Sperm exhibit extraordinary levels of morphological diversification across the animal kingdom. In songbirds, sperm have a helically shaped head incorporating a distinct acrosomal membrane or "helical keel," the form and extent of which varies across species. The functional significance of this helical shape, however, remains unknown. Using scanning electron microscopy, we quantified inter- and intraspecific variation in sperm head morphology across 36 songbird species (Passeriformes: Passerida). Using phylogenetic comparative methods, we investigated the relationship between sperm head morphology and both sperm swimming speed and the frequency of extra-pair young (EPY). We found that species whose sperm had a relatively more pronounced helical form (i.e., long acrosome, short nucleus, wide helical membrane, and a more pronounced waveform along the sperm head "core") had faster-swimming sperm. We found no evidence of a relationship between interspecific variation in sperm head morphology and EPY, although we did find that among- and within-male variation in sperm head traits were negatively correlated with EPY. Applying principles of fluid mechanics, we discuss how the helical form of the sperm head may influence swimming speed, and suggest that further studies considering aspects of sperm morphology beyond sperm length are needed to improve our understanding of sperm structure-function relationships.

18Capsule: Providing peanuts on bird feeders was shown to attract more individuals and more 19 species than providing cheese or bread. 20Aims: To investigate how the provision of different human-derived foods affects visitation rates 21 of urban birds at bird feeders. 22Methods: A fully replicated study design was set up in parkland, offering a binary choice from 23 three food types (peanuts, bread and cheese), on bird tables. Birds were observed using a scan-24 sample method. 25Results: Peanuts attracted more visits and a greater diversity of species than cheese or bread. 26This preference was strongest for Blue Tits and Great Tits, whereas Robins visited all food types 27equally, and Blackbirds preferred cheese. Bread was the most consumed food type when 28 measured in mass, but this could be linked to varying bite sizes. 29 Conclusion:Our results indicate that most birds preferred to visit the most protein-and energy-30 rich food, but that some birds still choose the carbohydrate-rich bread. The findings indicate that 31 peanuts, rather than household scraps like bread and cheese, attract the highest number of bird 32 species as well as individuals to bird tables. The findings are of interest to the public and to 33 organisations providing information on bird feeding for recreational purposes. 34 35

In a rapidly changing world, it is important to understand how urban environments impact wildlife. For example, supplementary feeding of birds, though well‐intended, might have unexpected negative effects on the health of individual animals. Sunflower seeds are commonly provided in garden bird feeders, but they contain high levels of linoleic acid (LA), an omega‐6 polyunsaturated fatty acid (PUFA). Omega‐6 PUFAs are associated with increased oxidative stress, which can damage cell membranes, and in particular sperm cells. We assessed the level of LA in the blood of two seed‐eating finch species, greenfinches Chloris chloris and hawfinches Coccothraustes coccothraustes, caught in and in environments with direct access to sunflower seed feeders (Norway), and compared these with the level of LA in a smaller number of individuals sampled in in a rural area with low incidence of sunflower seed feeders (Czech Republic). Furthermore, we investigated the relationship between the proportion of LA in the blood (as well as the proportion of 10 other fatty acids) and sperm quality (the frequency of sperm head abnormalities and sperm swimming speed). We found that both finch species, but particularly greenfinches caught near feeders, exhibited levels of LA that were considerably higher than those previously reported for other wild birds. We also found that the proportion of LA was positively correlated with the frequency of abnormal sperm heads (sperm missing the acrosome), while there was no significant effect of fatty acid composition on sperm swimming speed. Our results indicate that the sperm quality of finches may be negatively affected by a high intake of sunflower seeds, adding to a growing body of research showing that supplementary feeding may have detrimental side effects for urban animals. This is particularly relevant for the greenfinch, which is currently affected by disease and population declines.