What are Swamp Harriers?
Swamp Harriers or “Swampies’ as some farmers now call them, are slow flying ‘Birds of Prey’ or Raptors [the word Raptor means to seize and carry away] whose wing span is around a meter across, they are a ground nesting bird. They are usually seen flying, gliding or hovering at ‘stall speed’ only a few meters above a grain or fodder crop, along fence lines or over swampland, or occasionally on a roadside collecting carrion etc. They are masters at vertical take-off and landing. The hen bird is the larger weighing in at around •850 kg- •900 kg; males are about 2/3rd the size of the female.
What value are they to farmers?
Since European settlement with land clearing and swampland draining for improved pastures, this has somewhat altered the common nesting habit and subsequent Harrier feeding patterns on aquatic fauna like the Native and Eastern Swamp Hen plus many others, along with generous volumes of introduced feral herbivores like rabbits, rats and other vermin, not to mention the odd feral cat, Harriers will only usually take on live prey approx. half their own weight, every attempt at food capture has the potential for bird injury especially during chick rearing. Fresh roadside carrion is often on their menu, food-wise, Harriers have never had it so good in Tasmania!
Are they a threatened species?
Not yet, but a number of factors do exist that could tip the balance! The effect of rabbit control virus on the mainland in particular could be affecting their potential to maintain condition, remember they are competing with other carnivores while there to build up much needed energy reserves before migrating back to Tasmania (across Bass Strait) after over-wintering on the Australian mainland. Modern harvesting equipment and high operational speeds [the old type ‘finger’ mower usually traveled at about 1.5 Km per hr. – plenty of time to observe an active nest site or an alighting parent and take evasive action], the new modern slashing (hay-harvesting) mowers move at around 25Km per hour, ‘new’ breeding nest site alternatives in agricultural harvest crops is another contributing factor since their traditional marshland has been changed to irrigation storage dams. Incubating birds in particular also have a tendency to ‘sit tight’ in crops being harvested at night such as canning peas, both the nest and birds usually fall victim to the harvesting equipment.
Why are nests often found in crops and not traditional swampland?
Swamplands are prime areas to establish new irrigation dams for higher crop productivity and pasture improvements. Because the birds preference for aquatic food types and delivery of same to their demanding chicks, it is more energy efficient to ‘short-haul’ so it makes sense to live next to the ‘supermarket’, some waste-sites like thistle, Gorse and blackberry thickets are occasionally chosen probably because there is a perceived chance of disturbance during crop production?
Surveys have recovered nest sites not only the traditional hay and cereal crops but also in sophisticated field plantings of pyrethrum, opium poppies and canning peas.
What age do adult birds breed and what does the nest look like?
Generally the female in particular is around three to four years, from one to six years their general plumage colour progresses from a dark brown to lighter with more distinct facial disc markings, eventually reaching almost a fawn on older birds their life expectancy around ten years. Females are slightly larger than males with broader heads, longer legs and heavier talons. A bond pair will begin to construct a ground platform nest of dry grasses and sticks until it measures about 60 cm across or about the size of a dustbin lid, dried evidence of the food types consumed by the chicks often litter the nest site.
How many eggs and chicks?
Between one and four depending on a few factors such as site choice, available food, age and experience of the hen bird in particular, it is thought that some pairs gain confidence to produce a larger clutch as familiarity and accrued knowledge of the landscape around them, plus a known availability of food supply improves. Another theory is that confidence of a breeding pair comes with possible familiar recognition of previous nest areas – where banding recoveries have indicated an after winter migratory return. Incubation period is between 21 and 23 days, average chick number is three, occasionally a forth and weaker bird is consumed by the others as food in reserve.
When does nesting begin?
After about two or three months of courting and bonding with aerial displays usually above or within site of the intended nest location, construction begins around September – October.
How long do the chicks stay in the nest?
Overall about six to seven weeks, the more advanced young flying first, but often remaining up to another fortnight or so around the nest paddock area, still very reliant on parental hand outs. However at around five weeks they have a habit of ‘crop wandering’ short distances within the surrounding cover and away from the actual nest platform, usually seeking shade from the direct summer sun of January.
How can farmers assist to locate their nests in crops?
On the average a pair will occupy a ‘territory’ site of around 5 ha. or 10 acres, some males have been known to have two females on the go at once!, as a ‘short-haul’ is preferred returning with food to the nest. Observed aerial displays in the early months of Oct.-Nov. and repeated nest site activity, crop ‘rouging’ and crop harvest investigation and are all known methods to assist in nest finding.
Can the nest be moved to a safer place in the paddock?
Not without great risk of desertion, if standing cover surrounding the nest has been accidentally removed, a couple of hay bales high a meter or so down either side of the nest and facing East to West is the next best thing to provide shade and cover, allow an exit route at either end.
Will the adult birds desert the nest if disturbed?
During incubation or early hatching – [up to about two weeks] is the crucial period of desertion, especially with inquisitive farm dogs or cattle let in too early to feed on a stubble paddock where a nest has been skilfully avoided during harvest. Portable electric fences are most effective in this instance as long as the lower wire is well off the ground.
Can the nest be moved to a safer place in the paddock?
Not without great risk of desertion, if standing cover surrounding the nest has been accidentally removed, a couple of hay bales high a meter or so down either side of the nest and facing East to West is the next best thing to provide shade and cover, allow an exit route at either end.
How much space should be left around the nest site?
It is amazing how much cover loss during harvest some parent birds will tolerate, but too much is better than too little. Depending on the angle of approach with harvesting equipment, a three meter or ‘tractor length’ width either side of the nest is ideal, remember standing vegetation to provide Summer shade is all important as young birds are sometimes left for long hot periods by hunting parents as they grow.
How important is shade if the nest site is left exposed?
Very, as soon as the fledglings are able to leave the nest platform, on very hot days they will seek a cool shady squat close to the earth, this also helps to conceal them when the parent birds are away hunting. ‘Supplementary’ protection methods are mentioned above.
Where do they migrate?
Although the odd adult may be seen to over-Winter in Tasmania, a short migration to the eastern seaboard of the Australian mainland is their normal pattern and behavior. As a result of bird banding activity, records have revealed recoveries as far away as Fiji. April or May the birds begin their migration north across Bass Strait, returning around July – August, (in time to cash in on the ‘Plover’ or Masked Lapwing hatching). It is estimated up to 60% of juvenile Swamp Harriers are lost in the first year, the rabbit control virus methods on the mainland could also contribute further to this figure.
What information is gained from banding?
Populations and movements, species measurements and weights, longevity on band recovery is all useful data to build up a general profile.
What if you should locate an active Harrier nest on your farm?
Brightly flag a radius of at least a ‘tractor length’ (5×5 + meters) around the active nest so as to allow any farm or harvesting machinery ample warning to take evasive action to protect the site.