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Ecological relationships between Common Murres, Uria aalge, and Thick-billed Murres, Uria lomvia, at the Gannet Islands, Labrador. I. Morphometrics and timing of breeding

T. R. BIRKHEAD Zoology Department, The Universiry of Sheffield, Sheffield, England S10 2TN AND D. N. NETTLESHIP Canadian Wildlife Service, Bedford Institute of Oceanography, P.O. Box 1006, Dartmouth, N.S., Canada B2Y 4A2 Received August 6, 1986

BIRKHEAD, T. R., and NETTLESHIP, D. N. 1987. Ecological relationships between Common Murres, Uria aalge, and Thick-billed Murres, Uria lomvia, at the Gannet Islands. Labrador. I. Morphometrics and timing of breeding. Can. J. Zool. 65: 1621-1629. Several aspects of the breeding biology of Common Murres (Uria aalge) and Thick-billed Murres (Uria lortivia) in 1981-1983, at the Gannet Islands, Labrador, are described. At this colony there were ca. 60 000 pairs of Common Murres, and 1400 pairs of Thick-billed Murres. Common Murres were slightly heavier and had significantly shorter wings, and longer. narrower bills than Thick-billed Murres. The timing of laying varied between years (late breeding in one year was associated with late ice breakup), but the median laying date of Common Murres was consistently earlier (by up to 10 days) than that of Thick-billed Murres. For both species median laying dates fell between mid and late June each year. were larger and relatively longer than Thick-billed Murre eggs, but in both species fresh egg weight was about I I% of adult body weight. Incubation periods were similar in each species (33 days), but chick-rearing periods were longer in Common Murres (24 days) than in Thick-billed Murres (21 days) in all years. Seasonal pattems of colony attendance were broadly similar in the two species each year, except that Common Murres showed a consistent increase in numbers between laying and chick departure. and tended to remain at the colony for less time after chicks had departed cop pared with Thick-billed Murres. All of both species left the colony by mid-September each year.

BIRKHEAD, T. R., et NETTLESHIP, D. N. 1987. Ecological relationships between Common Murres, Uria aalge, and Thick-billed Murres, Uria lomvia, at the Gannet Islands, Labrador. I. Morphometrics and timing of breeding. Can. J. Zool. 65 : 1621-1629. Plusieurs aspects de la biologic de la reproduction des Marmettes de Troll (Uria aolge) ct des Marmettes de Briinnich (Uria lomvia)ont fait l'objet d'une etude de 1981 a 1983 dans les Iles Gannet au Labrador. Environ 60 000 couples de Marmettes de Troll et 1400 couples de Marmettes de Bninnich faisaient panic de cette colonic. Les Marmettes de Troll etaient legerement plus lourdes et avaient les ailes significativement plus courtes et lc bee significativement plus long et plus etroit que les Marmettes de BrUnnich. Le moment de la ponte a vane dune armee a l'autre (une année, la ponte tardive etait associde a la disparition tardive des glaces), mais Ic milieu de la periode de ponte des Marmettes de Troll s'est toujours produit plus tot (jusqu'a 10 jours plus tot) que le milieu de la periode de ponte des autres marmettes. Chez les deux especes, le milieu de la periode de ponte a toujours eu lieu entre la mi-juin et la fin de juin. Les oeufs des Marmettes de Troll étaient plus gros et relativement plus longs que les oeufs de l'autre espece, mais, chez les deux especes, la masse d'un ocuf frais correspondait a environ 11% de la masse de radulte. La durée de la periode d'incubation s'est averee semblable chez les deux especes (33 jours), mais la periodc d'elevage des oisillons durait plus longtemps chez les Marmettes de Troll (24 jours) que chez les Marmettes de BrUnnich (21 jours). La presence des marmettes aux colonies suivait a peu pres la meme tendance chez les deux especes chaque armee, mais le nombre de Marmettes de Troll augmentait toujours entre la ponte et Ic depart des oisillons et les rnarmettes de cette espece avaient tendance a quitter la colonic plus rapidement apres le depart des oisillons. Tous les oiseaux avaient quittd la colonic a la mi-septembre. (Traduit par la revue]

Introduction similar species replace each other in different latitudinal and lnterspecific competition is notoriously difficult to demon- oceanographic zones (Murphy 1936; Fleming 1941; Ashmole strate unequivocally in nature (see Strong et al. 1984). Lack 1971; Sealy 1973) and this pattern has been explained in terms (1971) stated that competition was most likely to occur between of competitive exclusion (Lack 1971). Within the family closely related, morphologically similar species occurring (Alcidae), at least two pairs of species show this type of pattern: sympatrically, and suggested that observed ecological differ- (i) , Fratercula corniculata, and Tufted Puffin, ences between such species constituted evidence for inter- Fratercula cirrhata, in the North Pacific (Sealy 1973); and (ii) specific competition. At best this approach provides only Common Murre, Uria aalge, and Thick-billed Murre, Uria circumstantial evidence for competition, as all species-pairs are lomvia, in both the North Pacific and North Atlantic (Tuck likely to show some ecological differences, which may or may 1961). The Thick-billed Murre has a more northerly distribution not be the result of competition, and may or may not result in than the Common Murre, but the two species' ranges overlap resource partitioning (Wiens 1984). considerably in both oceans (Tuck 1961; Nettleship and Evans Studies of colonial have provided a large amount of 1985). Common Murres and Thick-billed Murres are morpho- information on resource partitioning, in terms of both breeding logically and ecologically similar, and Lack (1971) assumed habitat and chick diets (Belopol'skii 1957; Ashmole 1963; that the relatively small differences in habitat and diet which Bedard I969a, 19696; Carrick and Ingham 1970; Croxall and existed between them indicated the occurrence of competition. Prince 1980). In several families, morphologically However, almost all of the information regarding ecological

1622 CAN 1 ZOOL. VOL. 65. 1987

dj(c3 0

GROSWA TER BAY

OUTER GANNET ISL AND GANNET 64 N ISLANDS

-GRADY

SANDWICH, BAY CARTWRIGHT

FIG. I. Location of the Gannet Islands and Outer Gannet Island, Labrador. segregation and competition between the two species is based the islands, but are least numerous on GC5 and GC6 (D N. Nettleship, on qualitative observations (Kaftanovskii 1938; Sergeant 1951; unpublished data). Alcids comprise over 99% of all seabirds breeding Uspenski 1956; Belopol'skii 1957; Tuck 1961; Swartz 1966; at the Gannet Islands and Outer Gannet, and Common Murres (total, Williams 1974; see also Spring 1971). In contrast, both species 63 000 pairs) and Atlantic Puffins, Fratercula archer, (50 000 pairs), in allopatric parts of their ranges have been the subjects of are the most abundant species (Table 1). detailed quantitative studies (Common Murre: Birkhead 1977; During the 19th century alcid numbers along the entire Labrador coast were greatly reduced by human exploitation, and numbers of all Birkhead and Hudson 1977; Hedgren 1979, 1980, 1981; Harris species remained low until the 1950s, when they started to recover and Wanless 1985; Thick-billed Murre: Gaston and Nettleship (Bent 1919; Tuck 1961; Nettleship and Evans 1985). Numbers of most 1981; Birkhead and Nettleship 1981, 1982; Birkhead et al. species probably arc still increasing (D. N. Ncttleship. unpublished 1985; Gaston 1985; Gaston et al. 1985). data). In 1952, Tuck (1961) made the following estimates of the The aim of the present study was to compare simultaneously population sizes of Common Murres and.Thick-billed Murres at the several aspects of the breeding biology of the two murre species Gannet Islands and Outer Gannet: Common Murre. 2750 and 8900 breeding at a single location, the Gannet Islands, Labrador. This pairs, respectively (total 11 650 pairs); and Thick-billed Murre. 15 and colony lies near the northern limit for Common Murres in the 300 pairs, respectively (total 315 pairs). Subsequent censuses showed a western Atlantic (Nettleship and Evans 1985). The results are marked increase: in 1983, values were Common Murre, 38 345 and 24 742 pairs, respectively (total 63 087 pairs), and Thick-billed Murre, presented in three papers; this, the first one, provides a 964 and 441 pairs, respectively (total 1405 pairs) (Table 1). Although background to the study and presents details of the morpho- techniques used in the two censuses differed (Nettleship and Evans metrics of each species and the timing of the major events in 1985; and unpublished data), the change in numbers indicates a their reproductive cycles over 3 years. The second paper substantial increase in both species over the 30-year period. examines differences in habitat use and how this influences breeding success in the two species (Birkhead and Nettleship Study areas 1987a). The third paper is a comparison of the diet and food Data presented in this and subsequent papers were derived from observations made primarily on GC4, where we established two intake of the chicks of each species (Birkhcad and Nettleship Common Murre and three Thick-billed Murre study plots. The plots 1987b). were in a small cove on the west side of the island. an area selected because we could enter blinds overlooking the murre breeding areas Study area and methods without causing any (detectable) disturbance. Plot CM-A held about The Gannet Islands (53°56' N, 56°32' W) lie 50 km southeast of the 250 pairs breeding on a series of broad ledges near the foot of the cliffs. entrance to Groswater Bay, Labrador (Fig. I ). The archipelago CM-B held about 70 pairs on narrow cliff ledges about 10-4(1 m above comprises six islands (which we referred to as the Gannet Clusters. sea level; this plot was one of the few areas where Common Murres following Austin (1932), and numbered GCI to GC6). These islands bred on cliff habitat and in close proximity to Thick-billed Murres (see range in area from 4.4 ha (GC4) to 125 ha (GC6). Islands GC! to GC4 Fig. I in Birkhead and Nettleship 1987a). The three Thick-billed all lie within 500 m of one another; GC5 lies 500 m to the southwest of Murre plots were all located between 5 and 40 m above sea level on GC4, and GC6 (also known as Western Gannet) lies 1.5 km to the west cliffs. Plot TBM-A held 60 pairs breeding on relatively long ledges of GCI. Seven kilometres to the north of GCI lies Outer Gannet occupied almost entirely by conspecifics. On plot TBM-B about 80 (54°00' N, 56°32' W). The Gannet Islands and Outer Gannet are low pairs of Thick-billed Murres bred on narrow cliff ledges among lying (with a maximum height of 66 m above sea level), composed of Common Murres (plot CM-B). and plot TBM-C was occupied almost granite, and vegetated by dwarf heath scrub. Seabirds breed on all of entirely by Thick-billed Murres (about 100 pairs) on numerous small BIRKHEAD AND NETTLESHIP: I 1623

TABLE I. Population estimates of seabirds breeding at the Gannet Islands and Outer Gannet Island, Labrador 1983; numbers are breeding pairs (D. N. Nettleship, unpublished data)

Gannet Outer Islands Gannet Total

Northern Fulmar, Fulmarus glacialis 13 3 16 Leach's Storm-Petrel, Oceanodrotna leucorhoa" 14 14+ Great Black-backed Gull, Lars marinu? 100 10. I10 , Alca torda 4 928 472 5 400 Common Mum, Uria aalge 38 345 24 742 63 087 Thick-billed Murre, Uria lomvia 964 441 1405 Atlantic Puffin. Fratercula arctica 41 311 8 394 49 705 Black , grylle 35 5 40 Black-legged , Rissa tridactyla 52 61 113

"Rough estimates only.

FIG. 2. Sea-ice conditions during early June in the vicinity of the Gannet Islands, Labrador, 1981 to 1983. Breakup was more protracted in 1982. Hatched area, 1/10 to 9/10 ice cover; stipple, open water. Redrawn from Atmospheric Environment Service (Environment Canada) maps. ledges. Plots TBM-B and TBM-C were directly above the sea, but the made on breeding birds (identified by the presence of a brood patch). cliff where TBM-A was located was separated from the sea by 50 m of Before egg laying we assumed that all birds caught were breeders, as rock, part of which was occupied by Common Murres. behavioural observations at study plots indicated that this was true. In Observations were made in 1981 (18 June to 6 September). 1982 (24 all cases samples of a particular species were obtained on a single day: May until mid-September), and 1983 (25 May until all murres had left and samples for the two species were usually obtained on consecutive in mid-September). days. We attempted to obtain samples from prelaying (before any eggs had been laid). incubation, and chick-rearing periods (between median Spring. ice conditions and summer climate dates; sec below), but this was not always possible because birds were Between late December and May the waters around the Gannet difficult to catch at certain times (particularly during the prclaying Islands are ice covered. Ice breakup in spring usually occurs in May, period). but the timing varies between years (U.S. Naval Oceanographic Office Timing of breeding was determined from observations made for 6 to 1968). During the present study, breakup commenced in mid-May in 14 h each day, using standardized techniques described in detail by 1981 and 1983, but not until early June in 1982 (Fig. 2). Sea Birkhead and Nettleship (1980). For each site where an egg was laid, temperatures in the area reach a mean maximum of 9°C (in August; we recorded the timing of egg laying, hatching, the chick's departure U.S. Naval Oceanographic Office 1968). Between 1981 and 1983 air from the colony, and breeding success (the proportion of pairs rearing a temperatures during the murres' breeding season (early June to early chick that left the colony). Only data accurate to 24 h (see Gaston and September) rarely fell below freezing, and on a few occasions rose to Nettleship 1981) were used to calculate incubation, egg-replacement. 30°C. Mean daily temperatures during the warmest part of the year and chick-rearing periods. These data were calculated by subtracting (July to early August) averaged about 10°C each year. The spring of the first date (e.g., laying) from the second (e.g., hatching). In 1981 1982 (late May to early June) was about 4°C cooler than in either 1981 observations commenced on 18 June. by which time egg laying in both or 1983, and was associated with delayed ice breakup. Mean daily wind murre species had started. We estimated laying dates in that year from speeds were similar each year, with no obvious seasonal pattern, and hatching dates using mean incubation periods of 33 days Ifor both annual means varied between 9 and 14 km/h. Fog occurred on average species) determined in 1982 and 1983 (see Results). I day in 6 in 1981, I day in 4 in 1982, and I day in 20 in 1983, but it Counts of all individuals of each species at the study plots were made never prevented us from making observations at rnurre study plots. daily at 12:00 throughout the breeding season. Precipitation occurred as rain or fog, rarely as snow, and averaged Eggs were measured on GC3 (Common Murre) and GC' (Thi y k- 2.1 cm per calendar month. Precipitation in July 1981 (4.8 cm) was billed Murre) 12 days after the first eggs of each species w.re laid. lg. over twice that recorded in any other month of the study. avoid measuring any replacement eggs, which arc smaller iliirkhead and Nettleship 1984a). Length and breadth were measured with vernier Study methods calipers (to the nearest 0.1 mm) to provide indices of volume (length x For morphometric information birds were caught alive, measured. ) and shape ((breadth x 100)/length). Frt sh egg weights weighed, banded, and released. Common Murres were caught on GC3 breadth = were estimated from linear measurements using regression equations in and Thick-billed Murres on GCI. No birds were collected, so the sex Birkhead and Nettleship ( I984a). All statistical tests are two-tailed. ratios of our samples are unknown. We recorded body weight (to the nearest 5 g), wing length (flattened chord, to the nearest mm), and bill Results dimensions (using vernier calipers, to the nearest 0.1 mm): culmcn length, depth and width at the gonys, and gape width at the corner of the Morphometrics mouth. All measurements during incubation and chick rearing were Thick-billed Murres had longer wings, and deeper, broader

1624 CAN. 1. ZOOL. VOL. 65. 1987

TABLE 2. Wing and bill dimensions (in mm) of Common Murres and Thick-billed Murres at the Gannet Islands

Common Murre Thick-billed Murre

SD SD

Wing length 212.3 4.72 64 217.5 4.80 60 Culmen length 42.2 2.22 185 35.0 2.16 130 Culmen depth 13.2 0.53 184 14.3 0.66 129 Culmen width 6.4 0.52 59 7.1 0.50 40 Gape width 21.6 1.11 30 22.8 1.05 49

NOTE: All differences between species are significant tr-tests. P < 0.0011

TA6I.E 3. Mean body weights (g; ± SD) of Common Murres and Thick-billed Murres, within and between years at the Gannet Islands

Prelaying Incubation Chick rearing

.i SD N i SD N SD N Common Murre 1981 980.3 60.9 30 908.7 64.0 24 1982 972.8 73.7 34 993.0 61.4 31 944.3 56.3 30 1983 967.6 59.5 44 974.7 56.1 31 960.4 50.1 30 Thick-billed Murre 1981 - 942.7 50.9 30 899.2 56.6 30 1982 - 971.4 60.2 30 903.0 47.0 20 1983 - 954.7 58.5 20 - -

NOTt: All (IllfercnQes het, een stages of the season within species are significant !ANON: or r- tests). except for Common Murre in 1983. 'No data.

bills with a wider gape than Common Murres (Table 2). Body colony in early to mid August. Between the median laying date weights are presented in Table 3. For measurements taken at the and the first chick departure (see Table 4), Common Murre same stage of the breeding cycle in the same year, Common numbers increased more rapidly than Thick-billed Murre Murres were always heavier (by 1 to 4%) than Thick-billed numbers (see Fig. 3). The differences in seasonal attendance Murres (but only two out of five possible comparisons showed patterns between the two murre species are likely due to the significant differences; t-tests, P < 0.05). varying presence of ( i) off-duty breeders and (ii)nonbreeders at There were no significant differences between years within the colony (see Gaston and Nettleship 1981). Since none of our species during the incubation period (ANOVAS) so we used the birds were individually marked, we could not distinguish 3-year means of these values to express values such as egg or between these two possibilities. chick weights as a percentage of adult body weight. These values were as follows: Common Murre, 982.7 g t 59.38 SD; Timing of breeding Thick-billed Murre, 956.5 g ± 57.1 SD (t 2.95, 170 di. P < In each year the median laying date of Common Murres 0.01). Within each species adult body weights differed between occurred significantly earlier than that of Thick-billed Murres different stages of the breeding cycle (Table 3), and in years for (Table 4). In 1981 the difference was 10 days (median test, P < which we obtained data, body weights were lower (by 4 to 8%) 0.001), in 1982, 2 days (P < 0.001), and in 1983.4 days ( P < during the chick-rearing period than during incubation. 0.001). For both species laying was significantly (P < 0.001) later in 1982 than in 1981 or 1983, presumably because of the Colony attendance late ice breakup in that year. The precise dates on which each species returned to the The temporal patterns of egg laying (Fig. 4), hatching. and Gannet Islands in the spring were determined only in 1982. In chick departure (see Fig. 5) also differed between the two :981 and 1983 birds of both species were present at the colony species. In each year the synchrony of egg laying was when observations began. In 1982 egg laying was late (see significantly greater in Common Murres than in Thick-billed below), and it seems likely that the birds' return was also Murres. The proportion of Common Murres laying in the first 12 delayed, as a result of late ice breakup. In that year observations days of the laying period was 0.75 in 1981. 0.94 in 1982. and commenced on 24 May, and on that date no birds had visited 0.89 in 1983. Corresponding values for Thick-billed Murres their breeding sites (as judged by the lack of droppings), were 0.52, 0.87, and 0.83 (all years, P < 0.05). Similarly. the although some birds were present on the water in the vicinity of number of days over which the middle 80% of females laid was the islands. Thick-billed Murres were first seen on their 6, 7, and 10 days for Common Murres and 22. 9, and 10 days for breeding sites on I June, and Common Mut-res. on 2 June. In Thick-billed Murres, in 1981, 1982, and 1983. respectively. In both species numbers increased from the start of egg laying until both species laying dates at the same individual breeding sites chicks and their accompanying adults first started to leave the (relative to the annual median date) were significantly less BtRKHEAD AND NETTLESHIP: I 1625

TABLE 4. Timing of the main events in the breeding cycles of Common and Thick-billed Murres at the Gannet Islands in 3 years

1981 1982 1983

Common Thick-billed Common Thick-billed Common Thick-billed Murre Murre Murre tvlurre Murre Murre

First egg (June) 6 13 20 20 10 12 Median laying (June) 15 25 24 26 15 19 First chick (July) 8 15 23 23 12 14 Median hatching (July) 16 26 26 28 18 22 First chick departure (Aug.) 2 7 14 12 4 6 Median chick departure (Aug.) 9 17 19 20 13 13

NOTE: Values are dates in the month indicated

THICK-BILLED NUR RE Murre eggs (t-tests, P < 0.001), but for both species the egg 1 0 0 constituted a similar proportion of adult body weight: Common 3 Murre, 11.5%; Thick-billed Murre, 11.2%. a 50 The shape indices (see Methods) of Common Murre and Thick-billed Murre eggs differed significantly: Common Murre, a: 60 61.7 (7.86 SD, N = 222), and Thick-billed Murre, 63.4 (2.71 a SD, N = 72) (t = 2.71, 292 df, P < 0.01), showing that o a 40 Common Murres laid relatively longer eggs than Thick-billed Murres. Belopol'skii (1957) found a similar difference between LI/ 20 murre species in Murmansk. In the present study there was a a considerable overlap in both size and shape, and a discriminant function analysis showed significant (P < 0.001) but incom-

-20 0 20 40 50 50 100 plete discrimination between species, with 65% of Thick-billed DAYS RELATIVE TO NED,AN Murre, and 70% of Common Murre eggs being correctly LAYING DATE classified (overall, 69%). This level of discrimination has little practical application for distinguishing the eggs of the two COMMON NuRRE species when they are mixed on ledges. I 0 0 a — 1981 Incubation periods a BO -• '962 Mean incubation periods for 1982 and 1983 combined did not

-L 1953 differ significantly between Common Murres (33.03 days ± c, 60 0,99 SD, N = 199) and Thick-billed Murres (32.99 days ± 1.55 0 SD, N = 114) (t = 0.25, 311 df, NS). Common Murre a 0 AO incubation periods did not differ significantly between years; z however, the Thick-billed Morre mean values did differ (P < z 20 0.05) by 0.66 days, but this was less than the degree of accuracy a (24 h) with which incubation periods were measured. b a

-20 0 20 40 60 a 0 Replacement periods The mean interval between loss of a first egg and the laying of Oars RELATIVE TO NE DIAN a replacement did not differ significantly between species when LA •- n NG DATE all years were combined: Common Murre (1981-1983), mean FIG. 3. Seasonal patterns of colony attendance in (a) Thick-billed 15.9 days (2.34 SD, N = 59); Thick-billed Murre (1982 and Murres and (b) Common Murres at the Gannet Islands. Values are (1 = 1.31. 90 df, 7-day running means as percentage of maximum numbers of birds, and 1983), mean 14.45 days (2.09 SD, N = 33) for each year are plotted relative to the median laying date (arrow) (see > 0.1). However, Common Murre mean values differed Table 4). significantly between year: (1981, 16.53 -_t.• 1.39, N = 19; 1982, 14.23 ± 1.79,N = 17; 1983, 16.61 ± 2.74,N = 23; F2 . 55, = 7.4, variable than between sites (Common Murre, F6.4 . 128 = 1.82, < 0.01), whereas those for Thick-billed Murres did not P < 0.01; Thick-billed Murre, F45.89 = 1.61, P < 0.01). (1982, 14.62 ± 2.50, N = 16; 1983, 14.29 1.69, N = 17; The differences between species in median chick-departure t = 0.45, 31 df, NS). In 1982 the difference between Common dates (Table 4) were smaller than between their respective Murres and Thick-billed Moires was not significant, but in 1983 laying dates as a result of the shorter chick-rearing period of the replacement period was significantly longer in Common Thick-billed Murres (see below). Murres (1 = 3.3, 38 df, P < 0.01). Egg si:e and shape Thick-rearing periods There was no significant interyear variation in the volume The mean chick-reari.„ period for 1981 to 1983 combined for indices of first eggs of either species (Table 5). In each year Common Murres was 23.9 days 2.17 SD (N = 394), and for Common Murre eggs were significantly larger than Thick-billed Thick-billed Murres it was 21.08 days ± 2.71 SD (N = 137) • ▪ ▪

CAN. 1. ZOOL. VOL. 65, 1987

1981 1982 1983

a s0 . 0 (N..,31) (M s 200) (N• 2/61 3 3 0

s/ • ?0 w 10 0 • 11'11'11'11'11

0 s0 0 IN 11) 0 1 0 IN= It, I a z 30 n 3 V • 20 I0

on en Jute Jun• JsIs Joe. 1, D A T C OAT( D • / FIG. 4. Temporal pattern of egg laying in Common Murres and Thick-billed Murres at the Gannet Islands from 1981 to 1983. Arrows indicate median laying dates.

19 81 1982 1983

1, 'TT^ 1 1" •1"•"•••"••••••• rill -1 91 - 1 • • 11' 41-111 r T y •-•-), • '11 • t • • • r

, • nn•• n :21 2 °

2', • .iT r1:1 If" r • f 11174{.11-P-47D-It••tr-rt- -" -"T Tr.-444114 41:1111-41 • FP' r r - r • q v" -11 ;1 11" "•••• /0 •• 111 lr 7• /t 1 / .s 70 70 • 11 l• .• 77 711 I. 1

000 0 0 t 4011

FIG. 5. Temporal pattern of chick departures in Common Murres and Thick-billed Murres from 1981 to 1983. in relation to wind speed (km/h).

TABLE 5. Volume indices and estimated fresh weights of eggs of = 11.0, 529 df, P < 0.001). For Common Murres. chick- Common Murres and Thick-billed Murres at the Gannet Islands rearing periods varied significantly between years (1981, 23.02 ± 1.61, N = 88; 1982, 23.72 ± 2.11, N = 175: 1983, Mean vol. Fresh wt. Wt. as `4 24.61 ± 2.39, N = 131: F2 . 391 = 15.5, P < 0.001), hut this index" SD N (g) adult wt. was not the case for Thick - billed Murres (1981, 21.72 3.44. N = 18: 1982, 21.02 ± 2.79, Common Murre N = 88; 1983, 20.37 ± 1.91, N = 31). In each year the chick-rearing periods of Common 1981 218.6 h 18.7 198 113.0J I i .5 Murres were longer than those of Thick-billed Murres, but the 1982 218.9 19.5 217 113.1 11.5 1983 215.3 20.0 250 111.5 11.3 differences were significant only in 1982 and 1983 (both P < 0.001). Thick-billed Murre In both species the age at which chicks left the colony was 1981 204.1' 18.3 72 106.9' 1 I .2 negatively correlated with their hatching date. In Common 1982 204.9 19.3 92 107.3 II 2 Murres the correlations were significant each year (1981, r = 1983 204.7 16.7 50 107.2 11.2 -0.616, 86 df; 1982, r = -0.394, 147 df; 1983• I = -0.380, 'Length x breadth'. 129 df; all P < 0.001), but for Thick-billed Murres only in 1981 'Differences between years: one-way A NOV A I 2.55. NS was the correlation significant (1981. r = - 0.710. 16 df, P < Difference between years: one-way ANOVA 2., = 0.04 NS.

'Fresh egg wt. = 0.44 vol. Index ♦ 16.85 if:lir: Ahead and Nettleship I 9840. 0.001; 1982, r = -0.159, 81 cif, NS; 1983. r = -0.143, 28 di, 'Fresh egg wt. 0.492 vol. index + 6.533 itlirkhead and NcItle,hiplk4(44.0. NS). •

BIRKHEAD AND NEMESHIP: I

ft A •

• • 1 O • AuguSt August

DATE DATE DATE Fm. 6. Temporal pattern of adult (II) and chick (0) departures at the end of the season for Common and Thick-billed Murres from 1981 to 1983. Values are cumulative percentages of maximum numbers of adults and chicks. Thick-billed Murres tended to remain at the colony for longer following chick departure than did Common Murres.

Departure of ;hicks and adults from the colony difference is unclear, but it may have been related to the higher In both species the departure of chicks from the colony was proportion of failed breeders among Thick-billed Murres; these more synchronous than egg laying. In each year the difference failed breeders remained at the colony longer than birds that between median departure dates of the two species was less than successfully reared a chick. In all years, over 99% of birds of between laying dates: for departure and laying, respectively, the each species had left the Gannet Islands colony by the Ist week differences were 8 and 10 days in 1981, 1 and 2 days in 1982, in September. and 0 and 4 days in 1983; see also Table 4. Synchrony at chick departure was caused by (i) the seasonal reduction in fledging age (above) and (ii) the effect of weather on departure: chicks Discussion left the colony only during calm conditions (Fig. 5). Differences in the morphology and breeding biology of In all years Common Murre chicks left the colony more Common and Thick-billed murres existed at the Gannet Island; synchronously than Thick-billed Murre chicks. The number of Common Murres were slightly heavier than Thick-billed days over which the mid 80% of Common Murre chicks Murres and had longer, narrower bills and shorter wings. departed was 4 in 1981, 6 in 1982, and 6 in 1983, while for Common Murres laid a few days earlier and with a higher degree Thick-billed Murre chicks, it was 12 in 1981, 15 in 1982, and 12 of synchrony than Thick-billed Murre in all 3 years. Common in 1983. Part of this difference between species was due to the Murres produced absolutely larger, relatively longer eggs than greater proportion of Thick-billed Murres producing replace- Thick-billed Murres, and had slightly longer replacement ment eggs, as a result of greater egg loss in this species (see intervals and chick-rearing periods than Thick-billed Murres. Birkhead and Nettleship 1987a), and therefore having a greater At the end of the season, adult Thick-billed Murres remained at spread of hatching dates. the colony after their chicks had departed for a longer time than We obtained departure weights for chicks of both species on Common Murres. Differences within species between years the same date on only a single occasion (12 August 1983). On also occurred: in 1982 laying was late and synchrony, greater, in this date over 50% of all chicks of both species left the study both species, than in 1981 or 1983. plots (and probably the colony as a whole). The mean weight of Despite the significant differences between species, none of Common Murre chicks was 246.3 g 23.91 SD (N = 50), and them (with the exception of morphological differences) can be for Thick-billed Murre chicks, 228.3 g ± 29.93 SD (N = 10); regarded as species specific, since the features examined varied, expressed as a percentage of adult weight these values were not either between years or between the Gannet Islands and colonic:, significantly different (Common Murre, 25.1%; Thick-billed elsewhere. For example, our data indicate longer chick-rearing Murre, 23.9%; t = 1.12, 58 df, NS). periods among Common Murres (24 days) than Thick-billed After the chick departed (with the male parent; see Harris and Murres (21 days) in all 3 years. However, at other colonies, Birkhead 1985), the female returned to the breeding site for where each species occurs allopatricaily, mean chick-rearing several days. Figure 6 shows the departure of adults in relation periods of 20 days for Common Murres and 24 days for to chicks. In all 3 years adult Thick-billed Murres remained Thick-billed Murres have been recorded (Hedgren and Linnman longer at the colony after chick departure than did Common 1979; Gaston et al. 1985). The differences that we observed Murres. By the time 95% of the chicks of each species had between species are likely, therefore, to be colony specific, and departed the mean number of adults present (expressed as the may be related to the two species breeding sympatrically. Here 3-year mean percentage of maximum numbers) was 13% in we discuss (i) interycar differences in timing of breeding within Common Murres (range between years, 10-22%) and 58% in species and (ii) interspecific differences in breeding biology Thick-billed Murres (range, 50-68%). The reason for this (see Birkhead and Nettleship I 987a). CAN. ). ZOOL. VOL. 65, 1987

Intetyear differences within species TABLE 6. Timing of Common Murre breeding at the Gannet Islands Both Common Murres and Thick-billed Murres laid rela- and mean air temperatures before egg laying in different years tively late in 1982, when ice breakup was late and spring temperatures were relatively low. The effect was more marked Mean air temp. in Common Murres whose median laying date was later by 9 Year Timing (°C)a Source days compared with 1981 and 1983. In Thick-billed Mun:es the 1952 Late —0.76 Tuck 1953, 1961 median laying date in 1982 was only 1 day later than in 1981, 1978 Late 1.53 A. R. Lock, unpublished data but 7 days later than in 1983. The date on which first eggs were 1979 Early 10.17 D. N. Nettleship, unpublished data laid by Thick-billed Murres was also relatively late in 1982: 1981 Early 6.15 This study 1981, 13 June; 1982, 20 June; and 1983, 12 June. The 1982 Late 0.83 This study difference between species in 1982 probably arose because ice 1983 Early 2.67 This study breakup occurred much later than when Common Murres would 1984 Late 5.72 D. N. Nettleship, unpublished data otherwise have laid, but closer to the time when Thick-billed °Temperatures are mean daily values for the last 16 days of Niv.y, taken from Cartwright Murres would have laid (see below). Several other studies have meteorological station (see Fig. I) and corrected for the Gannet Hands from data collected shown delayed onset of breeding associated with late ice at both locations in 1982 and 1983 (unpublished data). breakup and low air temperatures (Tuck 1961; Nettleship et a!. 1984; Springer et al. 1984). The precise way in which late ice Interspecific differences in breeding biology breakup influences timing of breeding is unknown, but there are There are several possible explanations for the difference in several possibilities. Extensive ice cover must reduce the area timing of breeding between the two murre species at the Gannet available for foraging, and may also influence the distribution Islands. One is that each species times its breeding so that the and abundance of food (Springer et al. 1984). However, low chick-rearing period coincides with the peak of food availability temperatures alone may influence food availability (Hedgren for each species, and that this peak occurs later for Thick-billed 1979), or may cause adults to direct more energy into Murres than for Common Murres. Although we know that chick maintaining body temperature than they would normally do. In diets differ significantly between the two species (see Birkhead other studies late ice breakup and late breeding have been and Nettleship 1987b), there is no evidence that the main prey of associated with other features that indicate reduced food each murre species shows a different temporal pattern of availability, such as a reduction in egg size (Bianki 1967; abundance or availability. The second possibility is that both Birkhead and Nettleship 1982; Nettleship et al. 1984). This was species attempt to breed as early as possible, but ,:re. constrained not the case in the present study because egg size in both species to different extents by food shortage during egg formation. The was very similar in all 3 years. The lack of a reduction in egg seasonal decline in breeding success (Birkhead and Nettleship size in 1982 may be explained by the following hypothesis. In 1987a) provides some evidence for this suggestion. If this is that year the sea ice disappeared very rapidly, around 6 June. If, true, the difference in timing of breeding may reflect differences before this, extensive sea ice had reduced the foraging area, but in food availability between species in the prelaying period. not the abundance or distribution of food, then the rapid There is some evidence for interspecific differences in feeding disappearance of the ice would have resulted in an immediate ecology before egg laying: in each year during the prelaying increase in food availability. It was striking that in both murre period the droppings of Thick-billed Murres at the colony were species, the first eggs (Table 4) appeared 14 days after the ice pink and composed of exoskeletons, whereas those disappeared, a period which agrees closely with the estimated of Common Murres were either yellow or white, presumably time to form an egg (as judged by replacement intervals; see also reflecting a diet. Indeed, Common Murres frequently Birkhead and del Nevo 1987). In addition, the sudden increase brought display fish to the colony at this time. During incubation in food availability, following the dispersal of the ice, would the droppings of both species tended to be white. Although these account for the highly synchronous pattern of egg laying observations suggest a difference in early season diets, it is still observed in that year. For Common Murres at least, egg laying not known whether this reflects a difference in food availability synchrony in 1982 was higher than recorded in any previous for the two species. If food is relatively scarce early in the study (see Birkhead 1980; Hedgren 1980). season, the numerical dominance of Common Murres would Our results suggest that the 1982 breeding season was result in a disadvantage for Thick-billed Murres and could atypical in its' lateness. However, the timing of ice breakup and explain their later onset of breeding. We have insufficient data of the Common Murre's breeding season is more variable on the to test this idea, but in the two papers tliat follow (Birkhead and Labrador coast than in many other areas. Observations made Nettleship 1987a, 1987a) we examine evidence that competi- during short visits to the Gannet Islands in years other than 1981 tion between Common Murres and Thick-billed Murres occurs, to 1983 provided a qualitative assessment of how timing of both for breeding sites and for food during chick rearing. breeding compared with the years 1981 and 1983. For example, in 1952, Tuck (1961) visited the Gannet Islands and recorded Acknowledgements the first Common Murre chicks hatching on 17 July, which This research was funded by the Canadian Wildlife Service suggests a relatively late breeding season (see Table 4). Overall, (CWS) and is associated with the program "Studies on northern from 7 years' data, there were three "late" seasons (which were seabirds," Seabird Research Unit, CWS, Environment Canada associated with low spring temperatures) and four "early" ones (Report No. 206). We are grateful to Drs. A. Burger, R. D. (Table 6); in other words, 1982 could not be regarded as Elliot, A. J. Erskine, A. J. Gaston, and M. P. Harris for helpful atypical. comments on the manuscript and also to the many individuals Late breeding in 1982 was not associated with a noticeable who assisted us during our murre studies at the Gannet islands, reduction in breeding success in either species (see Birkhead especially S. D. Johnson, R. McLagen, and E. Verspoor. and Nettleship 1987a), but there was evidence of a reduction in Thanks also to J. Pellatt for drawing the figures. 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