A History of the Birds of New Zealand.
Sterna Nereis. — (Little White Tern.)
(Little White Tern.)
Sternula nereis, Gould, P. Z. S. 1842, p. 140.
Sterna parva, Ellman, Zool. 1861, p. 7473.
Sterna nereis, Pelz. Verh. zool.-bot. Gesellsch. Wien, xvii, p. 818 (1867).
Sterna minuta, Finsch, J. f. O. 1867, pp. 337, 347.
Ad. ptil. æstiv. suprà dilutè cinereus, tectricibus alarum dorso concoloribus: remigibus intùs albis, extùs cano lavatis, primariis duobus externis extùs nigricantibus, scapis albis, pennis minoribus versùs apicem albis, secundariis intimis dorso concoloribus, dorso postico et uropygio cum supracaudalibus albis: caudâ albâ: pileo postico et nuchâ cum regione oculari et supraparoticâ nigris: fronte latâ, genis et facie laterali et corpore subtùs toto albis: rostro lætè flavido: pedibus flavis, unguibus nigricantibus: iride nigrâ.
Ad. ptil. hiem. similis ptilosi æstivæ, sed pileo summo albo nigro vario: nuchâ nigrâ.
Juv. fronte et pileo cinerascenti-albis fuscescente variis: lineâ crescente ab oculo postico circà nucham productà nigrâ: suprà dilutè cinereus, plumis versùs apicem fasciâ irregulari brunneâ transnotatis: rostro flavicantibrunneo: pedibus sordidè flavis.
Adult in summer. Forehead and along the base of upper mandible white; spot in front of each eye, crown of the head, and nape black; throat, fore neck, and all the under surface silvery white; hind neck, shoulders, back, and upper surface of wings delicate silvery grey, darker on the primaries; rump and tail, with the upper and lower coverts, pure white. Irides black; bill bright yellow; tarsi and toes yellow, the claws darker. Length 9 inches; wing, from flexure, 7·5; tail 3 (median feathers 1 inch shorter); bill, along the ridge 1·25, along the edge of lower mandible 1·35; tarsus ·6; middle toe and claw ·7.
Adult in winter. Differs in having the crown of the head white, mixed with black, darkening outwards, the nuchal collar being entirely black.
Obs. In some examples the first primary is margined on the outer web with black; in others it is of a uniform dark grey.
Young, Forehead and crown greyish white, mottled with dusky; from the eyes a crescent of greyish black, which encircles the occiput; the plumage of the upper parts silvery grey, mixed with white, and many of the feathers with an irregular wavy mark of dark brown near the tip; the smaller wing-coverts greyish brown; underparts white, as in the adult. Bill yellowish brown; feet dull yellow. The tail is less acuminate at the sides than in the fully adult bird.
This is the smallest of our Terns, and is the southern representative of the Sterna minuta of Europe. It is tolerably common on all our coasts, and occurs also very plentifully along the shores of Western Australia.
It is very active in its movements, flies high, turns in the air with facility, and dips into the water after its prey in a very adroit manner. When resting on the sands it appears, owing to the page 76 shortness of its tarsi, to be actually lying on its breast; but it seldom remains long in this position, being far more restless than the other species. Rising silently, it mounts in the air, and having marked out a fishing-ground, hovers first to one end of it and then to the other, repeating the circuit with the most regular precision. It is less sociable than the other Terns, never assembling in flocks, but always associating in pairs, usually hunting together in silence but with an occasional call-note, sounding like crek-crek. Sometimes four are seen in company, but this only represents a family party, the additional members being the young birds of the year.
During the breeding-season it is very clamorous, especially when its nesting-ground is invaded or even approached. It deposits its eggs on the bare shingle, without any attempt at forming a nest, merely selecting a natural depression suited to its own size; and the colour of the eggs harmonizes in a remarkable manner with their surroundings.
There is nothing more interesting in the study of oology than the systematic way in which the colouring of eggs (and particularly those of sea-birds) is adapted to their natural environment.
Captain Mair has furnished me with a remarkable instance of this law of assimilative colouring for protective purposes. In December, 1875, he visited the Rurima Rocks, in the Bay of Plenty, and found large numbers of Larus scopulinus breeding there. In some localities the nests—roughly formed and lined with feathers—were placed in the thick masses of wild spinach or in the midst of “sand-fire.” In all such cases he observed that the eggs which these nests contained were splashed over their entire surface with large green blotches, thus assimilating their colour to the surrounding vegetation; whilst other eggs (belonging to the same species), deposited on the white sand in the immediate vicinity, had a totally different appearance, being of a light stone-colour, and so marked as to harmonize exactly with their sandy surroundings.
It is difficult, however, to account for the very intricate marking that distinguishes the eggs of Larus bulleri from those of its near allies, the breeding-habits of these birds not being, so far as I am aware, in any way dissimilar. An egg of the last-named species in my son’s collection is of a creamy stone-colour, with a broad irregular inky zone near the larger end, splashed on its edges with umber-brown, the rest of its surface marked, in a very eccentric way, with widely-spread hieroglyphics of the same dark colour. Possibly these markings are intended to simulate minute fragments of seaweed.
But assuming this protective resemblance to be a chief factor in determining the natural colours and markings on the surface, it is indeed very curious to observe how sometimes the eggs in one nest, produced at short intervals and all subject alike to the same conditions as to their future safety, differ from one another in their coloration. There can be no doubt that the colouring of birds’ eggs, which is chiefly due to animal matter deposited on the surface of the shell and capable of being rubbed or scratched off, must be to a large extent influenced by the state of the producer’s health and by any special sensations to which the bird may be subjected shortly before the extrusion, for it is well known that, even in the case of many birds that produce highly-coloured eggs, the hard shell is found to be perfectly white only the day before it is laid. Even Mr. Hewitson, who, in his ‘Eggs of British Birds’ (Intr. p. viii), declines to admit the general rule that the varied and beautiful hues which adorn the eggs of birds are given as a protection against discovery and destruction, is constrained to say:—“That there are several instances in which the eggs of birds are admirably adapted to and closely resemble in colour the ground upon which they are deposited, I have frequently found, much to my annoyance, when in search of them; and these are just the instances where such protection is most necessary, and where contrasting colours would lead to detection; such is the case amongst those birds which, making little or no nest, deposit their eggs, for the most part, upon the bare ground, or the shingle of the sea-beach, and leave them uncovered on the least alarm.”
This species usually lays two eggs; these are of a regular oval form, measuring 1·4 inch in length by 1·05 in breadth, and are of a yellowish white, the whole surface marked with obscure spots of purplish grey.