This is probably the most familiar amphibian in Ireland. Adult common frogs vary significantly in colour from brown, through olive, to yellow. They have a pale belly and often prominent black markings on the head, back and sides, and dark bars on the hind legs. Adults reach approximately 8cm long. Females each lay between 1000 and 2000 eggs which swell to form characteristic spawn ‘clumps’ or aggregated ‘mats’ of jelly-like eggs. Common frog tadpoles are greenish brown in colour and tend to gather around pond margins during sunny weather.
Life-Cycle and Behaviour:
Common frogs hibernate for a short period from approximately mid-November to mid-January. Some hibernate in pond sediments, others in terrestrial hibernacula. Males assemble at their breeding ponds in late-winter/early-spring to await the females. Having bred, both sexes disperse from the spawning area but may stay around the pond or move into surrounding suitable habitat. Common frogs tend to feed on land at night, and take a range of invertebrate prey. Although they tend to frequent pond margins, they can also travel substantial distances over land between ponds. Tadpoles feed on a variety of micro-organisms, detritus and algae. They complete their metamorphosis and emerge from their natal ponds 10-15 weeks after hatching.
Common frogs exploit a wide range of wet habitats and will breed in small puddles, ditches, the edges of large lakes and even slow-flowing sections of rivers. Although the tadpoles are vulnerable to fish predation, the adults do not appear to avoid ponds containing fish, but tend to spawn in the more inaccessible areas.
Common frog surveys focus on recording vocalising males and counting spawn clumps and mats. Griffiths, Raper and Brady (1996) present a method for estimating the number of breeding animals and hence the size of the local population by counting spawn clumps. Where difficulty in differentiating spawn clumps that have coalesced into a larger spawn mat is encountered, this method also provides a technique for establishing the number of clumps by measuring spawn mat area. It is imperative that biosecurity measures are taken when carrying out amphibian surveys in order to avoid the incidental spread of vector borne diseases (such as crayfish plague) between waterbodies. This includes cleaning, appropriate disinfection and thorough drying of all equipment between surveys undertaken on different waterbodies and watercourses.
Optimum Survey Period:
Vocalising males should be surveyed between late-January and early-February. Spawn clumps and mats should be surveyed and identified during February.
Mitigation, compensation and enhancement:
Where it is considered appropriate to relocate frogs, adults can be captured using pitfall traps, drift fences and by netting, whilst spawn can be moved during the early part of the breeding season. Amphibian-proof fencing close to ponds can be effective in preventing frogs gaining access to the most hazardous parts of construction sites. Permanent fencing can be used to guide frogs to purpose-built tunnels and other safe crossing structures.