A long-term study of egg dimensions in the Tree Sparrow Passer montanus in relation to climate change

Abstract

Recent studies have illustrated that increasing temperatures have had considerable effects on species and entire ecosystems. In this study I examine long-term trends of egg dimensions (length, breadth and volume) in the population of Tree Sparrows (Passer montanus), investigated during 36 years (1980–2015), in relation to spring temperatures. This species is a common, sedentary, hole-nesting and socially monogamous passerine bird in research area. Research took place in Mokrice area in north-western Croatia. Average temperatures in spring (mean March–April) have significantly increased during the monitored period of time. In population in north-western Croatia there was no increase or decrease in egg dimensions. Correlations between mean spring temperatures and mean egg length, and egg breadth and egg volume were not significant. Spring precipitations (March–April) did not change over the years and did not affect egg dimensions. Data on changes in egg dimensions are relatively scarce in comparison with data on other breeding parameters in birds and, in some cases, controversial ; some authors report increase in egg dimensions while others report decreases in egg dimensions in different bird species and populations as a response to climate warming. Thus, changes in egg dimension are a very complex issue in bird demography because there are many traits apart from climate that have impact on egg size, such as female condition, genetic and habitat characteristics etc., so further investigations on this subject are necessary.