Effects of Backpack Radio Tags on Tawny Owls

Abstract

Radiotelemetry is an important tool in many studies of bird behavior and ecology. Additionally, long-lived radio tags, mounted as backpacks, enable survival to be estimated with higher precision and less sampling bias than studies based on band returns (Kenward 1993; Kenward et al. 1999, 2000). Any tag will inevitably increase the energetic costs of flight due to the additional weight and aerodynamic drag (Pennycuick et al. 1989), but due to their larger size and weights, backpack radio tags have higher potential for affecting the birds negatively than leg bands or radio tags attached to the tail feathers (Calvo and Furness 1992). In backpack mounts, the harness may also increase energy expenditures during flight due to friction between harness and plumage (Gessaman and Nagy 1988), as well as decrease agility and increase heat loss through distortion of the plumage. Poorly fitting mounts may cause abrasions, and birds might become tangled in vegetation (e.g., examples in Foster et al. 1992). Special concerns about application of radio tags is required in studies of fledgling birds; backpack tags are more difficult to attach because extra slack in the harness is required to enable further growth, and wings are not fully developed at fledging, which may augment the effect of the weight load of the transmitter. Juveniles are therefore likely to be more sensitive to tag effects than adults. Accordingly, any study drawing inferences about demographic parameters from radiotagged individuals should address the potential influence of the tags on their estimates, especially if backpacks are used. Despite the limited number of field studies addressing tag problems, several have found radiotagging to have a negative influence on reproductive success (e.g., Foster et al. 1992, Marzluff et al. 1997, Gregory et al. 2003, Phillips et al. 2003), survival (e.g., Longcore et al. 1991, Reynolds et al. 2004), or both (e.g., Paton et al. 1991, Paquette et al. 1997; see Calvo and Furness [1992] for a review of tag-effects in studies published before 1991). Particular concerns have been raised about the effects of backpack tags on owls because all 3 studies (Paton et al. 1991, Foster et al. 1992, Petty et al. 2004) testing the effects of backpack tags on demographic parameters have found significant negative effects. Foster et al. (1992) found lower reproductive success of tagged individuals in 9 of 10 studies of spotted owls (Strix occidentalis) in Oregon, USA, and Washington, USA—4 of these differences being significant—although backpack tags did not appear to affect survival. In California, Paton et al. (1991) found female spotted owl survival affected negatively by backpack tags in 1 of 2 years, and there were negative effects on reproductive success in both of 2 succeeding years. As a consequence, capture– recapture data obtained from radiotagged individuals have been discarded from demographic analyses on spotted owls (Forsman et al. 1996, Franklin et al. 1996). Petty et al. (2004) reported that juvenile tawny owls in England fitted with backpack tags prior to fledging having substantially lower recruitment probability than a control group of untagged juveniles, as tested with capture–mark– recapture analysis. Based on this result and a review of published information about survival and recruitment rates of juvenile tawny owls and spotted owls with radio tags, they strongly opposed using backpack tags on juveniles in any Strix species before the impact of tags have been further investigated. Herein, I evaluate the effects of backpack radio tags on body mass, survival, and reproduction of adult tawny owls, surveyed over 6 breeding seasons (1998–2003) in Denmark. Based on data from a single year, I also assess the effects of backpack tags on survival of fledged juveniles as compared with an untagged control group.