Since male and female offspring may have different costs and benefits, parents may use sex ratio adjustment to increase their own fitness under different environmental conditions. Urban habitats provide poorer conditions for nestling development in many birds. Therefore, we investigated whether great tits (Parus major) produce different brood sex ratios in urban and natural habitats. We determined the sex of nestlings of 126 broods in two urban and two forest sites between 2012 and 2014 by molecular sexing. We found that brood sex ratio did not differ significantly between urban and forest habitats either at egg-laying or near fledging. Male offspring were larger than females in both habitats. This latter result suggests that male offspring may be more costly to raise than females, yet our findings suggest that urban great tits do not produce more daughters despite the unfavourable breeding conditions. This raises the possibility that other aspects of urban life, such as better post-fledging survival, might favour males and thereby compensate for the extra energetic costs of producing male offspring.

中文翻译：

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大山雀（Parus major）的城市和森林种群之间的后代性别比例是否不同？

由于雄性和雌性后代可能有不同的成本和收益，因此父母可以通过性别比例调整来增加自己在不同环境条件下的适应度。城市栖息地为许多鸟类的筑巢发展提供了较差的条件。因此，我们调查了大山雀（Parus major) 在城市和自然栖息地中产生不同的育雏性别比。我们通过分子性别鉴定确定了 2012 年至 2014 年间两个城市和两个森林地点的 126 只雏鸟的性别。我们发现，无论是在产卵时还是在羽翼未丰时，城市和森林栖息地的育雏性别比都没有显着差异。在两个栖息地，雄性后代都比雌性大。后一个结果表明，雄性后代的抚养成本可能比雌性更高，但我们的研究结果表明，尽管繁殖条件不利，城市大山雀并不会产生更多的女儿。这增加了城市生活的其他方面的可能性，例如更好的羽化后生存，可能有利于雄性，从而补偿产生雄性后代的额外能量成本。