Data from: Thick eggshells of brood parasitic cowbirds protect their eggs and damage host eggs during laying

Brood parasites lay thick-shelled eggs and numerous hypotheses have been proposed to explain the significance of this trait. We examined whether thick eggshells protect the parasite egg during laying events. We used eggs of the parasitic shiny cowbird (Molothrus bonariensis) and its hosts, the house wren (Troglodytes aedon) and chalk-browed mockingbird (Mimus saturninus) in South America and the eggs of the parasitic brown-headed cowbird (M. ater) and its hosts the house wren and red-winged blackbird (Agelaius phoeniceus) in North America. We experimentally dropped parasite eggs onto host eggs to simulate laying by the parasite, parasite eggs onto parasite eggs to simulate multiple parasitism, host eggs onto parasite eggs to simulate hosts laying from the height cowbirds lay, and stirred eggs to simulate jostling that may occur when cowbirds and hosts interact during laying events. We found that cowbird eggs were significantly less likely to be damaged than host eggs when they were laid onto a host egg and when host and cowbird eggs were laid onto them. There was minimal damage to eggs during jostling experiments, thereby failing to support the hypothesis that thick eggshells provide protection when eggs are jostled. These findings support the hypotheses that thick eggshells resist damage when laid from an elevated position, when additional cowbird eggs are laid onto them in multiply parasitized nests, and these eggs also damage host eggs when laid.

巢寄生鸟类（brood parasite）会产下厚壳卵，学界已提出诸多假说以阐释这一性状的进化意义。本研究旨在探究厚卵壳是否能在产卵过程中保护寄生卵。本研究使用了两类寄生牛鹂的卵：其一为南美洲的亮羽牛鹂（shiny cowbird，*Molothrus bonariensis*），其宿主为家鹪鹩（house wren，*Troglodytes aedon*）与白眉嘲鸫（chalk-browed mockingbird，*Mimus saturninus*）；其二为北美洲的棕头牛鹂（brown-headed cowbird，*M. ater*），其宿主为家鹪鹩与红翅黑鹂（red-winged blackbird，*Agelaius phoeniceus*）。我们开展了以下四组对照实验：将寄生卵坠落至宿主卵表面，以模拟寄生鸟类的产卵行为；将寄生卵堆叠至其他寄生卵之上，以模拟多重寄生现象；将宿主卵坠落至寄生卵表面，以模拟宿主以牛鹂的产卵高度进行产卵的场景；以及搅拌卵群，以模拟产卵过程中牛鹂与宿主发生互动时可能产生的挤撞情况。实验结果显示：当寄生卵坠落至宿主卵表面，或宿主卵、寄生卵被坠落至牛鹂卵表面时，牛鹂卵的破损概率显著低于宿主卵。挤撞实验中卵群几乎未出现破损，因此不支持“厚卵壳可在挤撞过程中为卵提供保护”这一假说。本研究结果支持三项假说：厚卵壳可抵御从高处坠落时造成的破损；在多重寄生的巢中，额外产下的牛鹂卵不会因堆叠而受损；同时牛鹂卵在坠落时可对宿主卵造成损伤。