NOTES ON THE FOOD HABITS AND FOOD DEFENSE OF THE ACORN

MICHAEL H. MACROBERTS

Hastings Reservation University of Jamesburg Route Carmel Valley, California 93924

The Acorn Woodpecker ( formi- specifics and heterospecifics; and 3) it com- C~VOW), the most social of North American pares some aspects of the food habits of the , is a common resident of mixed Lewis ’ ( Asyndesmus lewis), Acorn, and Red- woodlands from Oregon south through headed (Melanerpes eythrocephulus) Wood- California, east through , New peckers. Since the food habits of the Acorn and Texas, and south to Panama. These wood- Woodpecker have been the subject of several peckers live in year-round groups. Each group previous papers (see literature cited by Beal consists of 2-10 of both sexes and ap- 1911, Ritter 1938, and Bent 1939), a repetition parently all ages (Ritter 1938). In most of these earlier works is avoided. groups males are reported to outnumber females, but this apparent discrepancy in sex STUDY AREAS ratio may be explained by the fact that ju- The study entailed 40 hr of observation be- veniles possessthe adult male plumage during tween 20 October and 15 December 1966 of at least part of their first year (Ritter 1938). two Acorn Woodpecker groups 2.5 mi. NW of Territory and food defense, nest hole con- Orinda, California, near the southern end of struction, and the care and feeding of nestlings San Pablo Reservoir, and about 500 hr of appear to be accomplished jointly by all mem- observation from 11 April to 15 September bers of the group. Aside from these rather 1968 of 11 groups at the Frances Simes sketchy points, little else is known about the Hastings Natural History Reservation, Mon- social organization of these woodpeckers. terey County, California. The groups studied In its food habits, the Acorn Woodpecker is at Orinda lived in mixed oak-pine woodland, one of the most atypical of all woodpeckers. while those studied at Hastings lived in foot- During the fall and winter the birds feed hill and canyon-bottom mixed oak woodland. extensively on acorns and other mast which they store in prepared holes in the surfaces FOODS OF THE ACORN WOODPECKER of and pines. This aspect of their ecology has been repeatedly described (see literature STORED ACORNS cited in Ritter 1938 and in Bent 1939), but The present study adds little to previous because of a predominance of interest in this descriptions of acorn storage, except to de- unusual habit, other aspects of the wood- valuate the relative importance placed by peckers ’ food habits have been largely ne- other investigators on it as a food source for glected by most naturalists. This is unfortu- the Acorn Woodpecker. nate since it has led to the assumption that About half of the 11 groups studied at the food habits of these birds are far less Hastings had exhausted their stored acorns diversified than is actually the case. In addi- before observations began there in April. This tion to storing and consuming dried acorns, was due largely to acorn failure in all but the the woodpeckers feed extensively on mature black oak () the previous and immature green acorns before storage year. Nevertheless, each group, irrespective begins during the fall, and are proficient fly- of the state of its acorn supply, maintained catchers and sapsuckers. its own territory and subsisted on other foods The purpose of this paper is threefold: 1) throughout the summer. Those groups which it describes some previously unreported or in- had acorn stores did not use these to any large completely described aspects of the food extent but, like those groups without stores, habits of the Acorn Woodpecker; 2) it de- subsisted on other foods. scribes the defense of these foods from con- It is apparent therefore that during the summer the woodpeckers can subsist on and 1Present address: Department of Zoology, Parks Road, Oxford, England. prefer foods other than stored dried acorns, The Condor, 72,:1Q6-204, 1970 11961 NOTES ON THE FOOD HABITS OF THE ACORN WOODPECKER 197 and that, even if stores remain from the previous winter, these may not be extensively used.

IMMATURE AND MATURE GREEN ACORNS Ritter ( 1938:29,84) mentioned that the Acorn Woodpecker uses immature acorns as food, but the present study suggests that their im- portance in the birds ’ diet is greater than has previously been supposed. In the groups observed at Hastings, immature acorns were eaten (especially those of the live oak, Quercus agrifolia, the black oak, Q. kelloggii, FIGURE 1. Acorn storage branch (center, with the valley oak, Q. lobata, and the blue oak, centimeter scale) and sapsucking branches (right and Q. d0ugZa.G) beginning in late July and con- left). Note differences in diameter of acorn storage tinuing through early September, without any and sap holes. The sapsucking branches used for this storing being involved. The immature nuts photograph were dead and desiccated and con- sequently nonfunctional at the time they were removed were picked, carried to “anvils,” split open, from the tree. and the meat immediately consumed. In the second week in September, the wood- peckers at Hastings began storing mature The sap holes were more or less uniformly green acorns. During this time they con- spaced. The distance between holes was ap- tinued to feed extensively on acorns, but in- parently independent of branch diameter, with stead of immediately eating all they picked, a mean nearest-neighbor measurement (mm) they stored some. These immature acorns of 24.7 (SD = 5.2, N = 131) and 21.6 (SD = 5.4, (and later, mature green acorns) were the N = 107) in two independent samples. This major food source of all the groups studied is approximately the same distance as between at Hastings throughout August and early acorn storage holes drilled in dead smooth sur- September, regardless of whether or not a faced trees (e.g., sycamores or valley oaks). particular groups’ acorn stores from the pre- The mean nearest-neighbor measurement vious year had been exhausted. (mm) between storage holes was 24.2 (SD = 4.3, N=73) and 20.5 (~~=3.6, N=llO) SAPSUCKING in two independent samples. The mean of the A third major food source is sap. The six entire set was 22.7 (SD = 5.0, N = 421). Vari- most studied Hastings groups had one or two ance analysis of the four samples shows that sap trees which they defended from other neither sap nor storage holes can be dis- birds. (It is not known whether the other tinguished by separation distance (F = 2.8, five groups maintained sap trees. ) In each P < 0.05). This suggests that a unitary be- case, these were live oaks (Q. agrifolia) lo- havioral mechanism may be involved in de- cated within 100 m of the groups’ main storage ciding the spacing pattern of storage holes tree. Live oaks were never used for acorn as well as sap holes. Figure 1 shows a sap- storage. sucking and an acorn storage area for com- The woodpeckers drilled small holes 5-15 parison. mm in diameter and 3-19 mm deep on the Since there are two possible ways that sap upper surfaces of middle and upper canopy and storage holes can be excavated to produce branches of live oaks. The branches in one a uniform spacing pattern, I shall briefly de- tree into which sap holes were drilled were scribe the process used by the woodpeckers. 2.5-15 cm in diameter. The diameter and On a certain section of limb, a will begin depth of the sap holes are related to the di- excavating a hole. After this hole is begun, ameter of the limb in which they are ex- but before it is finished, this bird or another cavated. Large limbs support wider and one will begin excavating a second hole at deeper holes than do small limbs. This is due the “correct” distance from the first. Other to the depth of the bark since the sap holes holes are excavated in a similar manner, each always penetrate through the bark but not into adjacent to, but at the “correct” distance from, the wood. In one particular live oak, about its nearest neighbor. Since hole construction 1800 sap holes were counted, although only a is a relatively slow process, many birds will part of the tree was used by the birds for this have participated in the excavation of a purpose. particular hole before its completion. Con- 198 MICHAEL H. MACROBERTS

sequently, since the birds are continually Woodpeckers engage almost exclusively in making new sap and storage holes, there are “specific flights” or catch only one insect per numerous holes in various states of prepared- flight. In addition, the Acorn Woodpecker ness. never forages for insects directly on the The woodpeckers repeatedly visited and ground or in low brush as is common in the probed into these sap-filled holes. I would Lewis ’ Woodpecker. estimate that during the height of the sap- I feel, however, that previous researchers sucking period (June and July) each hole have underestimated the importance of fly- was visited, on an average, 4-10 times per catching as a foraging method of the Acorn hr, but since different holes were differentially Woodpecker. At Hastings flycatching was a used, it is difficult to make a more precise common activity throughout the study, espe- estimate. In the process of excavating these cially during the nestling period when the sap holes the woodpeckers may eat some of newly hatched birds were fed on insects the excavated bark, but whether or not this caught in this manner. Although I did not in fact occurs is not known. attempt to quantify the time spent flycatching Whether or not sap is a year-round food or the amount of food obtained by it, fly- source for these birds is unknown. Further catching was undoubtedly the major foraging research on an annual basis will be necessary method of these birds throughout April and to establish its seasonality. I did not become May, only to be replaced in importance by aware of sapsucking until the early part of sapsucking in June and July. June, but from the number of sap-producing During the nestling period, the woodpeckers holes in the sap trees at that time, this food temporarily store insects. After each sortie, source had been of major importance for a the bird would return to its hawking perch long time before I became aware of it. and pound the captured insect into a crevice. Fisher and Peterson (1964:35) mention This was repeated several times before it car- that the Acorn Woodpecker sucks sap from ried all of the insects to the nestlings. Bock sapsucker holes, but in my study area they (1968:97) has reported that the Lewis ’ Wood- drilled their own sap holes and used these pecker likewise stores insects. The explana- exclusively. Since the red-headed race of tion he proposes is that “such behavior would Sphyrapicus uarius is migratory in the Hastings increase the ability of Lewis ’ woodpeckers to area, none was observed during the present capitalize upon a temporarily superabundant study, and consequently it is not known food source by shortening the time between whether the Acorn Woodpecker robs sap from each capture” when feeding the young. This these birds during the winter months when explanation certainly seems plausible for the sapsuckers are present. Acorn Woodpecker as well. As immature acorns became an important food source in August and September, the OTHER FOOD SOURCES birds ceased frequenting their sap trees. By According to Ritter ( 1938:31) the Acorn the end of the study the woodpeckers seldom Woodpecker has all but abandoned “the entered sap trees and ceased protecting them traditional woodpecker way of getting nour- from other birds. This indicates that sap- ishment from the insect world. . .” (i.e., by sucking is a seasonal activity, but just when boring and gleaning for insects and insect it begins is in doubt. larvae). I am in complete agreement with Ritter on this point as I never definitely ob- FLYCATCHING served birds gleaning and never observed an The behavioral aspects of flycatching by the Acorn Woodpecker drilling for any length of Acorn Woodpecker have been adequately time at anything other than sap holes, acorn described by Bent (1939) and by Ritter storage holes, nest holes, and at various objects ( 1938). The behavior is almost identical such as acorns which were split open before to the flycatching behavior of the Lewis ’ being eaten. In addition, Beals’ (1911) Woodpecker described by Bock (1968, in analysis of stomach contents indicates that press) and to that of the Red-headed Wood- wood-boring insects and larvae are almost pecker (Bent 1939) except that the Acorn and entirely absent from the birds ’ diet. Red-headed Woodpeckers seldom execute Ritter (1938:28) stated that “on some oc- what Bock ( N&3:81 ) has called “non-specific casions I have seen the birds picking at some- flights” (i.e., remaining in the air for several thing in the oaks, when the spring buds are minutes while taking several insects before well advanced, in such a way that they ap- landing). Instead, Acorn and Red-headed peared to be feeding on the buds, but of this NOTES ON THE FOOD HABITS OF THE ACORN WOODPECKER 199

TABLE 1. Rate of supplanting of heterospecifics by Acorn Woodpeckers in Group As’ storage trees at Orinda, California, during 16.5 hr of observation.

No. passesto No. woodpeckers Interactions supplant intruder involved Interactions accompanied Smcies No. Rate/hr mean Ki”J?% mean range by “kanit-cut”

Fox squirrel 123 7.5 7.3 l-73 3.2 l-5 113 (Sciurms niger)

White-breasted Nuthatch 39 2.4 1.1 l-3 1.0 I (Sit& carolinensis)

Stellers’ Jay 36 2.2 1.7 l-5 1.3 14 (Cyanocitta stelleri)

Plain Titmouse 9 .5 1.3 14 1.0 ( Parus inornutus)

Red-shafted Flicker 9 .5 1.1 l-2 1.1 l-2 (Colaptes cafer)

Scrub Jay 3 .2 1.0 1.0 ( Aphelocoma coerulescens)

Rufous-sided Towhee 2 .l 1.0 1.0 (Pipilo erythrophthalmus)

Nuttalls’ Woodpecker 1 .06 1.0 1.0 ( Dendrocoposnut&&i)

Starling 1 .06 1.0 1.0 1 (Sturnus vulgaris)

Ruby-crowned Kinglet 1 .06 1.0 1.0 ( Regulus calendula )

Unidentified small birds 21 1.3 1.2 l-2 1.05 l-2

Total 245 14.88 115

I have never been quite certain.” I observed area of defense in this latter case is much the same of behavior at Hastings during larger and less localized than that defended the latter part of April and early May, but, from heterospecifics. like Ritter, I was never able to discern just what it was that the woodpeckers were getting HETEROSPECIFIC DEFENSE from among the oak blossoms. Most of the discussion which follows is based According to Ritter (1936) and Beal (1911) on observations made of the Orinda A group other items eaten by the Acorn Woodpecker in- in the fall of 1966. clude various fruits and the eggs of other birds. Acorn Woodpeckers supplant heterospecific Although I never saw a woodpecker eat any of birds and small mammals that enter their these items, I did see one eat a lizard (prob- storage trees. Supplanting usually consists of ably Sceloporms occidentalis). Whether the one or more woodpeckers making one or more lizard was alive or dead when it came into aerial passes at the intruding until the the woodpeckers’ possession is unknown. The intruder leaves, at which time it may or may bird carried it to an “anvil,” pounded on it, not be pursued. In this type of interspecific and consumed at least part of it. interaction, physical contact is seldom made between the woodpecker and the intruder. DEFENSE OF FOOD SUPPLY Supplanting may be accompanied by a vocal- The members of each group supplant hetero- ization (“karrit-cut”) which apparently func- specifics from their acorn storage and sap- tions to alert and attract the attention of other sucking trees, the acorn-bearing oaks from group members to the intruders’ presence. which they gather acorns, their roosts and Table 1 summarizes data on supplantings by nest holes, and occasionally from anvils and members of Group A (5 birds) of hetero- hawking perches. In addition, the members specifics that entered their storage trees during of each group supplant any conspecific of a 16.5 hr of observation on six days in November different group that enters their territory. The and December 1966. At Orinda, all hetero- 200 MICHAEL H. MACROBERTS

specifics which entered the storage trees were oreganos), and Ruby-crowned Kinglet (Regu- supplanted (one exception; see below). Con- IUS caEen&u). The following entered the sequently table 1 not only gives the frequency storage tree and were supplanted: Nuttalls’ of supplantings but also indicates the fre- Woodpecker ( Dendrocopos nuttallii), Hairy quency with which heterospecifics entered the Woodpecker ( Dendrocop vdlosus), Plain storage trees. Titmouse (Parus horn&us), and White- The woodpeckers were always successful in breasted Nuthatch (Sitia carolinensis) (twice). quickly supplanting trespassing birds. This The second set of observations (24.5 hr ), usually required a minimum of effort, a single conducted also between 36 August and 5 Sep- woodpecker making a single aerial pass at the tember, were of a group which had not ex- intruder. Fox squirrels (Sciurms niger, an in- hausted its acorn supplies. Two heterospe- troduced species), on the other hand, were cifics entered this groups’ storage tree: a often persistent in their efforts to steal nuts Sparrow Hawk ( Falco sparverius), and a Red- and would remain under continual attack tailed Hawk (Buteo iamaicends). The wood- while frantically attempting to extract an peckers attempted to supplant the Sparrow acorn, On one occasion five woodpeckers Hawk by making aerial passes at it; in addi- made 73 aerial passes at a squirrel before it tion, the hawk repeatedly flew at the wood- was able to extract an acorn and leave the peckers. The woodpeckers simply fled from storage tree. the Red-tailed Hawk. No bird was definitely seen taking any of During the Hastings study, only one tree Group As’ acorns and it is probable that some squirrel (SC&us gTiseu.9) was seen in any of the species supplanted never used acorns tree used by the woodpeckers. In this case, as food. Only fox squirrels and Stellers’ Jays four woodpeckers (all of the members of the (Cyanocitta stelleri) were seen with acorns, group) made repeated passes at the squirrel but in those cases involving the jays it is not and ultimately supplanted it from the acom- certain that these came from the woodpeckers ’ bearing oak. This tree was also a hawking and stores. anvil site. Between 36 November and 4 December, Based on these data and on my unquantified the squirrels “took” one of Group As’ four observations, the frequency of heterospecific storage trees. I did not see how this was ac- supplantings at Hastings associated with complished, but between these dates the heterospecific intrusion into storage trees was woodpeckers ceased attempting to supplant very much less than that observed at Orinda. heterospecifics that entered this tree. On 4 This can probably be attributed to three major December, during one observation period, factors. The first and most obvious reason there were four squirrels feeding on the few for the difference is that the rate at which acorns remaining in this tree. During each heterospecifics entered the woodpeckers ’ successive visit between 4 and 10 December storage trees was far lower at Hastings than the woodpeckers continued to lose ground. at Orinda. Orinda Group A maintained four The last time I saw Group A they were still large storage trees and no group at Hastings defending one storage tree, having lost the maintained anything approximating this ex- others. On my visit of 15 December, all in- tent of storage area. Consequently, by chance dividuals of Group A were gone and their one would expect more heterospecific in- acorn stores had been completely exhausted truders at Orinda than at Hastings. Second, by the squirrels. some heterospecifics at Orinda made major Two sets of comparative observations were efforts to rob the woodpeckers ’ stores. This conducted at Hastings on the frequency of was especially true of tree squirrels which heterospecific supplanting associated with in- were far more common at Orinda than at trusion into acorn storage trees. The first set Hastings. For example, at least five squirrels of observations (26 hr) was conducted be- were always seen in the immediate vicinity of tween 36 August and 5 September on a group Group As’ storage trees while at Hastings only of six birds which had exhausted its acorn one tree squirrel was ever seen in the vicinity supplies. During the observation period nine of any groups’ storage trees. What relation- heterospecifics entered the groups’ storage ship, if any, exists between the fact that at tree and five were supplanted. The following Orinda the squirrels were an introduced birds entered the storage tree but left of their species and the rate of competition between own accord within a few seconds: Western the woodpeckers and squirrels is unknown. A Tanager (Piranga Zudouiciana), Western Blue- final reason may be found in the fact that bird ( %&a mexicanu), Oregon Junco (Bunco all of Orinda Group As’ storage trees could NOTES ON THE FOOD HABITS OF THE ACORN WOODPECKER 201

TABLE 2. Rates of aggressive interaction between Acorn Woodpeckers and heterospecifics in sap trees dur- ing 24.5 hr of observation in two Hastings Reservation colonies.

No. passes to Interactions supplant intruder Mean no. Interactions woodpeckers accompanied Species No. Rate/hr mean range involved by %mit-cut”‘

Nuttalls’ Woodpecker 71 2.9 1.1 1-2 1.01 9 (Dendrocopos nuttdii)

Annas’ Hummingbird 31 1.3 1.0 1.0 1 (Calypte unna)

Plain Titmouse 7 .3 1.0 1.0 0 (Parus inornutzrs)

Mourning Dove 1 94 1.0 1.0 0 (Zen&ha macfoura)

Brown Towhee 2 .08 1.0 1.0 0 ( Pipilo fuscus)

House Finch 1 .04 1.0 1.0 0 (Carpodacus mexicanus)

Wrentit 2 .08 1.0 1.0 0 (Chamaea fasciuta)

Unidentified small birds 7 .3 1.0 1.0 1

Total 122 5.04 11 be entered via arboreal pathways from closely heterospecifics from acorn-bearing valley and adjacent trees. At Hastings, on the other hand, black oaks but often had great difficulty .@-I most of the storage trees could not be entered supplanting these intruders from live oaks except by crossing open areas on the ground. since the woodpeckers are relatively inept at Consequently, access to Group As’ storage moving quickly among the small, densely trees was an easy matter for arboreal mam- knitted terminal branches of these trees. Jays, mals. however, can easily maneuver in this type of Unharvested acorn supply. In addition to foliage. Consequently, if the woodpeckers the apparent year-round defense of acorn could not fly at the intruder, they would hop storage trees, the woodpeckers during the late along the branches until close to the intruder summer and fall defend the acorn bearing oaks who then generally flew out of or to another from which they harvest acorns (Ritter 1938: part of the tree. 98ff). At Hastings, several species, especially Sap trees. A third area defended by the the Scrub Jay ( Aphelocoma coermbscem) and woodpeckers is their sap trees. Table 2 records Stellers’ Jay, were supplanted from these oaks. the data for 24.5 hr of observation of two dif- However, since the woodpeckers used a large ferent sap trees on the Hastings Reservation number of oaks simultaneously and since it 21-26 June. The rate of interaction between was impossible to watch all of them, I was not the woodpeckers and heterospecifics recorded able to collect many data on this aspect of food in table 2 probably accurately reflects the rate defense. I would estimate that, in an “average” with which heterospecifics entered the sap group at Hastings, one jay entered and was trees, since all heterospecifics which entered chased from these acorn bearing oaks every were chased out. 2 or 3 hr of observation. This apparently low The woodpeckers often had difficulty in rate of interspecific interaction was un- driving birds from the sap trees, for the same doubtedly due to the abundance of acorns reasons that they had difficulty in driving and the uncommonness of heterospecific com- jays from the acorn-bearing live oaks. All petitors for these food sources in the study heterospecific birds who entered these sap area. Ritter ( 1938:lOl) recorded an instance trees were far more adept in maneuvering in in which interaction between jays and wood- the canopies of live oaks than were the wood- peckers over unharvested acorns was appar- peckers. In many cases the woodpeckers ently very severe. could not get a clear aerial path to the in- The woodpeckers usually had little or no truder and had to hop and climb about among difficulty in quickly supplanting jays and other the branches in order to get close to the in- 202 MICHAEL H. MACROBERTS truder. The woodpeckers apparently pro- curred between 3 and 5 hr after sunrise. In tect these sap trees only during the summer some cases the intruder(s) remained on the months, the period when they are feeding other groups’ territory for several hours until on sap. finally driven off. Conspecific intrusion, how- Although I do not have comparative data ever, rarely led to contact or fighting because from other localities, it seems that the rate of the intruders normally retreated soon after interspecific aggressive interaction between the territorial owners showed signs of attack- the Acorn Woodpeckers and other birds over ing them. sap “stores” was high. About five intruders Only one case was observed in which one per hour were chased from these trees. group actually took over an area occupied by Hawking perches, anvils, nests, and roost another group. In this case, one group of five holes. Lastly, the woodpeckers supplant birds defended one half of a storage tree and heterospecifics from their hawking perches their own sap tree while another group con- and from the immediate vicinity of their sisting of six birds defended the other half anvils, roosts, and nest holes. Only those sup- of the same storage tree and their own sap plantings associated with anvils and hawking tree. Members of each group frequently in- perches can be said to have any relation to vaded the territory of the other and fighting food defense. In these cases, the presence of between members of the two groups was a heterospecific at a hawking site could inter- frequent. In some instances, two or more fere with flycatching, and birds near anvil grappling birds would fall from the top of the sites might be attempting to retrieve acorn storage tree to the ground before becoming crumbs left from previous meals. Additionally, disentangled. After these two groups had been as the woodpeckers sometimes store insects observed for about one week, one of them left near their hawking perches, defense of these and the other occupied the entire area. Very areas might be associated with defense of these likely, before I began observing these groups, insect stores. When anvils and hawking one of them had initially occupied the entire perches are in trees which are protected for area and the other had managed to occupy other reasons (i.e., storage trees), it is difficult and defend a part of that area for themselves. to determine exactly what is being defended. Ultimately, either the intruders or the residents were ousted. DEFENSE OF TERRITORY FROM CONSPECIFICS DISCUSSION OF FOOD DEFENSE In addition to defending their storage trees, Interspecific and intraspecific aggression is a sap trees, unharvested acorn bearing oaks, common feature of the Acorn Woodpecker roosts, nest holes, anvils, and hawking perches existence, and competition over food is un- from heterospecifics, the members of each doubtedly the primary functional reason for group defend a territory from conspecifics. this aggression. This type of competition is This area of defense, which includes all loca- direct, with the competing individuals fre- tions defended from heterospecifics, is much quently coming into physical or near-physical larger than that defended from heterospecifics contact. This makes “it possible not only to and encompasses a minimum of 5 acres. A identify all competitors but also to quantify conspecific intruding on any part of this area the relative comparative effects of the various was always pursued until driven out of the species involved” (Bock X%8:126). It should territory. Territorial defense has not been be stressed, however, that all heterospecifics discussed in earlier literature on the Acorn that are supplanted by the woodpeckers are Woodpecker. not necessarily involved in any food competi- At Hastings the intrusion of conspecifics tion with them. Apparently at Orinda and at into other groups ’ territories varied markedly Hastings all heterospecifics which entered between groups and at different times of day. the woodpeckers ’ storage trees and sap trees In some groups no intruders were seen, In were supplanted with little or no discrimina- others intrusion was a daily occurrence. Dur- tion made between intruders. Consequently, ing 366 hr of observation of four groups, 159 at least in some cases, the frequency with cases of conspecific intrusion were recorded, which a particular species was supplanted did of which at least 18 cases involved two or not necessarily indicate the degree of competi- more intruders. Ninety-one per cent of all tion between it and the woodpeckers, but only conspecific intrusion occurred before noon the frequency with which it, for whatever (57 per cent of the observations were made reasons, entered the woodpeckers ’ defended before noon). Most conspecific intrusion oc- areas. For example, it is certain that Nuttalls’ NOTES ON THE FOOD HABITS OF THE ACORN WOODPECKER 203

Woodpeckers, Annas’ Hummingbirds (Calypte summer breeding habitat of Lewis ’ Wood- annu), and Plain Titmice enter the wood- pecker as follows: “one nearly universal char- peckers ’ sap trees for the purpose of get- acteristic of their breeding habitat is that of ting sap; but Mourning Doves (Zenaidura openness. Some trees are necessary for nesting macroura) and Brown Towhees (Pipilo fu.s- and as hawking perches, but a dense conif- CUS), which were also chased from these same erous forest or a woodland with closed trees, showed no signs of attempting to take canopy would restrict vision and aerial ma- the woodpeckers ’ food. Likewise, at Orinda neuvers and eliminate the brush and grass the fox squirrel was by far the major com- understory which supports important insect petitor, but, from a purely tabular point of prey populations. . , .” This type of habitat view, all birds combined would appear to have appears to be very similar to that of the Red- been equally important as competitors. Never- headed Woodpecker (see Bent 1939). theless, as Bock ( 1968, in press) has indicated The winter habitat of Lewis ’ and Red- for Lewis ’ Woodpecker, those species most headed Woodpeckers must have areas for interested in robbing stores are the ones that storage and an adequate supply of mast. Con- come the most often, and likewise they are sequently, they are found at this time of year driven off more intensively by the wood- in the immediate vicinity of oak woodlands, peckers. In the case of heterospecifics, how- commercial nut orchards or other nut-produc- ever, it is clear that the woodpeckers ’ be- ing trees (Bock 1968; Kilham 1958a). During havior is designed not only to protect food the winter Lewis ’ Woodpeckers and Acorn stores and sources but nesting and roosting Woodpeckers are often found in the same sites as well. area and competition between them is exten- The proximate factors stimulating con- sive (Bock lQ68). Neither of these species specific intrusion into the territories of other is sympatric with the Red-headed Wood- groups are not known. In no case was a con- pecker. specific intruder seen taking food within the In favorable habitats both the Lewis ’ and territory of another group. This, however, does Red-headed Woodpeckers will remain year not mean that the maintenance of group ter- round; the fact that the former are sometimes ritories in these birds and the defense of this in pairs indicates that, when the habitat con- area from conspecifics of other groups is not tains sufficient supply of both winter and related to the defense of food within the summer food sources, these woodpeckers are boundaries of the territory. The members of resident (Bent 1939; Kilham 1963; Bock 1968). each group gather all, or certainly the major The Acorn Woodpecker on the other hand portion, of their food from within their own uses many of the same foods, and in a very territory, and by ousting intruding con- similar manner to Lewis ’ and Red-headed specifics, they undoubtedly protect these food Woodpeckers, but is resident and lives in sources as well as their nesting and roosting groups. Its habitat (usually mixed oak wood- sites from conspecifics. land) consists of one or more closely adjacent storage trees that are usually separated by INTERSPECIFIC COMPARISON open grasslands from other trees. In addition, OF HABITATS other open areas, such as land cleared for The Lewis,’ Red-headed, and Acorn Wood- pasture, are usually found in each groups’ peckers have many similarities in food habits territory. Within easy flying distance of the (Beal 1911; Bent 1939; Bock 1968, in press; storage trees are acorn-bearing oaks which in Kilham 1958a, 195813, 1963). All flycatch, most instances would normally include trees store mast, and defend their stores, each in a suitable for sapsucking. The Acorn Wood- slightly different way, from conspecifics and pecker habitat, then, contains one or more heterospecifics. Unlike the Acorn Wood- trees suitable for storage, acorn production, pecker, however, neither the Lewis ’ nor the roosts, nesting holes, and, at least in some Red-headed Woodpecker lives in groups. Gen- cases, for sapsucking, and an area (or areas) erally speaking, each individual stores and suitable for flycatching and hawking perches; feeds on its own mast during the winter, thus, a heavy-canopied woodland or dense maintaining a territory at this time. During coniferous forest is not suitable. This single the spring, both the Lewis ’ and Red-headed habitat combines elements of both the Lewis ’ Woodpecker usually migrate to summer and Red-headed Woodpeckers’ winter and breeding grounds where they flycatch and summer habitats. feed on various fruits. If these three species have similar food Bock ( lQ68:11-12) has characterized the habits, the question naturally arises as to why 204 MICHAEL H. MACROBERTS they are so different with respect to social and nest holes, and occasionally their anvils and migratory behavior. It is impossible at and hawking perches from heterospecifics, present adequately to answer this question, and a larger area, the territory, from con- since the comparative ecological and be- specifics. havioral research is incomplete. Nevertheless, The study further indicates considerable the sedentary nature of the Acorn Wood- apparent similarity in food habits and habitat pecker is undoubtedly dependent on the pres- among the Acorn, Red-headed, and Lewis ’ ence of year-round food sources occurring in Woodpeckers, but with obvious differences in a nondispersed form. Although the Lewis ’ and social and migratory behavior. Red-headed Woodpeckers are dependent on highly concentrated and locally abundant food ACKNOWLEDGMENTS sources, as is the case in the Acorn Wood- I am particularly indebted to 0. P. Pearson, Director pecker, their migratory behavior suggests that of the Museum of Vertebrate Zoology, University of few habitats contain both mast and insects in California, Berkeley, and to J. Davis, Director of the Frances Simes Hastings Natural History Reservation, sufficient quantities to permit year-round oc- for permission to use the facilities at the Hastings cupancy. Another important question arises Reservation. and to W. T. Hartman for nermiting at this point. The Acorn Woodpecker is highly me to enter East Bay Water property nea; Orinda, social year round, whereas neither the Lewis ’ California. In addition I would like to thank G. W. nor the Red-headed Woodpecker is. Bock Barlow, C. Bock, R. Dawkins, and 0. P. Pearson for their helpful criticism of the manuscript, and to ex- ( 1968:141, 142) has suggested “that the com- press grateful appreciation to G. R. Bond and to my munal behavior of M. formicivors may have wife, Barbara, for their assistance in the field and in evolved specifically as a means of protecting the preparation of this paper. acorn stores.” This may in fact be the case, LITERATURE CITED but unfortunately answers to questions con- cerning the adaptive features of Acorn Wood- BEAL, F. E. L. 1911. Food of the woodpeckers of the United States. U.S. Dept. Agr. Biol. Surv. pecker sociality will have to await more de- Bull. 37. tailed study of the social organization and BENT, A. C. 1939. Life histories of North American genealogical relationships of group members. woodpeckers. U.S. Natl. Mus., Bull. 174. BOCK, C. E. 1988. The ecology and behavior of SUMMARY the Lewis Woodpecker. Ph.D. Thesis, Univ. California, Berkeley. The results of this study indicate that the food BOCK, C. E. In press. The ecology and behavior of habits of the Acorn Woodpecker are more the Lewis Woodpecker. Univ. California Publ. complex than has previously been supposed. Zool. FISHER, J., AND R. T. PETERSON. 1984. The world Instead of subsisting almost exclusively on of birds. MacDonald, London. stored, dried acorns, the birds eat immature KILHAM, L. 1958a. Sealed-in winter stores of Red- and mature green acorns, flycatch, sapsuck, headed Woodpeckers. Wilson Bull. 70: 107-113. probably do very little boring or gleaning for KILHAM, L. 1958b. Territorial behavior of winter- ing Red-headed Woodpeckers. Wilson Bull. insects and insect larvae, eat something as- 70: 347-358. sociated with oak blossoms, and occasionally KILHAM, L. 1963. Food storing in Red-bellied eat fruit, birds ’ eggs, and lizards. In addition, Woodpeckers. Wilson Bull. 75:227-234. the members of each group defend their acorn RIPER, W. E. 1938. The California Woodpecker storage and sap trees, the acorn bearing oaks and I. Univ. California Press, Berkeley. from which they gather acorns, their roosts Accepted for publication 24 March 1969.