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THE DESIGN, CONSTRUCTION AND OPERATION OF HELIGOLAND TRAPS.

BY H. G. BROWNLOW.

( I ) INTRODUCTION. HELIGOLAND traps have been briefly described by Lockley (1947), Hollom (1950) and in the New Naturalist (1948). They were originally developed by Dr. Weigold from the netting traps used to catch thrushes by the Heligoland islanders, and described in Gatke (1895). The objects of this paper are to give fuller details of these traps than have hitherto been published, and to serve as a guide for anyone wishing to build such traps in future.

A cardinal point that should be borne in mind by anyone making or using traps of any kind is that kindness to birds is the first consideration. No damage should be done to birds in any circum­stances, and the minimum possible shock and fright should be inflicted on birds trapped.

(2) GENERAL DESCRIPTION. A Heligoland trap consists of a tapering wire netting enclosure,

open at the wide end, and closed at the narrow end by a collecting box with a transparent back, which appears to birds driven into the trap as a way of escape, and induces them to enter the box. Traps are set up over places where birds collect to feed, rest and roost. They are designed to catch larger numbers of birds than are obtained in smaller portable traps. They involve considerable outlay of effort, materials and expense, and are, therefore, only-worth erecting at places where information of genuine scientific value may be obtained from the results of trapping. Such places, in general, lie on known migration routes, or at other points where migrants are known to concentrate.

(3) SITING. A trap is best sited at the end of a fairly large patch of cover.

If sited in the middle of cover, there will be a greater tendency for birds to by-pass the trap. The nature of the cover, and the position of the trap relative to it, should be such that birds may easily be driven through it to the trap.

The trap should be protected from wind. A high wind in the cover at the mouth of the trap usually means no birds. The ideal site is thus in a hollow, protected from all winds. If that ideal cannot be obtained, the trap should be protected from the winds prevailing at the migration seasons. Before construction starts, of course, the siting should be confirmed by observation that the selected cover is used by birds in reasonable quantities, and variety of species.

The best cover is bushes with some trees. High trees in the cover immediately in front of the trap are undesirable, because birds perching in them may fly over the trap. Paths should lead through

388 BRITISH BIRDS. [VOL. XLV.

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the cover towards the trap mouth, and, particularly near and under the trap, there should be no patches of very thick cover from which birds cannot easily be driven. The bushes in the trap mouth should not exceed five feet in height. If they are higher, they necessitate a higher roof to the trap, thus adding expense and to the danger of birds breaking back over the trappers' heads.

If the site allows of choice of orientation, the mouth should face the local direction of movement of migrant birds at the season which is likely to be most prolific.

(4) DESIGN—GENERAL (See Figs* 1 and 2). The dimensions of the trap will depend on the site, the number

of trappers that will normally be expected to work the trap, and on the resources available for construction. A trap with a mouth wider than 25—30 feet is difficult to manage without several well-drilled trappers who have had plenty of experience on the trap. About five yards of frontage per trapper is as much as can efficiently be managed. A long, narrow trap is, generally speaking, preferable to a short widely splayed one, because in a narrow one, birds are less likely to break back past the trappers.

At some sites, wing-walls of wire netting extending outwards from the mouth of the trap are beneficial. They tend to prevent birds by-passing the trap.

At the open end of the trap, and at suitable points further in, baffles (D. in Figs. 1 and 2), both horizontal and vertical, should be provided. These are strips of wire netting, 1—3 feet wide, fixed all round the mouth of the trap, and similarly further in, and sloping inwards at about 60° to the main wire netting of the trap so as to make a " lobster-pot " effect. They prevent the escape of a large proportion of birds that break back, and amply repay the trouble and expense of fixing them.

A wire should be stretched horizontally across the mouth of the trap and about a foot below the roof to act as a perch for flycatchers (Muscicapa sp.).

The frames supporting the wire netting are like a series of soccer goal posts of equal or slightly diminishing height, and of decreasing width. The height at the mouth should not exceed 7—10 feet. They should converge to a frame about 4—6 feet high and about 6 feet wide. The trap should present birds driven into it with a " point of no return " beyond which the transparent back of the collecting box appears the only or the obvious way of escape, and, in any case, a preferable one to breaking back to the mouth. The " point of no return " illusion is created by changes of direction in the remainder of the trap both in horizontal sense and also in the vertical sense, by making the remainder of the trap slope upwards to the collecting box. Changes of direction have the additional advantage of slowing birds down so that they do not fly at full speed against the back of the collecting box. The birds must, however, be led easily and naturally to the collecting box,

390 BRITISH BIRDS. [VOL. XLV.

Changes of direction must not, therefore, be too sudden and sharp. Doors, to close behind birds driven into the trap are essential, at at least one point, best at or just outside the " point of no return " and, preferably, also at the collecting box opening. Trappers should be able to see clearly birds nearing the main door.

Before the collecting box is reached, therefore, the following additional parts are required (see Figs, i and 2)—

(a) a length of wire netting passage, A, clear of cover or with only very thin cover converging to

(b) a frame, B, constructed as a door frame with a well-fitting door. The opening and closing of this door is best controlled by a wire leading to a point further back in the mouth of the trap. This door is called the main door.

(c) a small, rising and sharply converging passage, called the " funnel" leading from the main door to the opening of the collecting box. The relative positions of the collecting box and the funnel should be such that the sky, or at any rate as much light as possible, can be seen through the glass back of the box from the funnel. The centre line of the passage, A, should be at a slight angle to the centre line of the mouth, and the funnel should be at an angle of about 30° to 45° to it in turn. Traps with only one change of horizontal direction are satisfactory and cheaper, but two changes of direction assist the " point of no return " illusion and lead birds more naturally to the collecting box. The control of the main door is best from a point further back so that birds are not followed too closely. If they are too much flurried before they get into the collecting box, they may damage themselves against the wire netting.

It is a great convenience if a door, called " the trapper's door ", is provided in the side of the netting near the main door. It saves trappers having to go round by the mouth of the trap to get to the collecting box.

Additional lures may be provided in the mouth of trap, such as a bird table with food, and drinking water. Water is a much greater attraction if moving. The easiest way to provide moving water is to hang a container over a small concrete pool and to make a hole in the container with a wood plug slightly ill-fitting so that water drips at the rate of about one drop per two or three seconds. Turnip seed is a cheap and efficient bait for finches (Carduelis, Chloris and Fringilla sp., etc.). Bread crumbs are attractive to many species.

When a trap is being constructed, it will save mush future trouble and expense if a carpenter is employed to make and set up the frame of the main door, and to fix the main door. The trap can then be built outwards from the frame. A good wren-tight door will result.

(5) THE COLLECTING BOX. This is a box with an opening to the funnel and with a trans-

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parent back. It is provided with a hole about 6 inches in diameter, closed by a sliding shutter or by a cloth sleeve, and through which birds can be taken out by hand.

Opinion at at least two observatories favours very large catching boxes, because large flocks of birds are sometimes caught at one drive, and if the collecting box cannot contain the whole flock some birds will damage themselves against the netting of the funnel. A very large box is, however, extremely inconvenient on the more frequent occasions when only one or only a few birds are caught, and may lead to prolonged efforts to catch a bird with the hand. This breaks the cardinal rule of kindness to birds. The ideal arrangement would probably be a glass-lined funnel to hold the flock, and a small and convenient collecting box. Expense rules out that solution, however, and each trapper must work out a compromise to suit the catches he is likely to get.

If a bird is to be caught quickly and easily, the horizontal dimen­sions of the bottom of the box should not exceed 18 inches X 18 inches, and the trapper must be able to see through the back of the box his hand and the bird he is catching. The box should be at a convenient height from the ground for working. The shutter or sleeve should be 3 feet—4 feet 6 inches from the ground. The glass of the upper part of the box should slope, because a slope tends to deflect birds downwards to the bottom of the box, and because the force of the impact of a bird flying against the glass is reduced by the slope. To be effective the slope must be at least 60°, preferably 45 °.

A " small-bird " box should be attached to the main collecting box. The partition between the boxes should consist of a grille of vertical wires or rods i£ inches apart. The " small-bird " box should be at a lower level than the main box, so that birds do not tend to get back through the bars when they see the trapper's hand. This arrangement enables a small bird chased into the trap by a hawk to escape, and also allows small birds caught simultaneously with large birds to escape from trampling.

There are many variants in box design. Figures 3 (a) and (b) show a type of box that has proved satis­

factory at Cley Observatory. Fair Isle Observatory uses automatic boxes as shown in Figure 3 (c). If such boxes are used they MUST be visited very frequently. A small bird may be killed by an hour or two of confinement in a box. Spurn Observatory uses boxes with no internal shelf, but with a hinged plate at the front of the box which is pushed back till it impinges on the glass back, by means of a stiff wire pusher operated from the funnel, (see Fig. 3d). The Swedish trap at Oland has a curved transparent plastic back. Fig. 3 (e) shows a simple type of box that has proved success­ful at Stavanger.

(6) VARIANTS ON THE USUAL DESIGN. (a) " Gully " traps have proved very successful on Fair Isle. The

VOL. XLV.] HELIGOLAND TRAPS. 393

upper ends of steep sided and narrow gullies, or ravines, have been roofed with wire netting, and main doors, funnels and collecting boxes fitted at the upper end.

(b) Double traps, with mouths facing in opposite directions, have been made at Spurn and Fair Isle. That at Spurn was sited at a point where an old chalk bank disappears into a large patch of sea buck-thorn and elder. It was designed to catch such birds as Wheatears (GEnanthe cenanthe) and Meadow-Pipits (Anthus pratensis) with the mouth facing the bare chalk bank, and cover-loving birds with the other. It was also designed to be convenient for both migration movements, the directions of which are well defined on a peninsula like Spurn. The two mouths shared a common funnel at right angles to each, but this arrangement was not altogether satisfactory. The change of direction was too sudden for driven birds, and a great many made no attempt to enter the collecting box. The double trap at Fair Isle (plate 82) is on a stone wall and was originally two straight traps, butted back to back. The mouths are rather wide and short. Birds entered the mouths quite satisfactorily, but nearly all broke back past the driving trappers. At both observatories each trap was later fitted with a separate funnel, properly angled. The bad effect of the lack of any " point of no return " illusion in the Fair Isle traps was most marked, and the modifications resulted in greatly increased effect­iveness.

(c) Portable Heligoland traps have been developed by several ornithologists. All are designed to be carried in an ordinary car, and to be capable of erection in a period of the order of one hour. Mr. R. Chislett has a funnel made of light wood frames covered with wire netting and makes a mouth of bamboos and strawberry-netting. Colonel R. S. P. Bates uses triangular frames of stiff wire with wire netting soldered to them. The frames are bent through about a right angle about the central axis. Each frame is fitted with a small collecting box thus making a simple trap with no funnel and with a mouth of triangular section. Dr. K. B. Rooke has made a portable trap specially designed for the shore at Portland. The mouth is made up with guyed steel uprights carrying string netting. This trap is larger than the others described. The time of erection is of the order of one day. One of the earliest was that of Herr A. Schifferli, of Sempach, Switzerland. It can be erected on a foundation of stakes in a reed-bed, or similarly on stakes to take birds from the top of a high hedge. The mouth is made of iron frames, 2 metres by 1 metre, covered with wire netting. The frames are joined by string " snake " lashings. If a two-frame width or height is required, the frames are joined by diagonal sticks, " snake " lashed to the netting. The funnel consists of similar tapering frames joined by rings, so that it will " matchbox " flat for packing. When erected, it is kept square by lashing to the frames of the mouth, and to the collecting

394 BRITISH BIRDS. [VOL. XLV.

box. The collecting box is made of stiff wire with wire netting soldered to it. It has a central shelf, a removable glass back in slides, and a sliding door at the bottom.

Herr Schifferli's trap is shown in Fig. 4.

FIG. 5. CONSTRUCTION of WOOD FRAMES

(7) DETAILS OF DESIGN. (a) Framing. The frames may be made of a variety of materials

and, in most cases, design will be dictated by stocks available or by the type of material most easily obtained. The neatest finish will be given by timber, preferably squared, and properly framed and jointed (see Fig. 5) or by small gauge water pipe or electrical conduit connected by the standard bends, T-joints, etc. With care, however, a wren-proof wire covering can be arranged round lashed junctions of poles or round the joints of tubular scaffolding. If timber is used, it, or at any rate the bottoms of posts, should be creosoted. It is best to get the timber treated by creosote in a pressure plant, but such plant is not common, and transport charges may be prohibitive. Timber posts should be sunk at least 18 inches into the ground in good ground. In very loose sand it may be

VOL. XLV.] HELIGOLAND TRAPS. 395

necessary to sink them as much as five feet. Metal posts should each be set in a concrete block of not less than i cubic foot volume poured in a hole so that the top of the concrete is flush with ground level.

(b) The wire netting. The finer the mesh of the wire, the greater will be the expense, and the greater the resistance to wind and snow. A variety of mesh sizes will therefore probably be used in construction of the trap. The funnel is the part of the trap where birds will make the most strenuous efforts to escape. The netting of the funnel and of the main door must be at most \ inch mesh if Wrens {Troglodytes troglodytes) and Chiffchaffs (Phylloscopus collybita) are to be retained. The netting of the mouth may be § inch or f inch and of the wing walls i inch. The roof of the trap may also be of coarser mesh than the walls, provided that a strip about one foot wide along the edges of the roof is of the same mesh as the walls. A coarse mesh for the roof is desirable, to avoid trouble in the event of heavy snow. The gauge of all wire used should be about 19 gauge. Netting of finer gauge does not last well. In trap construction, trenches about 6 inches deep should be dug right along both sides of the trap before the wire is put on. The bottom of the wire should be set at the bottom of the trenches, which should be filled in when wiring is completed. Adjacent widths of netting should be overlapped for a distance at least three times the mesh (e.g. f inch netting should overlap at least z\ inches). The best joint is made by " sewing " with fine wire. A light person can sit on a plank supported by the frames to pass the " sewing " wire back when sewing the roof joints.

(c) Doors. Moving parts are always likely to be a source of trouble and to involve careful maintenance and the more com­plicated they are, the greater are the likelihoods. The simplest arrangement for the main door is to hinge it to the top horizontal cross member of the frame so that the door falls to close. Gravity cannot go wrong. If the frame is sloped back towards the collecting box at a slope of about 4/1, the door will be kept closed by gravity. It can be controlled by a wire led back to the control point by small pulleys, rings or tubes, or be kept open by a vertical stick which can be pulled away by a cord. Wire or fishing line is much more satisfactory than ordinary cord for control cables. Cord stretches, and contracts when wet. Wire remains the same length.

If the door is hinged to an upright, more complicated arrange­ments will be needed to control it. Avoid patent spring closers like the plague. They are not normally designed for use in the open, and quickly go wrong. The main door itself should be a stout frame covered with the same netting as that of the funnel. It is most important that arrangements be made to prevent birds getting behind open doors. Either the opened door should fit closely to the wire of the trap, or a light false roof of wire should b6 made to which the open door can closely fit. The simplest

396 BRITISH BIRDS. [VOL. XLV.

control for the trapper's door is made by boring a hole through the upright of the frame at a convenient height, pushing a metal rod through the hole and bending its ends at right angles close to the upright so that they can swing to embrace the closed door. A nail on the inside and on the outside of the door on which they can rest in the closed position completes the device, which is easily operated from either side.

(d) The funnel. A most important point that is always over­looked by inexperienced constructors is that the wire mesh of the funnel MUST come close to the collecting box opening (sep Fig. 6).

FIG. 6. WRONG RIGHT

The part of the floor of the funnel which rises from the ground at the bottom of the collecting box opening is known as the ramp. The ramp should be opaque, and strong enough to take the weight of a trapper who wants to get his hand to the collecting box opening, to deal with recalcitrant birds or to operate the pusher of a Spurn-type box. Even in the biggest traps, the funnel need be no bigger than just large enough to contain one trapper comfortably inside the closed main door. If the funnel is unnecessarily long, the advantage of the bends in slowing birds down is lost, and in a big funnel birds do not easily see the apparent way of escape at the collecting box. The funnel should be kept clear of vegetation. Bushes in a funnel are an intolerable nuisance.

(e) The collecting box. This may be made up from boarding, or may consist of a wood frame covered with light boarding, asbestos sheeting or any other material available. The transparent back panels may be simply glass sheets sliding in grooves, or a rebated framing carrying glazed wooden panels. It is necessary that the transparent panels be easily removable, so that the trap can be put out of action when it is not used for lengthy periods. If the trap is not being used for two or three hours, it may be temporarily put out of action by leaving one of the doors closed, preferably the door of the box opening, because this may be opened after birds have been driven into the trap and the main door has been closed. A piston for reducing the size of a large box consists of a board shaped to the vertical section of the collecting box lower compart­ment and firmly fixed to a handle protruding through a tube bearing secured to the side of the box. The piston head should be £ inch

VOL. XLV.] HELIGOLAND TRAPS. 397

clear of the sides and bottom of the box, and should have strips of rubber fixed to its edges to sweep the bottom and sides. The piston should be kept in place by the fit of the handle in the tube bearing. Such an arrangement reduces the chance of pinching a bird's wing or leg by the action of the piston, (see Fig. 7).

FIG. 7 . PISTON FOR REDUCING SIZE OF LARGE BOXES

Various substitutes for glass have been tried in collecting boxes. Perspex sheets soon lose their transparency and are not so easily cleaned as glass. Wire gauze such as is used to fly-proof larders was successfully used in Egypt, but quickly rusts in the damp climate of Great Britain. It is a wise precaution to keep some of one of these substitutes as a " spare " so that a first-aid repair can quickly be done if a glass is broken.

(f) Shutters and sleeves. Both have their disadvantages. Unless they are very stoutly constructed, shutters are apt to shrink or warp and become loose, and even when they are well-fitting, the occasional accidental escape of a bird while it is being taken out of the box is more likely than with a sleeve. Shutters should be made so as to slide vertically, not horizontally. Horizontal shutters are apt to be blown open by the wind. A sleeve, on the other hand, while more efficient, becomes wet and uncomfortable to use in bad weather. A suggested way of overcoming that disadvantage is shown in Fig. 3 (b). A small pent-roof is fixed over the opening, a hook-screw is fixed between the opening and the roof, and a button hole is made near the end of the sleeve. The sleeve can then be hooked up under cover after use. Part of an old flannel trouser leg makes as good a sleeve as anything.

(g) Accessories to the Trap, (i) A hut or some building with table and chairs sited close to the trap is almost essential. I t is useful to keep ringing registers, to store rings, trap glasses when not in use, books, tools, spares, clapnets and portable traps, bags of seed for bait, and the mixed bag of oddments which observatories acquire. The building must have a sound floor on which a dropped ring may quickly be found.

(ii) One or more portable collecting boxes are very useful for transporting a multiple catch of birds to the ringing place. A

398 BRITISH BIRDS. [VOL. XLV.

box about 9 inches by 9 inches by 9 inches is suitable. It should be provided with a carrying handle or sling, and a sleeve. A useful addition is to have one side of the box open and fitted with grooves to take two slides, one glazed and one opaque. A bird can then be kept quietly in the dark, and inspected by lifting the opaque slide. If birds have to be carried more than a few yards, linen or muslin bags are preferable to boxes.

A bank of small boxes with opaque shutter openings is another useful accessory. Some system of marking to differentiate between occupied and empty boxes is essential

(iii) The following tools are suggested :— 1. Rule with stop at the millimetre zero, and dividers for measuring

birds. Pliers of various sizes. 2. Tools for trap maintenance :—

Spade, fork, garden shears, scythe, billhook, handsaw, tenon saw, hammer, screwdriver, pliers side-cutting, tinsmiths' shears (for cutting wire netting).

3. The following spares are suggested :— Perspex or wire-gauze for repairs to broken glasses of collecting boxes, fine sewing wire, nails, screws, staples, spare hinges, wire netting.

(8) OPERATION. The driving technique will vary with every trap, and with the

species of bird being caught. It must be worked out for each trap. There are, however, certain general principles applicable to all traps and which are suggested for inclusion in trap rules.

(a) Approach to the trap should always be in the normal direction of driving birds. A route for leaving the trap which is well clear of the drives should be worked out and enforced.

(b) Trappers must never stand about in the mouth of the trap. (c) At every drive one trapper must be responsible for resetting

the trap, e.g. opening trapdoors and shutting the trapper's door. (d) Rings must be used in proper numerical sequence, and the

rules on the back of the B.T.O. Ringing Committee's schedules must be strictly obeyed.

(e) Other forms of trapping, such as clapnets and portable traps, should be done, well clear of the driving area of a Heligoland trap.

(f) Birds should be driven into the trap with the minimum possible noise and violence. Birds like warblers (Phylloscopus sp. and Sylvia sp., etc.), Hedge-sparrows (Prunellamodularis), Robins (Erithacus rubecula), and other cover-loving birds can usually be gently shepherded almost all the way into the box. Birds of the thrush family (Turdidce) require rather more forceful driving and finches, lured into the trap mouth by food or water, can usually only be trapped by a sudden rush. As a general rule noisy driving is only permissible when far from the trap. The nearer to the trap the quieter should be the driving, except, of course, for a final

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rush on "the mouth of the trap which is sometimes necessary. (g) When birds are in the mouth of the trap, trappers should

keep in line with each other. Birds break back much more easily if trappers are out of line.

(9) PLANTING OF COVER. Plants bearing berries or seeds that are likely to attract birds

should generally be planted in and near the mouth of the trap. Lists of trees and shrubs suitable for various habitats, and valuable information on planting and on cuttings is given in Rowe (1951). Of the 37 species or genera of shrubs listed in these books as suitable for very exposed parts of the coast some have already been established in the extreme conditions usual at observatories. Escallonia and Veronica, both evergreens, have done well at Skok-holm. Experience at Heligoland, the Isle of May, Spurn and Cley, has shown that Elders (Sambucus sp.), Privet (Ligustrum sp.), Sea Buckthorn (Hippophce rhamnoid.es), and Sycamore (Acer pseudo-platanus) are about the only woody plants that have been tried which are capable of surviving at first, though others may do so when the elders have grown up enough to protect them. Other plants that may prove useful are the Tree Mallow (Lavatera arborea), which has been successfully established at May, Skokholm and Cley, but is unlikely to grow north of the Firth of Forth, and the Hop (Humulus lupulus), which, at the Isle of May, has done splendidly as a climber up wire or poles, and the Tea Plant (Lycium chinense), which grows luxuriously on the east coast and has been success­fully used at Cley. Small plants likely to attract birds are Yellow Loosestrife (Lysimachia vulgaris), Mullein (Verbascum thapsus), Thistles (Carduus sp.), Sea Rocket (Cakile maritima), Sweet Vernal Grass (Anthoxanthum odoratum) (has been known to attract Scarlet Grosbeaks (Carpodacus erythrinus), at Fair Isle).

(10) ACKNOWLEDGEMENTS. I am much indebted to Messrs. W. B. Alexander, K. Williamson,

G. H. Ainsworth, and R. A. Richardson who have kindly read this paper and given me very helpful criticisms and suggestions, to Mr. Williamson also for permission to use the photograph illustrating this paper and to Mr. G. C. Johnson for drawing the figures.

REFERENCES. GXTKE, D R . HEINRICH. (1895). Heligoland as an Ornithological Observatory.

Translated from the German by R. Rosenstock. HOLLOM, P. A. D. (1950). Trapping methods for bird ringers. B.T.O.

Field Guide, No. 1. LOCKLKY, R. M. (1947). Letters from Skokholm. Dent. R O W E , W. H. (1951). Tree and Shrub growing. Faber and Faber. R O W E , W. H. (1951). Trees and Shrubs and how to grow them. Penguin.

British Birds, Vol. xlv, PI. 82.

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