Overhead Line Bird Kill

It has been long known that some river birds are killed by collision with the overhead lines that cross the Ashley River,  but this has not previously been properly documented.

Seven overhead lines cross the river in the area between about a kilometre upstream of the Cones Road bridge and 1.5 kilometres upstream of State Highway One. These consist of 3 pylon lines and 4 normal power lines. Transpower owns the pylon lines and one of the normal power lines; the others appear to be owned by Mainpower.

On 27 January 2019 a number of dead black-billed gulls were found under the power lines 400m upstream from the “Railway” gull colony and 600m downstream from the railway bridge. It was then decided to check the remaining lines; this was done on 29 and 31 January and 4 February. The results are shown in the table and map below. Traverses were done just upstream and downstream from each set of lines and any dead birds within 50m of them should have been located.

Species

Number

Black-billed gull (BBG)

22

Duck

1

Paradise duck

1

Pied oystercatcher (SIPO)

1

Pied stilt

2

Royal spoonbill

2

Total

29

Overhead lines and birds killed
Overhead lines and birds killed

The correlation between overhead line location and dead birds is clear. Dead birds are at times seen elsewhere along the riverbed, but during a bird survey on 31 January from Groyne 2 to the estuary, none were seen other than under the lines. The great majority of bird carcasses were found within 50m of the lines, a SIPO 150m from lines could possibly have died from another cause.

Black-billed gulls are the most abundant species killed by the lines. Four of the 6 BBG beneath or close to the lines just down from the railway bridge were partly eaten juveniles, it is possible that they could have been taken there by harriers, but this does not seem likely. Some of these were partially eaten, but the remains looked more like consistent with being eaten by rats. Under these lines there is vegetation which could host rats.

All but one of the 13 BBG under the lines 400m upstream from the Railway colony were adults, these were intact. These gulls seem to have struck the lines when leaving from or returning to the colony. Each dead adult would probably have resulted in a chick starving to death. Two BBG were found under the westernmost power lines, one was a juvenile with it’s head eaten away. No dead gulls were found under the lines 600m up from the Toppings colony.

Perhaps the most disturbing aspect of the overhead line bird kill is the 2 spoonbill found under the westernmost pylon lines. Spoonbill are normally a very rare sight along the Ashley – but there often seems to be a resident population of around a dozen at the estuary. Nine of these were seen along the upper part of the river on 31 January. They appear to have come up the river to take advantage of easy prey in the shallowing water. The westernmost pylons have a thin wire running along the top – this is much less visible than the transmission wires.

Dead spoonbill beneath pylon lines
Dead spoonbill beneath pylon lines
Same pylons: note the thin high wire at the top
Same pylons: note the thin high wire at the top

The dead paradise duck was found under the easternmost pylons on 31 January. It was being eaten by a black-backed gull. On 4 February the only evidence left was a few feathers and some gory stones. Obviously to properly investigate the number of birds killed by lines it will clearly be necessary to check regularly. A male paradise duck was seen under the westernmost pylons, seemingly with an injured wing. However, it was able to fly.

The bird kill needs to be evaluated in terms of the total population present. On 16 November 2019 there was a total of 2,909 birds counted on our annual survey between kilometres 10 and 13 – an area that encompasses 5 of the 7 overhead lines. This included 2,800 BBG – mainly at the “Railway” colony. A similar count on 31 January 2020 gave 114 for the same area – many of these attracted by the easy fishing where the river was drying out.

Conclusions and Recommendations

  • Quite a large number of birds are being killed by the overhead lines with black-billed gulls being the most frequent victims.
  • No black-fronted terns were found, these patrol the river more than other species, but usually at lower speeds and below the level of the wires.
  • No dead wrybill were found. We only have about 8 pairs nesting along the Ashley, if any were to be lost to wires, this would be serious.
  • Next nesting season we should regularly look for birds under the lines – at least fortnightly. Some dead birds will no doubt get washed away by the river.
  • We shouldn’t encourage birds to nest close to lines by clearing weeds off islands near them. Before the “Railway” colony was formed, the gulls had started nesting very close to the sets of pylons upstream from Cones Road – this could perhaps have led to many deaths.
  • It is perhaps possible for the power companies to make the lines more visible; this could be explored when we have more information.

Grant Davey