Differential Predation on Eggs in Clutches of Northwestern Crows: the Importance of Egg Color’,2

The Condor92:695-701 0 The CooperOrnithological Society 1990

DIFFERENTIAL PREDATION ON EGGS IN CLUTCHES OF NORTHWESTERN CROWS: THE IMPORTANCE OF EGG COLOR’,2

NICOLAASA. M. VERBEEK Department of Biological Sciences,Simon Fraser University,Burnaby, British Columbia V5A lS6, Canada

Abstract. In the majority of clutches of Northwestern Crows (Corvuscaurinus) one of the eggsis frequently slightly to much paler than the others. I addressthe question why this may be so and I test the hypothesis that this egg, once a predator is at the nest, draws attention to itself and is thus subject to selective predation by conspecifics.In three-, four- and five-egg clutches,last-laid eggswere significantlysmaller than all the other eggsin each clutch and second-laid eggswere generally the largest. In 74% of the clutches in which a pale eggwas present this was the last-laid egg, while the first-laid egg was the palest one in 18% of the clutches.The larger the clutch, the greater the chance that one of the eggswas a pale one. In completed clutches,the first egg to be taken by a predator was significantly more often (a) the first- or last-laid egg in the clutch than a central egg and (b) the palest egg.The mean volume of depredatedeggs did not differ significantlyfrom the others so size per se was not the reason why they were taken. Last-laid eggsare particularly expendable as they have a significantlyreduced hatching successdue to asynchronoushatching. Key words: NorthwesternCrow; Corvidae;egg color; eggsize; dcflerentialegg predation; hatchingsuccess.

INTRODUCTION nest, and is therefore subject to selective pre- Several experimental studies have shown that dation compared to the other eggsin the clutch. egg color provides camouflage and enhances an METHODS egg’s chance of escapingpredation (Tinbergen et al. 1962, Croze 1971, Montevecchi 1976b, Ber- All eggswere numbered with India ink in the tram and Burger 198 1). Other studies(Dice 1947, sequencein which they were laid. If I found a Mueller 1968, Ohguchi 1981) have shown that nest for the first time and it contained two or “the odd one out” is subject to selective preda- more eggsthen these were given the same num- tion. ber. For instance, if there were three eggs,all were Northwestern Crows (Corvus caurinus) lay a inscribed with the number 3. Subsequenteggs in clutch of three to five eggswith a mean of 4.0 -t that clutch were numbered in the order in which 0.7 eggs (Butler et al. 1984). The background they were laid, i.e., 4 and 5 in the case of a five- color of the eggsranges from light blue to dark eggclutch. Once the clutch was completed all the green with blotches of brown to black. In some eggsfor which the laying sequence was known clutches all eggsare of the same color but in the were measured with vernier calipers to one dec- majority of clutches one of the eggsis slightly to imal place. Egg volume was determined using very much lighter than the others.Holyoak (1970) the formula: volume = length x width2 x 0.5 1 reported the same phenomenon in the Carrion (Hoyt 1979). Eggswere laid in a row, on a brown Crow (C. corone) and a number of other species cloth that resembled the color of the nest lining, of Corvus. with the first-laid eggon the left and the last-laid In this paper I test the hypothesisthat the light- eggon the right and a color photograph was taken colored egg,because it is the odd eggin the clutch, of the clutch. The slides were later projected and draws attention to itself once a predator is at the the eggswithin each clutch scored for color relative to each other. I tried to measure and photograph on the same occasion to reduce disturbance but for various reasons this often was 1Received 30 November 1989.Final acceptance20 April 1990. impossible. When a female crow is flushed from 2This paper is dedicated to Frank A. Pitelka on the the nest she will leave silently and settle in a occasionof his retirement. nearby tree top from where she monitors the

WI 696 NICOLAAS A. M. VERBEEK

TABLE 1. Volume (X f SD) of eggsin completed clutchesof three, four, or five eggs(number of each clutch size in parentheses)of Northwestern Crows on Mitlenatch Island, in which the volume (cm’) and the sequence of each egg in the clutch was known. Statistical comparisonswere done using standardized data (see text).

Clutch size

(n L, (n ,428)

1st egg 17.08 + 1.52 15.77 + 1.23 15.99 + 1.66 2nd egg 17.12 & 1.37 16.22 + 0.82 16.52 ?Z1.43 3rd egg 16.14 ? 1.40 15.95 * 1.11 16.28 * 1.44 4th egg 15.28 +- 1.01 15.96 +Z 1.52 5th egg 15.25 + 1.37 K 16.78 f 1.44 15.88 t- 1.23 16.00 + 1.55

*Significance levels of paired comparisons (Tukey test) within clutches as follows: three-egg clutches: 1 vs. 2, ns; 1 vs. 3, P < 0.05; 2 “s. 3, P < 0.01; four-egg clutches: 1 vs. 2, P < 0.02; 1 vs. 3, ns; 1 vs. 4, P < 0.01, 2 vs. 3, P < 0.01; 2 vs. 4, P .z 0.001; 3 vs. 4, P < 0.001; live-egg clutches: 1“ S.2. P < 0.02: I vs.3. ns: I vs.4. ns: I vs. 5. P < 0.01: 2 vs. 3. ns: 2 “S. 4, P < 0.01; 2 vs. 5, P < 0.001; 3 vs. 4, P < 0.01; 3 vs. 5, P-c 0.001; 4 vs. 5; P < 0.00’1. situation. If you stay near the nest too long she order (for three-egg clutches F = 10.50, df = 2, will return to the area and make alarm calls, 51, P < 0.001; four-egg clutches F = 14.42, df which may attract her mate and other crows. = 3, 108, P < 0.001; five-egg clutchesF= 12.55, Furthermore, eggswere photographed in bright df = 4, 90, P < 0.001). Using adjusted values, light, which often meant that I had to take them the mean egg volume of first and second eggsin to the forest edge. In some cases the distance three-egg clutches was significantly (Table 1) involved was such that I ran the risk of giving larger than the mean volume of third eggs(Tukey the crow the impression that I had left the area. test). Similarly, secondeggs differed significantly The crow finding the nest empty on her return from first, third, and fourth eggs in four-egg would likely have abandoned it. This problem clutches and from first, fourth, and fifth eggsin could only be avoided by having an assistantstay five-egg clutches. In three-, four-, and five-egg near the nest while I photographed, but an as- clutches last-laid eggswere significantly smaller sistant was not always available. Thus, in some than all others in their respectiveclutch size (Ta- cases I was forced to measure and photograph ble 1). Clutches of three, four, and five eggsdif- on separatedays, which meant that in some cases fered significantly in mean eggvolume (ANOVA predation occurred before the eggs were mea- F = 4.17, df = 2, 62, P < 0.025). Mean egg sured or photographed,whichever occurred first. volume in three-egg clutches was significantly Sample sizes for the various components of this larger than in four-egg clutches (Tukey test, q = study are thus not the same. Once the eggswere 4.01, df = 2, 62, P < 0.01) and five-egg clutches measuredand photographed,the nest was visited (q = 2.94, df = 2,62, P < 0.05), but mean volume as little as possible (every 3.98 + 1.42 days) to in four- and five-egg clutches did not differ sig- keep disturbance to a minimum, until all eggs nificantly. had hatched or had had ample time to do so. Relative to the mean volume of each clutch, The incubation period is 18 days (Butler et al. secondeggs were largest in five-egg clutches and 1984). The data were gathered on Mitlenatch smallest in three-eggclutches (Fig. 1). First eggs Island, British Columbia, during 1983-1986. in four- and five-egg clutches and fourth eggsin five-eggclutches were as heavy as the mean weight RESULTS of all eggs in their respective clutch size. The EGG VOLUME heaviest eggswere the first and second in three- The mean intraclutch eggvolume differed in the egg clutches and the second and third in four- order of laying in each clutch size (Table 1). As and five-egg clutches (Fig. 1). interclutch variation obscuredthe real difference between successiveeggs within each clutch I did EGG COLOR an ANOVA on adjusted values, obtained by sub- In those clutches (n = 82) for which the color of tracting the intraclutch mean volume from the the eggsin the laying sequencewas known, the volume of eachegg within the clutch. Eggvolume last-laid eggwas the palest in 53 clutches(64.6%), (Table 1) was significantly influenced by laying followed by the first egg (n = 13, 15.9%), while PREDATION ON NORTHWESTERN CROW EGGS 697

TABLE 2. The positionof the palestegg within each of 82 clutchesof three,four, or five eggsof the North- westernCrow on MitlenatchIsland. +3 . No. of clutches in which all eggs No. of clutches in which the palest had the egg was number Clutch No. of same size clutches color 1 2 3 4 5 3 23 5 4 2 12 4 36 4 5 3 1 23 5 23 1 4 0 0 0 8

The degreeto which the light-colored egg differed from the other eggsin each clutch was vari- able. Clutches in which all the eggsin the clutch were of the same color or in which one egg was only slightly paler than the others, predominated in nests with three eggs.The largest percentage of very pale to extremely pale eggswere found in five-egg clutches, while the proportion of me- dium-colored eggsremained about the same in each clutch size (Fig. 2). PREDATION The potential predatorson Mitlenatch Island were coast garter snakes (Thamnophis elegans), river I otters (Lutra canadensis),Glaucous-winged Gulls 1 2 3 5 (Larus glaucescens),and Northwestern Crows NUMBER OF EGG IN &Cti themselves.As Mandarte Island, where this study FIGURE 1. Intraclutch variation in egg size of was begun, lacked snakes, yet egg predation oc- NorthwesternCrows in clutchesof three(n = 18),four (n = 28), and five eggs(n = 19). curred there, I think it highly unlikely that garter snakestook crow eggson Mitlenatch Island. Ad- ditionally, the crowsare quite capable of catching very few central eggs(all other eggsin a clutch and killing garter snakes(Butler 1979, Jamesand except the first and the last) (n = 6, 7.3%) were Verbeek 1984, unpubl. data). Otters may have the palest eggin the clutch (Table 2). In 10 clutch- taken the eggs from some ground nests but as es (12.2%) the eggswithin each clutch were of they would almost certainly have taken the whole the same color (Table 2). The highest proportion clutch they are of no concern in this study (see of these occurred in three-egg clutches (2 1.7%) below). Gulls would certainly eat crow eggs if and the lowest in five-egg clutches (4.3%) with they came across a nest, and they would likely four-egg clutches lying in between (11.1%). The take all the eggs,but as the crowsnested in clumps relative frequency of pale eggsamong first, cen- of trees and in dense shrubbery the gulls had no tral, and last eggs(Table 2) differs significantly accessto the nests. This leaves the crows them- (x2 = 167.8, df = 2, P < 0.001) from expected. selves. I have seen crows in the act of stealing Excluding clutcheswithin which all eggswere of crow eggsand on many occasions I have seen the same color (Table 2), significantly more first empty crow eggshellsat sitesto which crows take eggswere the palest eggin the clutch than central eggsto deal with them at leisure (Verbeek 1982). eggs(x2 = 11.5, df = 1, P < O.OOS),while last- In this study I was interested in a subset of laid eggswere significantly more often the palest depredated clutches (n = 22) that were known than first-laid (x2 = 40.6, df = 1, P < 0.001) and to be complete, had not been depredated prior central eggs(x2 = 111.2, df = 1, P < 0.00 1, Table to completion and from which only a single egg 2). had disappeared upon my return visit. Whether 698 NICOLAAS A. M. VERBEEK

60- TABLE 3. Positionof thedepredated egg in thelaying sequenceof eggsfirst taken by NorthwesternCrows from 22 completedclutches of three,four, or five eggs.

Position of egg in the laying sequence Clutch No. of size clutches 1 2 3 4 5 3 6 2 1 3 4 10 4 0 5 6 3 : 0 ; 1

clutches were of the same color, therefore the predators could not have chosen on the basis of color, and in two clutches the depredated eggs were lost before I photographed them, hence I had no record of their color. Of the remaining 17 clutches, 12 (70.6% of 17 eggs)lost their palest egg (four first, seven last, and one central egg) and five (9.6% of 52 eggs)had eggsother than the palest taken (Table 4), which is significant CLUTCH SIZE (x2 = 25.6, df = 1, P -c 0.001). Of the 12 nests FIGURE 2. The proportionof clutchesin which(1) that lost their palest egg, eight had been scored all the eggswere of the samecolor or in whichone egg for the degree of paleness (for the other four I wasonly slightlypaler than the others(black bar), (2) one of the eggswas much to very muchpaler than the knew that the palest egg in each nest had disap- others(open bar), or (3) the differencein colorof the peared but I did not known how pale they were palestegg in the clutchwas intermediate compared to relative to the other eggs in their respective the othereggs in the clutch(stippled bar), in clutches clutch). Of the eight pale eggs,three were scored of three(n = 23), four (n = 36), and five eggs(n = 23). very pale and three extremely pale. To test whether eggsize was important, I compared the mean volume (15.83 + 1.24 cm3) of or not subsequent to my return visit to the nest the single depredated eggin each clutch (n = 10, additional eggsdisappeared from some of these because for the other 12 of the 22 single eggs 22 clutches(and some did), was of no interest to taken I did not have the volume) with the mean this study. From this subset I ask the question, volume (16.11 L 1.36 cm3) of the nondepredated given a complete clutch, which egg does a crow eggs (n = 28) in those same 10 clutches. The take first? As eggswere marked in the sequence comparison between means was done on stan- in which they were laid, results were tabulated dardized data as explained earlier. Although dep- (Table 3) to determine whether the missing eggs redated eggswere on average smaller than non- were taken from clutches at random or by selec- depredatedeggs, the differencebetween the means tive predation, thereby indicating that some eggs was not significant (ANOVA, F = 0.62, df = 1, were more vulnerable to predation than others. 37, P > 0.05). The relative proportion of depredatedeggs among first, central, and last eggs,in all clutches com- HATCHING bined, departs significantly from expectation (x2 Eggs that failed to hatch (infertile or the chick = 13.5, df = 2, P < 0.005). Significantly (x2 = did not manage to get out of the egg) remained 9.5, df = 1, P < 0.005) more first eggs(nine out in the nest for several days before they disap- of 22) than central eggs (four out of 44) were peared. Hatching failure was investigated in all taken by the predators and significantly (x2 = completed clutches(regardless ofwhether I knew 9.5, df = 1, P < 0.005) more last eggs(nine out their volume or not) that were not preyed on. In of 22) than central eggs. these clutches, the proportion of eggs that did Among the 22 completed clutches that lost a not hatch, 5 out of 48 (10.4 1%) eggsin three-egg single egg(Table 3) the eggswithin each of three clutches (n = 16) 14 out of 180 (7.77%) in four- PREDATION ON NORTHWESTERN CROW EGGS 699 eggclutches(n=45),and 16 out of 110(14.55%) TABLE 4. The number of completed Northwestern in five-egg clutches (n = 22), did not differ sig- Crow clutchesof three, four, or five eggs,in which the nificantly (x2 = 3.33, df = 2, P > 0.05). In three laying sequenceand the color of the eggsin each clutch was known, as well as the color of the one eggin each of 30 clutches in which eggsfailed to hatch I did clutch that was taken by a predator. not know the position of the unhatched egg in the clutch. In the remaining 27 clutches, 32 eggs No. of palesteggs No. of darkereggs failed to hatch (one secondand two last-laid eggs Clutch No. of Total Avail- Avail- failed to hatch in three three-egg clutches, three size clutches em able Taken able Taken first, one second, three third, and six last-laid 3 4 12 4 0 eggsin 12 four-egg clutches and three first, two 4 8 32 t 7 2: 1 second, two third, four fourth, and five last-laid 5 5 25 5 1 20 4 eggs in 12 five-egg clutches). Hatching success Total 17 69 17 12 52 5 was significantly influenced by the position of the egg in the laying sequence (x2 = 7.52, df = 2, P < 0.025). Significantly (x2 = 6.97, df = 1, advantage. There may be an additional disad- P < 0.005) more last-laid eggs (13 out of 27) vantage due to small size, namely differential failed to hatch than central eggs(13 out of 63) predation. and first-laid eggs(6 out of 27, x2 = 3.98, df = Egg loss can be quite high in Corvus: 27% in 1, P < 0.05). First-laid eggsand central eggsdid C. corone (Yom-Tov 1974) and 9.4% in the not differ significantly (x2 = 0.02, df = 1, P > Northwestern Crow (Richardson et al. 1985). In- 0.05) in the number of eggsthat failed to hatch. traspecificpredation is the main reason for this. To investigate the possibility that eggsize (vol- Wittenberg (1966) stated that most of the egg ume) might have influenced hatching successI loss in the Carrion Crow was caused by non- compared the mean volume of hatched (n = 170) breeding conspecifics.The same applies to the and unhatched eggs(n = 27) in all unpredated roving groupsof yearlings, as well as neighboring clutches in which the volume of all eggs was adults, in this study. Intraspecific predation is known. The means did not differ significantly. A thus common and it is made easier in North- similar comparison between the 85 heaviest western Crows because nests are close together hatched eggsand the 14 heaviest unhatched eggs (Butler et al. 1984) and the birds are well ac- and between the 85 lightest hatched eggsand the quainted with the location of each other’s nests 13 lightest unhatched eggsalso showed no sig- (pers. observ.). Yom-Tov (1974) too found that nificant difference. predation was higher in those areas where the intemest distance was small. Although female DISCUSSION crows sit tight when incubating, the average in- In a number of corvids (Slagsvold et al. 1984), attentive period of 5.6 + 3.3 min (n = 65, Butler including Northwestern Crows (Table I), the last- et al. 1984) seems more than long enough to laid egg in the clutch is on average noticeably allow another crow to take an egg. and statistically smaller than the others. The The possibility remains that female crows re- corvids share this phenomenon with larids (Ver- moved their own eggsbut this seems unlikely. beek 1982, 1988; Slagsvold et al. 1984). In ad- Yom-Tov (1976) showedthat Carrion Crows did dition, incubation starts before clutch comple- not remove the eggs of other Carrion Crows, tion. In three-eggclutches, incubation startswhen Rooks (C. jirugilegus),and domestic hens (Gallus on average 1.82 + 0.57 eggshave been laid and domesticus)when these were placed in their nest in four- and five-egg clutches incubation starts during incubation, starting 3 days after the crows after 2.43 f 0.77 and 2.50 + 0.50 eggs have had completedtheir own clutch. Yom-Tov (1976) been laid, respectively (Butler et al. 1984). Thus did find that when he introduced cracked eggs in all clutchesone or more young hatch later than they were removed. It may be suggestedthat the others and becausethey hatch from slightly paler eggsare not as sturdy as normal-colored smaller eggsthey may be substantially smaller eggs and therefore more likely to be damaged than earlier hatched young which may have been and thus removed by the female. When handling fed and growing for a day or more before later eggsand while writing numbers on them I did siblings hatch. Last eggsthus have a double dis- not find paler eggsmore easily damaged than the 700 NICOLAAS A. M. VERBEEK others. Unhatched eggsmight stay in the nest for of which frequently does not manage to hatch. up to a week before they were removed, presum- My results do not show that hatching successis ably by the parents (Bingal and Withers 1989). related to egg size (volume), as is the case in C. Some of these eggs were pipped but the chick coroneand Black-billed Magpies, Pica pica (Rof- had been unable to emerge. stad and Sandvik 1985). Hatching successin C. As Montevecchi (1976a) showed, C. brachy- caurinus thus appears to be dependent on the rhynchostends selectivelyto take small eggswhen position of the egg in the clutch and not on the presented experimentally with a choice between size of the egg. I am suggesting,as I did earlier large and small ones. Northwestern Crows prey for Glaucous-winged Gulls, Lams glaucescens heavily on the eggsof cormorants (Phalacroco- (Verbeek 1988), that theseeggs stand out because rax sp.) and are often seenflying with intact whole they are very pale. In both gulls and crows, in- cormorant eggsin their bills (Verbeek 1982). As traspecific predation is common. cormorant eggs are much larger than North- Clutches of three eggscontained proportion- western Crow eggs,the crows should be able to ally fewer pale eggsthan those of four- and five- carry off the entire range of available crow egg egg clutches (Fig. 2). Arguably, this may be an sizes. Although egg size per se thus does not ap- indication that females that lay large clutchesrun pear to be an important determinant of predation out of pigment. However, as 20% of first eggs in this study (mean volume of depredated eggs were the palest in the clutch this may indicate was not significantly different from that of non- that pigment production at times runs behind depredated eggs),this does not negate the pos- schedule or that it takes time for the pigment sibility that size is nevertheless important, par- glands to become functional. If females are run- ticularly as speed may be a factor during the act ning out of pigment, they are able to recover of predation. A stronger case can be made for quickly because eggsin 10 repeat clutches (laid the importance of color. 13.6 +- 1.2 days after the loss of the first clutch Presumably birds’ eggs are colored to cam- [Butler et al. 19841) are similar in color to those ouflage them in the nest, particularly in open in the original clutches(Holyoak 1970, pers. ob- ground nests. Tinbergen et al. (1962) and Mon- serv.). The notion that females run out of pig- tevecchi (1976a) provided evidence to support ment is not supported by an experiment I did this. Corvus caurinus lines its nest with strips of with Glaucous-winged Gulls, in which third eggs cedar bark (Thuja plicata) and this, along with were usually paler than the first two eggsin the a generally well-hidden nest, provides for a dark, clutch. If the first three eggswere removed in the shady background against which light-colored order of laying, and the gulls continued to lay, eggstend to stand out. In C. caurinus these pale then the last egg (sixth) was generally the palest eggsare typically the last laid in the clutch, and (Verbeek, unpubl.). Another possibility is that occasionallythe first-laid eggs(Table 2) precisely the processesresponsible for pigmentation are those eggsthat suffer the most predation (Table physiologically controlled. Both Paludan (195 1) 4, see also Verbeek 1988). It is not the size of and Parsons (1972, 1976) suggestedthat the lay- the depredated egg that is important in making ing and partial incubation of the first two eggsin it more vulnerable but apparently the color, par- three-eggclutches of Herring Gulls (L. argenta- ticularly the oddity of the egg’s color relative to tus) inhibits the processesof eggproduction thus the others in the clutch. resulting in the smaller third egg. A similar pro- AS crow eggsappear to suffer frequent intra- cessmay inhibit the functioning of the pigment specificpredation, I wondered which eggsin the glands resulting in reduced pigmentation (Ver- clutch were of the least value to the crows and beek 1988). Regardlessof the mechanism, run- therefore most expendable? This study and that ning out of pigment or physiological control, and of Richardson et al. (1985) shows that last-laid I do not envisage that the birds have the ability eggsin completed clutches have a significantly to deliberately produce a pale egg,most last eggs greater chance of not hatching when compared and some first eggsare paler than the others. This to first-laid eggsand central eggs.One reason for study clearly showsthat the position of some eggs this is that incubation begins before the clutch is in the clutch and their concomitant palenessre- completed and asynchronous hatching often results in reduced hatching successand selective sults in the neglect of the last-laid egg,the young predation. PREDATION ON NORTHWESTERN CROW EGGS 701

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