As an animal model of influenza, ferrets are uniquely capable of displaying clinical signs of illness similar to those of human influenza virus infection. To quantify lethargy, we previously established video monitoring as a more sensitive method than the commonly used manual scoring methodology for ferrets infected with influenza virus. Video monitoring is simple to set-up, but its adoption by other laboratories is restricted by the need to purchase costly commercial software, EthoVision® XT, to analyse activity. To broaden the use of video monitoring method in ferrets, we assessed Swarm Sight, a free open-access programme, for analysing activity changes in ferrets infected with seasonal influenza A(H1N1), A(H1N1) pdm09, A(H3N2) and B virus. Swarm Sight could differentiate between the various levels of lethargy associated with the infection of different influenza virus subtypes to a similar degree to EthoVision® XT. However, one major limitation of Swarm Sight is that it does not permit high throughput analysis, which considerably increases the time required to process video clips from experiments involving large numbers of ferrets. Despite this limitation, the open-accessibility and comparable results to EthoVision® XT make Swarm Sight a good alternative for researchers interested in using video monitoring to measure lethargy in ferrets.