Appearance and behaviour of immature Guillemots and Razorbills at sea P. Hope Jones and E. I. S. Rees he young of both Guillemot Una aalge and Razorbill Alca torda have an Tintermediately precocial pattern of development (Sealy 1972). That is, the juvenile is led out to sea by one adult, usually the male parent (Birkhead 1984), when its wing development is sufficient for fluttering down from the cliff and diving, but when it has reached only one-third to one-half of the adult weight (Birkhead 1977). Although the departure of the juveniles from the colonies has been described by several authors (e.g. Greenwood 1964), very little has been recorded about their life over the next few months. The departure has often been referred to as fledging, though Burger (1980) considered that the term fledging should be reserved for the stage when they actually fly and become independent of the adult. The primaries, secondar• ies and tail feathers are grown for the first time during the post-juvenile moult of wing-coverts and body feathers in July to October: the first proper flight is therefore achieved with the development of first-winter plumage. This paper describes some aspects of the appearance and behaviour of these immature auks seen at sea off western Britain in the period between early July and mid September, when the adults undergo their complete moult to winter plumage. Throughout this paper, the word 'adult' refers to indi• viduals at least 12 months of age (i.e. in their first summer or older). A series of cruises was made, mainly in the Irish Sea, during 1979-83 with the main aim of studying the distribution and abundance of the auks during the late summer, when the whole population is flightless. Noting appear• ances and behaviour was supplementary to the primary objectives, but the paucity of knowledge prompts us to publish data that are often more anecdotal than systematic. Most of the data presented here were gathered in 1983, but additional observations extending back to the early 1970s have also been drawn upon. Virtually all the observations were made from ships. The influence of the observer's relatively massive platform cannot, therefore, be ruled out; nor was it possible to observe birds for more than a few minutes, even when the ships were stationary. Reaction varied from near panic to apparent indif• ference. Occasional birds were seen actively to approach the stationary or slow-moving research vessels and to dive under them. Trawler-following by 370 [Brit. Birds 78: 370-377, August 1985] Immature Guillemots and Razorbills at sea 371 Guillemots in the Irish Sea has been reported by Hillis (1973) and Watson (1981). 11 seems that, particularly in areas where the birds are used to ships, and when the observer is on a vessel not moving too fast or throwing up a big bow wave, the behaviour is likely to be fairly normal. In the channels into the Mersey, we have seen auks apparently unconcerned by the passing of large ships. By contrast, in more remote localities off the west of Scotland, they seem to react at longer ranges. Distinguishing juveniles and first-winters from adults Throughout July, there was normally no problem in distinguishing the juvenile Guillemots at ranges up to 300m and beyond (with 10X binocu• lars). The main criteria were their small size, relative to the adults, which they usually closely accompanied, and their high-pitched calls. The call is so distinctive and penetrating that, when light and sea conditions make birds on the water inconspicuous, the juveniles are often detected from their calls before being spotted. Subsidiary distinguishing features stem from their behaviour and head colour. Through August, juvenile Guillemots continued to be distinguishable by the same four criteria, but by mid August juvenile Razorbills were accompanying their apparent guardians less closely or joining the parties of adults (first-summers and older) so that they became less obvious in the time available for scrutiny from moving ships. By mid September, the immatures of both species (then mainly in full first-winter plumage) had grown to be almost as big as the adults. Both birds of such couples were almost always in full winter plumage, so that, although on behavioural grounds it was often suspected that couples seen together were a first-winter and its guardian, they could not certainly be ascribed as such. When identified by call, the September first-winter Guillemots looked sleeker than the fuller-shaped adults, with no flank streaks; they also looked distinctly whiter about the head, though by no Table 1. Group size of Irish Sea Guillemots Una aalge, where the two age-classes for all individuals were firmly established, July-August 1983 Groups larger than nine may be under-represented because of the need for rapid recording as the ship moved past; there was little time for careful separation into age-classes, and many larger groups were separated only by species; those in the table were 11 and 26 (each with one juvenile) and 16 (with two juveniles). No. of accom• panying Number of juveniles adults 1 2 3 4 1 377 (75%) 0 0 0 2 67(13%) 24 (55%) 0 0 3 32 ( 6%) 8(18%) 3 0 4 11 ( 2%) 4(9%) 0 1 5 7( 1%) 3( 7%) 1 0 6 4( 1%) 1(2%) 0 0 7 3 1 ( 2%) 0 0 8 3 0 0 0 9 0 2( 3%) 0 0 >9 2 1(2%) 0 0 Total no. of records 506 44 4 1 372 Immature Guillemots and Razorbills at sea means all of them showed white on the nape. (Flank streaks on adult Guillemots in the Irish Sea varied from distinctly dark to barely visible.) Group size During at least the first month after the time they would have left the colonies, the juveniles of both species were usually found accompanied by only one adult. Tables 1 and 2 show numbers of juveniles and adults where the two age classes for all individuals were firmly established. September records have been omitted owing to the difficulties of positive age diagnosis in that month. The proportions were not different between July and August, so data for the two months have been combined. For 555 groups containing juvenile Guillemots, 68% comprised just one juvenile and one adult, whereas, for 211 groups of Razorbills, the equiva• lent proportion was higher, at 84%, though the difference between the species was not significant. Often, when more than one adult was grouped with a juvenile, it was obvious which of the adults was the real guardian, from the relative proximity of the two adults to the juvenile. In areas where there were large numbers of adults, the adult/juvenile pairs tended to swim slightly apart from the rafts of adults. Moult and head plumage Nestling Guillemots have completely dark heads in the down plumage; this is replaced by juvenile feathering which is dark over the crown and nape but white on the chin and throat. They depart from the colonies in this juvenile plumage. In the Irish Sea, during the summer of 1983, it became obvious that the juvenile plumage was variable, and we classified the head plumage into three categories (fig, 1). By September, all the identified immatures were in first-winter plumage, and were very similar to the winter adults, and presumably, in many cases, indistinguishable from them. Fig. 1. Categories of head plumage shown by juvenile and first-winter Guillemots Uriaaalge: left, dark; centre, intermediate; right, white During the period from mid July to mid September, adult Guillemots are progressing through their complete moult to winter plumage (Birkhead & Taylor 1977). Swennen (1977) showed, with captive birds, that this moult becomes progressively later in the summer with the increasing age of the birds, loss of primaries and secondaries starting in mid June for first- summers and in late July for third-summers. In the Irish Sea, first signs of wing moult were noted on 21st July on birds not with juveniles, but those accompanying juveniles rarely seemed to flap their wings, and we have no dates for commencement of wing moult in this group. Individuals with Immature Guillemots and Razorbills at sea 373 traces of winter-like plumage in the head feathering can be seen at any time of the summer; most of these may be sexually immature (first- to fourth- summers), though one case is known of such a bird incubating an egg (PHJ personal observation). The following categories of head plumage were used: (1) full summer plumage, without any trace of white visible on a dark head; (2) full winter plumage; and two intermediate categories: (3) s/w, closer to summer than winter, and (4) w/s for the converse. Razorbills do not seem to have been so well studied, but, for convenience, we classified them according to the same criteria. Table 2. Group size of Irish Sea Razorbills Aha tarda, where the two age-classes for all individuals were firmly established, July and August 1983 No groups over 8 included juveniles, but see caveat under table 1 No. of accom• panying Number of juveniles adults 1 2 3 1 177(88%) 0 0 2 14 ( 7%) 8 0 3 3( 1%) 0 2 4 5 ( 2%) 0 0 5 0 0 0 6 1 0 0 7 0 0 0 8 1 0 0 >8 0 0 0 Total no. of records 201 8 2 Head-plumage patterns for juveniles/first-winters of these species are shown, according to date, in table 3. Although suggesting that the dark phase develops by post-juvenile moult through intermediate to white in Guillemots, there remains the possibility that juveniles do not show sequential changes, but remain in one category until they moult through to first-winter plumage.