Impaired hatching success and male-biased embryo mortality in Tree Sparrows

Abstract

During the past 30 years, many species of farmland birds have declined dramatically in numbers in Northern Europe, a trend coinciding with a tremendous intensification of agriculture, although the exact causes of these declines remain unclear. One of the worst affected species is the Tree Sparrow (Passer montanus). We studied two Swedish Tree Sparrow populations during the years 1996-2004 and found that in both populations, almost half of all laid eggs remained unhatched. This led us to investigate whether the eggs failed to hatch because of: (1) eggs not being fertilised or (2) embryo mortality. Our analyses showed that all of the eggs investigated contained sufficient number of sperm for fertilisation and that they also had other visible signs indicating that fertilisation had occurred. Hatching failure was instead shown to result from embryo mortality. Using molecular techniques, we were able to determine that embryo mortality is more likely to affect male embryos than females and that the fledgling sex ratio was consequently highly female biased. The cause of this sexbiased embryo mortality remains unknown, but various potential explanations are discussed.

Male‐specific mortality biases secondary sex ratio in Eurasian tree sparrows Passer montanus

“…Most of the unhatched eggs did not show any sign of embryo development, even though they were fertilized. These results concur with previous studies (Birkhead et al., ; Svensson et al., ). The high mortality of male embryos has also been reported in a Swedish population of Eurasian tree sparrows (Svensson et al., ).…”

“…However, in several species of Passer birds, this percentage is much lower than in other species [e.g., 65% in Eurasian tree sparrows, Passer montanus (Svensson et al., ); 56% in house sparrows, Passer domesticus (Aslan, Yavuz, & Erdogan, ); and 67% in Spanish sparrows, Passer hispaniolensis (Marques, )], although there is large variation among populations. In Eurasian tree sparrows, males have higher mortality than females at the embryonic developmental stage (Svensson et al., ). Their mean clutch size is not large, approximately 5 per breeding attempt, but they nest 3 or 4 times and lay many eggs in a breeding season (Kato personal observation).…”

“…Svensson et al. () determined the sex only of developed unhatched offspring and did not investigate the sex of embryos that showed no sign of development. As mentioned, previous studies reported that SSM of embryos might be caused by two factors.…”

Unhatched eggs: methods for discriminating between infertility and early embryo mortality

“…The second part of this study was conducted on the Tree Sparrow, a small sexually monomorphic passerine in which high levels of hatching failure have been reported (Svensson et al. 2007).…”

“…2007). Our study population was one of those described by Svensson et al. (2007), located on organic farmland just outside Uppsala, Sweden.…”