In 1950, Rensch first described that in groups of related species, sexual size dimorphism is more pronounced in larger species. This widespread and fundamental allometric relationship is now commonly referred to as 'Rensch's rule'. However, despite numerous recent studies, we still do not have a general explanation for this allometry. Here we report that patterns of allometry in over 5300 bird species demonstrate that Rensch's rule is driven by a correlated evolutionary change in females to directional sexual selection on males. First, in detailed multivariate analysis, the strength of sexual selection was, by far, the strongest predictor of allometry. This was found to be the case even after controlling for numerous potential confounding factors, such as overall size, degree of ornamentation, phylogenetic history and the range and degree of size dimorphism. Second, in groups where sexual selection is stronger in females, allometry consistently goes in the opposite direction to Rensch's rule. Taken together, these results provide the first clear solution to the long-standing evolutionary problem of allometry for sexual size dimorphism: sexual selection causes size dimorphism to correlate with species size.

Birds provide excellent model organisms for testing functional explanations of sexual size dimorphism (SSD), since many species are exceptionally well-studied in nature. This chapter reviews four major functional hypotheses of SSD, and tests these using data on five morphometric traits from over 2,500 bird species. This comprehensive analysis reveals that SSD is male-biased in most avian species and families, and that allometry consistent with Rensch's rule occurs in significantly more avian families than expected by chance. Using cross-species analyses, the chapter shows that the pattern of SSD is most consistent with patterns of sexual selection, specifically with the Mating competition and the Display agility hypotheses. Sexual selection, however, is unlikely to explain all variation in SSD, and further work is essential on ecological use of resources and fecundity selection.

Body size is an important characteristic of animals, influencing physiology, life histories, and general ecology. Hence, it often needs to be taken into account even if the aim is to test for relationships among other traits. We provide a comprehensive data set on avian body sizes that would be useful for future comparative studies of avian biology. We extracted species-specific measurements on male and female body mass, wing length, tarsus length, bill length, and tail length from major ornithological text books and some other sources covering bird species of Africa, Australia, New Zealand, Antarctica, North America, and the western Palearctic. These measurements were matched with measures of egg and clutch sizes, and scores of mating system, sexual display agility, and the degree of intersexual resource division. We present morphometric data ranging from 2350 species (minimum, tail length) to 2979 species (maximum, wing length) where measurements for both sexes are known, some additional data where only one sex or unsexed birds have been measured, and explanatory data ranging from 1218 species (minimum, display agility) to 2603 species (maximum, egg mass). In total, 3769 species from 125 of 146 different bird families are included. We have used the data in comparative studies of avian sexual size dimorphism, where we test adaptive hypotheses concerning the influence of sexual selection, fecundity, and the degree of withinpair resource sharing. By publishing the data we intend to give easy access to a large data set containing variables relevant for a wide range of comparative studies on birds, thus saving researchers from the time-and resource-consuming data gathering process. In addition, the data set will function to point out species where baseline data on body size and relevant information on reproduction and behavior are currently lacking or of poor quality, thus stimulating the publication of such data.

1. Currently, the deployment of tracking devices is one of the most frequently used approaches to study movement ecology of birds. Recent miniaturization of lightlevel geolocators enabled studying small bird species whose migratory patterns were widely unknown. However, geolocators may reduce vital rates in tagged birds and may bias obtained movement data.2. There is a need for a thorough assessment of the potential tag effects on small birds, as previous meta-analyses did not evaluate unpublished data and impact of multiple life-history traits, focused mainly on large species and the number of published studies tagging small birds has increased substantially.

Summary1. The incubation-limitation hypothesis states that clutch size in shorebirds is constrained by the ability to incubate more than four eggs successfully. In order to test predictions from this hypothesis, clutches of northern lapwings ( Vanellus vanellus ) were here experimentally increased and reduced by one egg at the onset of incubation. Parental behaviour, incubation time, hatching synchrony, egg viability, hatching success and body condition of hatchlings was compared with unmanipulated clutches. 2. Nest desertion and egg breakage was not observed in any of the experimental nests, showing that clutch enlargement did not alter the parental behaviour adversely. 3. There were no differences among the experimental groups in male and female attentiveness, but the total nest attentiveness was lower in reduced clutches than in the other clutch sizes; nor did the parents' frequency of nest visits differ between clutch sizes. 4. Enlarged clutches were incubated longer than controls and reduced clutches. There were, however, no significant differences among clutch sizes in partial or total clutch predation rates. Hatchability of the eggs (proportion of eggs hatching of those present in nests at the end of incubation) was marginally lower in the enlarged clutches (92·5% vs. 100% in the two other groups). 5. Contrary to predicted, hatching success was highest in enlarged clutches. The numbers of hatched eggs calculated from Mayfield estimates of daily nest and egg survival rates were 0·93, 2·10 and 3·14 in three-, four-and five-egg clutches, respectively. 6. When controlling for egg volume, mean body mass of newly hatched chicks was lower in clutches with five eggs than in other nests. 7. We suggest that the lowered body condition of chicks could outweigh the benefits of laying five instead of four eggs in northern lapwings. Future studies of the selective mechanisms of clutch size in shorebirds should not only focus on the number of young that hatch, but also their quality.