Charuvi, A., Lees, D., Glover, HK., Rendall, AR., Dann, P. and Weston, MA.
For a given species, why does one bird fly when another doesn’t?
This process is often referred to as habituation – where some individuals become accustomed to repeated stimuli and therefore respond less. Habituation has been extensively discussed in relation to its importance in informing guidelines for mitigating the impacts of human disturbance on wildlife. However, most studies only consider behavioural responses of individuals, not the underlying physiological responses (Weston et al. 2012). Only a limited number of studies have considered the behavioural and physiological response of individuals to threatening stimuli. These studies are important as it may be that birds respond physiologically prior to showing any behavioural response, meaning our measures of the impact of negative stimuli (i.e. human usage of an ecosystem) may under-estimate true impacts. Therefore, in this study we investigated whether physiological costs were incurred prior to, or simultaneously with a behavioural response.
We simultaneously investigated the behavioural and physiological responses of masked lapwings (Vanellus miles) on Phillip Island. We made standardised approaches towards nesting lapwings; their behavioural response was measured through flight initiation distances (the distance at which the approaching human is from the bird when the bird sleft the nest; physiological responses were measured through placing a false egg containing a hear rate (HR) monitor into the individuals nest prior to the approach (where increased HR represents increased physiological cost).
We synchronised recordings of behavioural and physiological costs and compared the timing of behavioural and physiological responses. We identified two broad response types; a ‘startle’ and a ‘non-startle’ response. ‘Startle’ responses were where a behavioural or physiological response coincided with the appearance of a person; while ‘non-startles’ represent individuals who responded after the appearance and commencement of an approach of the investigator (Figure 2). Startle responses were an unexpected outcome.
We found that in non-startled individuals (N=14) a physiological response occurred prior to the initiation of a behavioural response (57.1% of individuals) or in some cases a behavioural ‘alert’ response occurred prior to a physiological response (42.9%). This finding supports the idea of a distinct ‘Physiological-Initiation Distance’ (PID) which can precede Flight-Initiation Distance (FID) – a behavioural response. Therefore, birds that habituate to human stimuli (i.e. bird watching) are likely to incur greater physiological costs, despite behaviourally showing limited responses.
Our results suggest that behavioural metrics of response are likely to underestimate responses of individuals to stimuli. Further studies are required on a broader range of wildlife to confirm the existence of a PID; thus enabling prescriptions to manage the consequences of human interactions with wildlife.