Several studies on Parids have noted a peak in the provisioning of spiders to nestlings in the early stages of development. It has been suggested that spider proteins are rich in the sulphur-containing amino acid (SAA) cysteine and that increased provisioning may serve to provide extra cysteine during rapid feather growth. Amino acid analyses were conducted on samples of arthropod groups known to feature in the diet of breeding tits. Cysteine and methionine (both SAA) levels were slightly higher in aranaea protein than in that of other arthropods. However, we argue that the increased level of SAA found in aranaea is insufficient to drive the observed diet selection and that the timing of peak aranaea provisioning does not support the feather growth hypothesis. Instead, we propose that observed patterns of provisioning are due to high levels of taurine present in aranaea and that taurine may be an essential nutrient during a critical stage of chick development.