Art. XXVIII.—A Bird-killing, Tree
[Read before the Wellington Philosophical Society, 26th September, 1883.]
About a month ago my brother mentioned that in a shrub growing in my father's garden at New Plymouth, two Silver-eyes (Zosterops) and an English Sparrow had been found with their wings so glued by the sticky seed-vessels that they were unable to move, and could only fly away after having been carefully washed.
Never having heard before of such an occurrence, and being anxious to learn more about this curious plant, I asked him to keep an eye upon it and let me know anything further he might observe. The following mail brought me some of the sticky seed-vessels and a few leaves, which I showed to Mr. Buchanan, who at once identified the shrub as Pisonia brunoniana, or sinclairii, a native of Whangarei Bay and a very rare plant, called by the Maoris Parapara. In a letter which accompanied the leaves, my sister told me that on the previous day she had rescued two more birds from the plant, and, to use her own words,—“Thinking I was doing a merciful act, I collected all the branches with seed on them I could lay hands on, and threw them into the ashpit. To-day the servant comes in to say that about a dozen ‘silver-eyes’ are glued to these branches, and a pretty piece of work we had to get them clear, for four or five of the sticky pods, at the lowest average, were clinging to each bird. When you look at the tree you can see tufts of feathers and legs where the birds have died, and I really don't think they could possibly get away without help. The black cat just lives under the tree, so that a good many fall to her share; but in revenge many pods get into her fur and she has to come and get them dragged out.”
This particular shrub is 10 or 11 feet high, and perhaps 20 in circumference. I am not aware what the usual height of the tree is when full grown, but probably not much more, as this one is in a sheltered position, in rich soil, and is, I should think, at least fifteen years old. The gum which causes the mischief is secreted by the seed-vessels when they attain full size, and is nearly as plentiful on them in their green state as when they become ripe. The seeds remain long in the sticky condition; but the gum does not exude from either the stem or leaves.
My first idea was that the birds were perhaps attracted by some sweetness in the gum, but my sister tells me that it does not appear to have any taste, and it certainly is not sweet.
My next impression was, that insects must be caught in the gum, and that the birds in seeking for them fell into the same trap. Upon inquiry, I learned that there were a number of insects sticking to the berries. My brother noticed mosquitos, spiders, house-flies, blue-bottle-flies, and the big brown blow-flies. He states, however, that he does not himself think the birds are attracted by the tree in any way more than that they find shelter there, and he adds that he is quite sure that no importance should be attached to the fact that the blight-birds were caught in the berries when thrown into the open ashpit, as he has frequently observed these birds in large flocks in an ashpit in his own garden in quest of scraps of potatoes, etc.
A friend to whom I mentioned the circumstance just told, remembers a shrub in Mr. James Russell's garden, at Auckland, being pointed out as remarkable for the same behaviour. There were tufts of feathers adhering to it also, and the shrub, if not of the same species, closely resembled the one at New Plymouth.* Mr. Buchanan, too, tells me that he and Dr. Hector recollect that when travelling to the north of Auckland, they were told of a tree which captured birds; but they did not pay any heed to what they regarded as a bit of Maori romance. It is clear, then, that Pisonia brunoniana is a confirmed bird-slayer, and that the specimen at Taranaki is not a depraved individual of a harmless species.†
I may, perhaps, also mention that Mr. Buchanan, after considerable search for this shrub, believes, or at least thinks, it probable that it is in its native state extinct, and is now only to be found cultivated in gardens.
The question of course arises,—Does the plant derive any, and, if so, what advantage from its sticky seed-vessels? As the leaves and stems do not exude gum, it surely cannot extract nourishment from its captives as does Dionæa and other carnivorous plants, for it is difficult to understand that any nutriment can be absorbed through full-grown seeds, which spring from the ends of the branches. If the flowers were viscid, fertilization might be promoted by the entangled insects; but it is the seeds alone which generate the viscid matter. Can it be that the seeds are sticky to ensure their being widely disseminated by means of the bird to whose plumage they attach themselves? If so, the plant has not been successful in its
[Footnote] * It is a Pisonia of the same species.
[Footnote] † Since writing this, I find that Hooker, in his Flora of New Zealand, describes Pisonia as a “small genus, chiefly of littoral tropical shrubs or trees with viscid cymes of fruit, sometimes armed with hooked spines in which small birds get entangled.” P. brunoniana has certainly no spines.
object, for, as just stated, it is nearly, if not quite, extinct in its original habitat. But I am too ignorant to give an opinion on this interesting point, and must leave it to others to decide. My part consists merely in calling attention to a curious fact in connection with Pisonia brunoniana which, so far as I am aware, has not been noticed in the Transactions of the New Zealand Institute.