis. Journal of Animal Ecology 89 (4): 1017–31.
Kempenaers, Bart, Pernilla Borgström, Peter Loës, Emmi Schlicht, and Mihai Valcu. 2010. “Artificial Night Lighting Affects Dawn Song, Extra-Pair Siring Success, and Lay Date in Songbirds.” Current Biology 20 (19): 1735–39.
Mueller, Jakob C., Heiner Kuhl, Bernd Timmermann, and Bart Kempenaers. 2016. “Characterization of the Genome and Transcriptome of the Blue Tit C Yanistes Caeruleus : Polymorphisms, Sex-Biased Expression and Selection Signals.” Molecular Ecology Resources 16 (2): 549–61.
Mueller, Jakob C, Stephen A Schlebusch, Yifan Pei, Manon Poignet, Niki Vontzou, Francisco J Ruiz-Ruano, Tomáš Albrecht, et al. 2023. “Micro Germline-Restricted Chromosome in Blue Tits: Evidence for Meiotic Functions.” Molecular Biology and Evolution 40 (5): msad096.
Santema, Peter, and Bart Kempenaers. 2021. “Offspring Provisioning by Extra-Pair Males in Blue Tits.” Journal of Avian Biology 52 (5): jav.02755.
Santema, Peter, Lotte Schlicht, Ben C. Sheldon, and Bart Kempenaers. 2021. “Experimental Evidence That Nestlings Adjust Their Fledging Time to Each Other in a Multiparous Bird.” Animal Behaviour 180 (October): 143–50.
Santema, Peter, Kim Teltscher, and Bart Kempenaers. 2020. “Extra-Pair Copulations Can Insure Female Blue Tits Against Male Infertility.” Journal of Avian Biology 51 (6): jav.02499.
Schlicht, Emmi, and Bart Kempenaers. 2021. “Origin and Outcome of Social Polygyny in the Blue Tit.” Ardea 109 (1).
———. 2023. “Age Trajectories in Extra-Pair Siring Success Suggest an Effect of Maturation or Early-Life Experience.” Journal of Evolutionary Biology 36 (9): 1213–25.
Schlicht, Lotte, Alexander Girg, Peter Loës, Mihai Valcu, and Bart Kempenaers. 2012. “Male Extrapair Nestlings Fledge First.” Animal Behaviour 83 (6): 1335–43.
Schlicht, Lotte, Peter Santema, and Bart Kempenaers. 2023. “Start and End of Daily Activity Predict Extrapair Siring Success Independently of Age in Male Blue Tits.” Animal Behaviour 198 (April): 21–31.
Schlicht, Lotte, Mihai Valcu, and Bart Kempenaers. 2014. “Thiessen Polygons as a Model for Animal Territory Estimation.” Edited by Stephan Schoech. Ibis 156 (1): 215–19.
———. 2015. “Male Extraterritorial Behavior Predicts Extrapair Paternity Pattern in Blue Tits, Cyanistes Caeruleus.” Behavioral Ecology 26 (5): 1404–13.
Valcu, Cristina-Maria, Richard A. Scheltema, Ralf M. Schweiggert, Mihai Valcu, Kim Teltscher, Dirk M. Walther, Reinhold Carle, and Bart Kempenaers. 2019. “Life History Shapes Variation in Egg Composition in the Blue Tit Cyanistes Caeruleus.” Communications Biology 2 (1): 6.
Valcu, Mihai, and Bart Kempenaers. 2008. “Causes and Consequences of Breeding Dispersal and Divorce in a Blue Tit, Cyanistes Caeruleus, Population.” Animal Behaviour 75 (6): 1949–63.