IBIS 127: 174-183 1985

Breeding success and the question of clutch size of Northwestern Crows Corvus caurinus

HOWARD RICHARDSON, NICOLAAS A. M. VERBEEK & ROBERT w. BUTLER'

Department of Biological Sciences, Simon Fraser University, Burnaby, British Columbia, Canada VSA IS6 and Canadian Wildlife Service, Box 340, Delta, British

Columbia, Canada V4K 3 Y3

Accepted 1 June 1984

The breeding biology of two island populations of the Northwestern Crow was studied in British Columbia over a period of five years. Both populations laid an average of four eggs per clutch, of which approximately 75% hatched. More young survived to fledge on Mandate than on Mitlenatch Island. The young from nests close (< 100 m) to the beach on Mitlenatch had a higher survival rate than those from further inland. The number of eggs lost or failing to hatch was lowest in clutches of four eggs. There was a non-significant trend for chick survival to be inversely related to clutch size. The net result was that clutches of five eggs produced no more young than did clutches of four eggs. It is suggested that the major limiting factor on clutch size is food availability but the decreased hatching success of any remaining eggs once hatching has started is also a contributing factor.

Many of the members of the genus Corvus are common, widely distributed and conspicuous, frequently benefiting from a close association with man. In Europe they have been extensively studied (Lockie 1955, Tenovuo 1963, Holyoak 1967, Wittenberg 1968, Yom-Tov 1974, Picozzi 1975, Loman 1977, 1980, Coombs 1978, Newton et al. 1983) but in North America, although there is some anecdotal information (Bent 1946, Goodwin 1976), there are surprisingly little published data, particularly of their reproductive biology (but see Emlen 1942, Verbeek & Butler 1981).

A number of problematic observations have consistently arisen in the European studies of Corvid reproduction. First, for many populations the largest clutches produce more young than the most common sized clutches, thus posing the question of why do not more birds lay large clutches (Lockie 1955, Owen 1959, Wittenberg 1968, Loman 1977). Secondly, the fledging success per egg laid is amongst the lowest for all passerines (Lack 1954). A low reproductive rate is not unexpected in such relatively long-lived species but why so many eggs are required to produce so few young is uncertain. Lastly, the highest mortality occurs in the first seven days after hatching (Zimmermann 1951, Lockie 1955, Koersveld 1958, Owen 1959, Holyoak 1967, Tenovuo 1963, Wittenberg 1968). Though starvation is suspected as the source of this mortality, it occurs at a time when the energy demand of the whole clutch is less than for a single young immediately prior to fledging (Yom-Tov 1974).

The purpose of this paper is to try to answer the questions posed above using the comparative data from our two study populations of the Northwestern Crow Corvus caurinus and from the European studies.

STUDY AREAS

MANDARTE ISLAND (48'38'N, 123"17'W)

Mandarte Island is about 600 m long, 100 m wide and 6.3 ha in area. The southwestern shore consists of 10-30 m high cliffs dropping directly into the sea.

00 19-1 0 19/85/020 1 74 + 10 $03 .OO/O 0 1985 The British Ornithologists' Union

1985 BREEDING SUCCESS OF NORTHWESTERN CROWS 175

These cliffs and the slopes immediately above offer nesting sites for about 550 pairs of Pelagic Cormorants Phalacrocorax pelagicus and 800 pairs of Double-crested Cormorants P . auritus (Verbeek 1982). The northeastern shore is less precipitous, with a series of rocky ledges as much as 15 m wide and exposed during low tides.

The crows nest mainly in a strip of trees and shrubs growing along the crest of the island for almost its entire length. Breeding territories tend to be arranged in a nearly linear fashion along the length of the island, with a single territory frequently spanning the island from the cormorant cliffs to the rocky inter-tidal zone. Thus most pairs have ready access to two major food sources-cormorant eggs (Verbeek 1982) and inter-tidal invertebrates (Zach 1978).

MITLENATCH ISLAND (49’57”, 125’00’W)

Mitlenatch Island (35.5 ha) is biotically and morphologically more complex than Mandarte (Brooke et al. 1983). The crows nest in small patches of shrubs, or on the fringes of larger clumps of trees and shrubs scattered over the island. Approximately 200 pairs of Pelagic Cormorants nest on the cliffs along the southern shore of the island (Campbell 1976). Two bays at the northeastern end of the island contain large inter-tidal areas. With their loose, fine-grained substrate, these bays are important foraging sites for the crows (Butler 1974, James & Verbeek 1983, Butler et al. 1984). Unlike the crows on Mandarte, with less than 50 m to travel to their major foraging areas, those nesting in the middle of Mitlenatch have to fly more than 250 m to reach either the cormorant cliffs or the inter-tidal beaches.

There are approximately 2000 pairs of Glaucous-winged Gulls Lams gzaucescens nesting on each of the islands (Campbell 1976). The crows feed on gull eggs, young chicks and regurgitated food whenever it is available.

METHODS

The data were collected on Mandarte and Mitlenatch Islands from 1976 to 1980 and 1977 to 1981, respectively. The islands were searched in late April and May to locate new or refurbished nests. Once found, these were visited at intervals of one to seven days. T o avoid unneccessary exposure of eggs and chicks, nests were not visited during wet and windy weather. Eggs in each clutch were numbered with India ink in the order of laying. The fate of each egg was recorded as either lost prior to hatching presumably to predators, failed to hatch or hatched. Since egg loss in the seven days prior to hatching was minimal, any egg disappearing after the first one in the clutch had hatched was assumed not to have hatched. Thus eggs that might have hatched but which were immediately eaten or ejected from the nest would be included here. Clutches with only one or two eggs (four out of 191 clutches) and those which failed to hatch at least one egg were omitted from the analyses. We suspected that these clutches resulted from our disturbance causing additional exposure to intra-specific nest predation or desertion.

Because of the small size of Mandarte and the nearly linear arrangement of the crows’ territories we almost certainly located all the nests present in each year. On Mitlenatch the number of nests located varied with the time spent searching. However, in no year was searching concentrated in one area and each year’s data contained similar proportions of nests from the different parts of the island.

Weather data for Victoria International Airport (5 km west of Mandarte) and Cortes Island (1 0 km north of Mitlenatch) were taken from the Monthly Record of Meterological Observations in Western Canada (Anon. 1976-83).

176 H. RICHARDSON ET AL. IBIS 127

RESULTS

DISTRIBUTION OF CLUTCH SIZES

For seven of the ten sets of observations (five years on each of two islands) the modal clutch size was four eggs. The exceptions to this were the unusually high incidence of three-egg clutches on both islands in 1977 and a modal clutch size of five eggs on Mandarte in 1979 (Table 1). The total data set from both islands consisted of 187 clutches, of which 101 (54%) were of four eggs.

Because of the small sample size in some years it was necessary to lump data to test for independence of clutch size distribution and years. T o avoid potential bias from arbitrary combinations, Chi-square tests of independence were performed on all possible combinations of two years data from one of the islands against the lumped data of the three remaining years. Since none of these combinations showed significant non-random variations the pooled data from each island were compared. Again there was no significant difference in the relative numbers of three, four and five-egg clutches layed on Mitlenatch and Mandarte ( x 2 = 1.65, df = 2, P > 0.5). Nor was there any significant difference in the distribution of clutch sizes from nests within 100 m and more than 100 m from the beach on Mitlenatch. The number of three, four and five-egg clutches in beach and inland territories was 5, 13, 8 and 21, 56, 17, respectively (x2 = 2.01, P > 0.3).

There was also no significant decrease in the average size of clutches during the breeding season. Laying started on Mitlenatch during the last week of April (Butler et al. 1984). The mean size (fs.e.) of 19, 47, 43 and 11 clutches laid, respectively, in four consecutive ten-day periods starting on 20 April was 4.3 + O . 18,4.0 *0.11,4.0 - +0.11 and 3.9f0.12. Although clutch size tended to decrease through the season, late clutches laid in the last ten-day period were not significantly smaller than those laid in the first period ( t = 1.55, 0.1 > P > 0.05).

FATE OF EGGS

Because of the small sample sizes on Mandarte, the number of eggs lost and failing to hatch was combined for data analyses. On both islands there was a high, but not statistically significant level of variability between years (Table 2). The greatest source of variability on Mitlenatch was the number of eggs lost prior to hatching, particularly the high loss in 1977 and 1979 and the low loss in 1980. On Mandarte the greatest anomaly was the very high egg failure in 1978.

TABLE 1

The number of Northwestern Crow clutches of 3 , 4 and 5 eggs on Mitlenatch and Mandarte Islands during the period of the

study

Clutch size

Mitlenatch Mandarte

3 4 5 Total 3 4 5 Total ~~

1976 2 7 3 12 1977 5 5 2 12 5 4 3 12 1978 2 8 3 13 3 8 3 14 1979 7 16 8 31 4 5 6 15 1980 8 25 8 41 4 8 2 14 1981 4 15 4 23

Total 26 69 25 120 18 32 17 67

1985 BREEDING SUCCESS O F NORTHWESTERN CROWS 177

TABLE 2

The fa te of Northwestern Crow eggs for each year of the study on Mitlenatch and Mandarte Islands

Mitlenatch' Eggs lost % Eggs not hatched % Eggs hatched 70

Mandarte2 Eggs lost/not hatched % Eggs hatched %

1977 7

15.6

1978 1979 1980 1981 5 18 8 7 9-4 14.4 4.9 7.6

11 11 18 34 15 24.4 20.8 , 14.4 20.7 16.3. 27 37 89 122 70 60.0 69.8 71.2 744 76.1

1976 1977 1978 1979 1980 8 8 20 I5 8

16.3 17-4 35.7 24.2 14.8 41 38 36 47 46 83.7 82-6 64.3 75.8 85.2

Total 45 9.4

89 18.6

345 72.0

Total 59 22.1

208 77.9

Notes: ' x 2 = 13.03, 0.5 > P > 0.1. 2 2 x = 9.32, 0.1 > P > 0.05.

The lumped data from the two islands showed a large, but not significant relationship between the fate of eggs and the island (x2 = 5.53, df = 2, 0.1 > P > 0.05). There was also no significant difference in the fate of eggs from nests on Mitlenatch close to the beach (within 100 m) and those further inland (Table 3).

FATE O F YOUNG

The proportion of young surviving from hatching to fledging on Mitlenatch ranged from 37% to 67% (Table 4). For most years of the study, survival was close to the lower value of 37% but the abnormally high success in 1977 resulted in a significant difference between years (x2 = 9-64, df = 4, P < 0.05).

The proportion of chicks surviving to fledge on Mandarte ranged from 37% in 1977 to 58% in 1978. Once again 1977 was an anomalous year, except on this island it was a year of abnormally low survival. Despite this there is no significant difference in chick survival between years (xz = 4-42, df = 4, 0.5 > P > 0.1).

A significantly higher proportion of young on Mandarte (51 o/o) survived to fledge

TABLE 3

Fate of Northwestern Crow eggs and young lost (taken by predators or starved) from territories located within 100 m (beach territories) and more than 100 m (inland territories) from the beach on Mitlenatch Island (% of total shown in parentheses)

Beach Inland Total

Eggs' Eggs lost 7 (6.5) 38 (9.5) 45 (9.4)

66 (17.9) 89 (18.6) Eggs failed 23 (21.5) Eggs hatched 77 (72.0) 268 (72.6) 345 (72.0)

Young' Young lost 32 (41.6) 175 (65.3) 207 (60.0) Young fledged 45 (58.4) 93 (34.7) 138 (40.0)

~~

Notes: ' x 2 = 1.85, P > 0.5. 'xZ = 14.05, P < 0401

178 H. RICHARDSON ET AL. IBIS 127

TABLE 4

The number of Northwestern Crow young that were lost (taken by predators or starved) or survived to fledge in each year of the study on both islands

Mitlenatch’ 1977 1978 1979 1980 1981 Total Young lost 9 21 56 77 44 207 % 33.3 56.8 62.9 63.1 62.9 60.0 Young fledged 18 16 33 45 26 138 % 66.7 43.2 37.1 36.9 37.1 40.0

Mandarte’ 1976 1977 1978 1979 1980 Total Young lost 19 24 15 23 20 101 Yo 46.3 63.2 41.7 48.9 43.5 48.6 Young fledged 22 14 21 24 26 107 % 53.7 36.8 58.3 51.1 56.5 51.4

Notes: x 2 = 964, P < 0.05. x = 4.42,0.5 > P > 0.1. 2 2

than on Mitlenatch (40%). This is so, even when the data for 1977, which is counter to the general trend, are included (1’ = 6-84, df = 1, 0.01 > P > 0.005).

A comparison of the different areas of Mitlenatch showed that beach nests fledged significantly more young than inland nests (Table 3).

FATE OF EGGS BY CLUTCH SIZE

In the absence of any significant differences between either islands or years, all the data were pooled to investigate relationships between clutch size and the probability of eggs hatching. There were significant differences in the fate of eggs from clutches of three, four and five eggs (Table 5). Most of this variability was due to the high proportion of eggs lost from three-egg clutches and the large number of eggs which failed to hatch in five-egg clutches.

FATE OF YOUNG BY CLUTCH SIZE

Since there were differences in nestling survival between the islands, the two sets of data were analysed separately. On both islands there was a tendency for chick mortality to be higher in larger clutches, but this trend was not significant (Table 6). However, because of this trend and the high hatching success of clutches of four eggs (Table 5), the number of young fledged from clutches of five eggs is not greater than that from clutches of four eggs (Mann-Whitney U-test, P > 0.3 for both islands). Clutches of three, four and five eggs on Mitlenatch fledged an average of 0.98, 1.22

TABLE 5

The number of Northwestern Crow eggs lost, failing to hatch or hatching from clutches of 3, 4 and 5 eggs (yo of total shown in

parentheses)

Clutch size

3 4 5 Total

Eggs lost 17 (12.9) 28 (6.9) 13 (6.2) 58 (7.8) Eggs not hatched 21 (15.9) 58 (14.4) 56 (26.7) 135 (18.1) Eggs hatched 94 (71.2) 318 (78.7) 141 (67-1) 553 (74.1)

Note: x 2 = 20.1, P < 0.005.

1985 BREEDING SUCCESS OF NORTHWESTERN CROWS

TABLE 6 The number of Northwestein Crow young lost ( to predators or starved) orfledgedfrom clutches of 3,4 and 5 eggs on Mitlenatch and Mandarte

Islands (yo of total shown in parentheses)

Clutch size

3 4 5 Total

179

~ ~

Mitlenatch' Young lost 29 (54.7) 130 (60.7) 48 (61.6) 207 (60.0) Young Hedged 24 (45.3) 84 (39.3) 30 (38.4) 138 (40.0)

Mandarte2 Young lost 16 (39.0) 51 (49.0) 34 (54.0) 101 (48.6) Young Hedged 25 (61-0) 53 (51.0) 29 (46.0) 107 (51.4)

Notes: ' x 2 = 0.74, P > 0.5. 2 x 2 = 2.23,0.5 > P > 0.1.

and 1.20 young, respectively. On Mandarte they fledged an average of 1.39,1.66 and 1.71, respectively.

FATE OF YOUNG BY CLUTCH SIZE A T HATCHING The combined data from the two islands showed a significant relationship between

the number of eggs which hatched in a clutch and the probability of the hatchlings surviving. The survival rate of young from small clutches was higher than for large clutches (Table 7). Similarly, clutches in which all the eggs hatched fledged a smaller proportion of chicks than those in which one or more eggs were lost or failed to hatch (Table 8).

MSCUSSION

The breeding biology of the Northwestern Crow shows some basic similarities to that of other species of the genus Corvus but differs significantly in a number of ways. The average clutch size of 4-0 eggs falls within the range of 3.9 to 4.5 eggs reported for most species (Table 9) but the range of clutch sizes is less than in these other species. The largest recorded clutch size, five eggs (Bent 1946, Gabrielson & Jewett 1959, this study), is only one egg larger than the modal size. All other Corvus species in North America and Europe occasionally lay clutches of seven or more eggs (Table 9).

TABLE 7

The number of Northwestern Crow young lost ( to predators or starved) and fledged from clutches hatching different number of eggs

Number of eggs hatched

1 2 3 4 5 Total

Young lost 7 31 I19 I27 24 308 % 37 42 56 60 69 56

Young Hedged 12 43 94 85 1 1 245 % 63 58 44 40 31 44

Total 19 74 213 212 35 553

Note: x2 = 12.3, 0.05 > P > 0.01.

180 H. RICHARDSON ET AL.

TABLE 8

The number of Northwestern Crow young lost and fledged in clutches which were reduced by predation and infertility

by none, one or more than one egg prior to hatching

Number of eggs not hatching

0 1 > I Total

IBIS 127

~

Young lost I 49 114 45 308 % 62 53 48 56

Young fledged 93 103 49 245 % 38 47 52 44

Total 242 217 94 553

Note; x 2 = 6.63.0.05 > P > 0.01.

Lack (1 954) pointed out the adaptive value of clutch size being related to territorial quality, food availability and hence to the physical condition of females prior to laying. Among corvids a positive relationship has been observed between clutch size and winter food supply (Yorn-Tov 1974, Newton et al. 1983), nesting habitat type (Wittenberg 1968, Loman 1977) and territorial quality (Yom-Tov 1974). In contrast, the Northwestern Crow shows a remarkable consistency in clutch size over both time and space. The crows of Mitlenatch and Mandarte leave the islands during the winter and probably forage in loose-knit flocks around the urban centres of nearby Vancouver Island. There may be less variability in the winter condition in these versatile foragers than in species with more restricted foraging patterns. Of course, social rank within a winter roost (Swingland 1977) or a foraging flock (Rohwer & Ewald 1981) can undoubtedly have deleterious affects on the nutritional status of subordinate individuals but the most seriously affected should be the immature birds. Alternatively, differences in physical condition may be expressed through earlier clutch initiation, rather than a larger clutch (Loman 1977), with a fitness advantage gained by the increased overwinter survival of early chicks.

TABLE 9

The average clutch size and range of clutch sizes layed by species of the genus Corvus

n Mean Range Location Source

C. caurinus C . brachyrhynchos

C. corone C. corone C. corone C . corone C. corone C. corax C . corax C . monedula C. mondedula C . monedula C . frugilegus C. frugilegus C . cryptoleucus

189 4.0 1-5 4-4 2-6

3-9 168 3.9 1-7 180 4.4 2-7 28 4.1 74 4.5 49 4.1 2 4 67 5.2 3-7

245 4.5 431 4-3 2-7

80 4.2 1 4 233 4.4 14 182 4.2 1-7 151 4.3 2-7

3-8

Canada N. America N. America Britain Sweden Scotland W. Germany Scotland Britain Wales Britain Switzerland Britain Britain En g I a n d N. America

This study Emlen 1942 Bent 1946 Holyoak 1967 Loman 1977 Yom-Tov 1974 Wittenberg 1966 Picozzi 1975 Holyoak 1967 Newton et al. 1983 Holyoak 1967 Zimrnermann 1951 Lockie 1955 Holyoak 1967 Lockie 1955 Bent 1946

1985 BREEDING SUCCESS OF NORTHWESTERN CROWS 181

The approximately 75% hatching success observed in this study is similar to the success reported for other corvids and passerines in general (Emlen 1942, Zimmermann 1951, Lack 1954, Lockie 1955, Tenovuo 1963, Wittenberg 1968, Loman 1977). Only Yom-Tov (1974) reports a radically different hatching success. Over 27% of the eggs laid by his Carrion Crows disappeared, presumably taken by members of the large flock of non-breeding birds which foraged in the nesting area.

In the Northwestern Crow the sources of egg failure differ with clutch size. A higher proportion of eggs are lost to predation from clutches of three eggs than from those with four or five eggs. Incubation usually does not start until the second or third egg is laid, up to which time the female is frequently absent from the nest (Butler et al. 1984) while the last eggs in a clutch are attended almost constantly from laying. The loss of an early, unattended egg represents a greater proportional loss from a clutch of three eggs than from a clutch of five. Clutches of five eggs have the highest proportion of eggs failing to hatch, perhaps due to inefficient incubation of, and the physical abuse suffered by, the unhatched eggs when the nestlings are fed. Clutch size is apparently constrained by the conflicting needs of protecting the first laid eggs from predation and limiting hatching asynchrony to avoid damaging unhatched eggs.

Once hatched, chick mortality follows a typical corvid pattern. On Mitlenatch, 51 yo of the chicks died within seven days of hatching, apparently having starved (see also Zimmermann 1951, Lockie 1955, Koersveld 1958, Owen 1959, Tenovuo 1963, Wittenberg 1968) but it is unlikely that food availability is the only cause of chick mortality: too many young die at a time when the parents should be able to cope with their energy demands. This premature mortality seems inconsistent with a brood- reduction startegy (Ricklefs 1965, O'Connor 1978) which will only be advantageous if future food shortages can be accurately predicted, or if maintenance of excess hatchlings is detrimental to the eventual survivors. It is also hard to envision the behavioural mechanisms that would produce this pattern of mortality. Wittenberg (1968) suggests that males may be insufficiently stimulated by the small, silent young to provide them with food. The strong selective pressure for efficient feeding, and an apparent lack of any constraints thereon, makes this an unlikely explanation. I t is more likely that some physiological or anatomical constraints may produce a developmental crisis such as in the transition from yolk to parentally provided food as the source of nourishment.

The differences in chick mortality between beach and inland nests on Mitlenatch (Table 3) and between our two study populations (Table 6) give some indication of the possible major sources of that mortality. Since the crows on Mandarte, the more southerly of the islands by 120 km, start to lay about ten days earlier than those on Mitlenatch (Butler 1980, Butler et al. 1984) a climatic influence on chick mortality was suspected. However, the monthly mean temperature near Mitlenatch is consistently higher than near Mandarte, and although the former is wetter there is no apparent relationship between rainfall and chick mortality. On Mitlenatch, the rainfall in June and July 1978 was only 40% and 9% of the norm, respectively. In 1979 precipitation was near average in these months, in 1980 it was 150% of normal in May, June and July while 1981 was one of the wettest summers on record. Despite this, the proportion of young surviving to fledge ranged between only 37% and 43 yo. Similarly on Mandarte, dry summers of 1977 and 1979 and the wet ones of 1980 and 1981 fledged 37%, 51%, 57% and 51%, respectively, of all young hatched. Consequently, climate was assumed not to be the source of the difference in chick mortality between the islands.

On Mitlenatch most chicks were weighed at regular intervals. This additional disruption of nests may be another source of egg and chick loss. However, egg loss is

182 H. RICHARDSON ET AL. IBIS 127

the same on both islands and it is suggested that disturbance should have the greatest effect during the egg stage and in the denser population on Mandarte. Yom-Tov (1974) stated that nestling crows were affected little by nest visits.

The final and most probable source of high chick mortality is the limited availability of food. Lockie (1955), Owen (1959), Tenovuo (1963) and Wittenberg (1968) also found starvation to be the major source of chick mortality. Most of the territories on Mandarte are adjacent to both the rocky inter-tidal and the cormorant colony. The extent to which the crows prey on cormorant eggs has been documented (Verbeek 1982). Relatively few gull eggs were eaten on either island (Verbeek 1982, pers. obs.), perhaps because the gulls are more mobile and aggressive than the cormorants in the defence of their eggs. In the absence of a large and conveniently located cormorant colony on Mitlenatch the crows must forage elsewhere. The most predictable, alternative food source is inter-tidal. Most of the perimeter of Mitlenatch has either a very limited, rocky inter-tidal shore or none at all. Only the mostly sandy beaches in the two bays provide plentiful inter-tidal food. The importance of these beaches is evident because a chick reared close to them has a greater probability of surviving from hatching to fledging than one reared in an inland nest (see Table 3). The observation that the loss of eggs from a clutch prior to hatching increases the probability that the remaining chicks will survive to fledge also supports the contention that chick survival is strongly influenced by the parents’ ability to bring enough food to the nest.

In some other corvids (Tenovuo 1963, Holyoak 1967) and in contrast to Lack’s principle, the most productive clutch is greater than the modal size. In the Northwestern Crow clutches of 5 eggs produced no more young than modal sized clutches of 4 eggs. This is so even if post-fledging, weight related differences in mortality are considered, as we have found the calculated asymptotic weight (Ricklefs 1967) of fledglings to be remarkably constant. Deviations from Lack’s predicted coincidence of modal and most productive clutch sizes are numerous and have been variously ascribed to differences in female age, experience and condition, territory quality and the need to reduce effects detrimental to future reproduction (see Klomp 1970 for review). Among European corvids there are frequently a small number of individuals able to lay more eggs and rear more young than the majority of the population (Table 9 and refs. therein). This presumably indicates significant variation in reproductive condition, commitment or food availability in these populations. The coincidence of modal and most productive clutch sizes, and the constancy and limited range of clutch sizes in the Northwestern Crow are in strong contrast to the European studies. This could be a reflection of a more conservative, long-term approach to the yearly allocation of reproductive effort by the Northwestern Crow. However, our observations of individually banded birds suggest little demographic difference between these and other populations. We have argued earlier that breeding success is influenced by food availability, yet there appears to be an upper limit to food availability or to the conversion of resources to progeny. It seems most likely that temporal limitations, by the tide, of access to a major food source and the reduced environmental diversity typical of islands (MacArthur & Wilson 1967) serve to limit and reduce the variability of foraging gains.

We wish to thank L. Graf, P. James, J. Kirbyson, L. Legendre, J . Morgan, C. Ritchie, J. Smith, A. Stewart and P. Toleikis for help in gathering the data. W. and A. LeChasseur provided indispensable logistical support. The Tsawaout and Tseylurn Indian Bands of Saanich, British Columbia and the Provincial Parks Branch, Victoria, British Columbia gave permission for the research to be carried out on Mandarte and Mitlenatch, respectively. The Natural Sciences and Engineering Research Council

1985 BREEDING SUCCESS O F NORTHWESTERN CROWS 183

provided funds through a research grant to NAMV and a post-graduate scholarship to HR. We also received support from the University Research Fund of the Canadian Wildlife Service and a President’s Research Grant from Simon Fraser University.

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