Summary 1.Researchers often attach transmitters and other devices to free-living birds without a clear understanding of potential deleterious consequences to their study organisms, and thus to their data. Studies investigating this topic have generally been limited to a single species or type of device. 2. To achieve a broader understanding we used a meta-analysis of 84 studies to ask: (1) Do devices have an overall effect on birds? (2) Which aspects of avian behaviour and ecology are affected? (3) What attributes of birds influence transmitter effects? (4) What attributes of devices influence their effects? (5) Are effects partially a consequence of capture and restraint? 3. We found a significant negative effect of devices on birds, both overall and for 8 of the 12 specific aspects analysed. The most substantial effects were that birds with devices had markedly increased energy expenditure and were much less likely to nest. 4. Effects were independent of attributes of the birds (sex, age, primary method of locomotion and body mass). We also found no evidence that proportionally heavier devices had greater effects, although researchers generally avoided using heavy devices. Breast-mounted and harness attachments increased device-induced behaviours such as preening, however, and the risk of deviceinduced mortality differed between attachment methods. 5. Other than foraging behaviours, no effects were a consequence of capture or restraint. 6. Synthesis and applications. We provide the first comprehensive evidence that transmitters and other devices negatively affect birds and may bias resulting data. Researchers should balance the benefits of using these techniques against potential costs to the birds and reliability of the data obtained.

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In males it is frequently testosterone (T) that activates the expression of sexually selected morphological and behavioral displays, but the role of T in regulating similar traits in females is less clear. Here, we combine correlational data with results from T and gonadotropinreleasing hormone (GnRH) manipulations in both sexes to assess the role of T in mediating sexually dimorphic coloration and morphology in the red-backed fairy-wren (Malurus melanocephalus). We show that: (1) natural variation in female expression of ornamental traits (darkened bills and red back feathers) is positively associated with age and circulating androgen titres, (2) females have the capacity to express most male-typical traits in response to exogenous T, including carotenoid-pigmented body plumage, shorter feathers, darkened bill and enlarged cloacal protuberance, but (3) appear constrained in production of male-typical melanin-pigmented plumage, and (4) low androgen levels during the pre-nuptial molt, probably because of low ovarian capacity for steroid production (or luteinizing hormone sensitivity), prevent females from developing male-like ornamentation. Thus, females appear to retain molecular mechanisms for hormonally regulated male-typical ornamentation, although these are rarely activated because of insufficient production of the hormonal signal.

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