Background  In birds, the trade-off between the quality and number of nestlings is one of the most important theories of the evolution of life history of birds, which is closely related with climate. We investigated variation in egg and clutch size of the Black Redstart (Phoenicurus ochruros) on the northeastern edge of the Qinghai-Tibetan Plateau in order to explore and test the relation between egg size and clutch size and the rules governing the variation in these two components with climate change.

Methods  We monitored each located nest daily to determine the exact laying order of the eggs and measured their sizes. The variations in egg volume and clutch size between 2008 and 2009 were studied by independent sample t tests. The difference in egg volume in the sequence of the laying order was tested by Kendall's W test and we employed Pearson correlations to investigate the relation between egg size and laying order. All statistical analyses were performed with IBM SPSS Statistics, version 19.0

Results  In 2008, the mean egg volume of the Black Redstart was 2.40 ± 0.20 cm³ (n = 50), and the mean clutch size 4.90 ± 0.57 (n = 11). In 2009, the mean egg volume and clutch size were 2.26 ± 0.18 cm³ (n = 74) and 5.20 ± 0.26 (n = 15) respectively. We found a significantly positive correlation between egg size and laying order (r = 0.397, p = 0.030), while egg size was strongly and negatively correlated with clutch size (r = -0.274, p = 0.002). Black Redstarts tend to lay smaller eggs but a larger clutch in a brood in a resource-rich year, whereas larger eggs but smaller clutches are produced in a harsh year.

Conclusions  The Black Redstart adopted a "brood survival strategy" to ensure that more offspring could survive. The Black Redstart' trade-off in egg volume and clutch size would guarantee that it will gain maximum benefit in any case.