WINTER ECOLOGY OF MIGRANT AND RESIDENT LEWIS ’ WOODPECKERS IN SOUTHEASTERN COLORADO

HARLO H. HADOW

Department of Biology Universitv of Colorado Boulder, VColorado 80302

The ecology of widely distributed in oaks (Quercus gambelii). The others remain heterogeneous habitat may be expected to resident and store corn kernels (see below), vary throughout their distribution. Such spe- often in the same cottonwoods in which they cies are necessarily generalists that must use nested. To compare the winter ecology of different food items, face various intensities migrant and resident , I studied inten- of competition and predation, and perhaps sively one pair of Lewis ’ Woodpeckers winter- even encounter relatively large climatic vari- ing on the plains and a second pair wintering ation in separate portions of their range. in the foothills (see fig. 10 for location of the Their behavior, and perhaps morphology, will areas), from October 1969 to April 1970. vary accordingly. Observations totaled 10,281 -minutes, ap- The opportunistic Lewis ’ Woodpecker proximately distributed equally between the ( Asyndesmus lewis), widely distributed over two pairs. Additional observations were made the western half of the United States, is an at two other locations in the foothills during example of one such generalist. Although that winter, and sporadically at all locations highly modifed for flycatching, with wide in the following 2 years. This paper describes gape (Spring 1965) and short, wide wings Lewis ’ Woodpecker migration, feeding ecol- ( Bock 1970)) Asyndesmus is sufficiently ogy, competition, and predation in the areas adaptable to feed almost entirely upon vege- described below and terminates with a dis- tative matter during certain months (Bock cussion of the recent range extension by 1970; this study). Bock (1970) has recently Asyndesmus onto the eastern plains of published a study of Asyndesmus in California Colorado. where it winters sympatrically with the simi- lar Acorn Woodpecker ( Melunerpes formici- STUDY AREAS vorus). The two species remain in contact The plains study area is located one mile SE of Olney in spite of intense competition because of Springs, Colorado, at Kings’ Center (R. 58 W., T. either a shifting ecological advantage that 22 S., sec. 33, Crowley County), elevation 4350 ft, favors first one and then the other, or the on the site of a hay mill that has been inactive for opportunistic habitat selection of Asyndesmus many years (figs. 1, 2, 10). A pair of Asyndesmus has wintered and bred for the past 3 years in a clump which limits contact to areas with large mast of nine mature cottonwoods. Corn fields and open crops. Colorado is outside of the range of the range surround the study area, and there are no other Acorn Woodpecker, and Colorado-wintering trees close to those around the mill. No other Asyndesmus are not sympatric with any Asyndesmus wintered or bred within a mile of this closely related species, so winter competition site. In 1969-70, migrant Lewis ’ Woodpeckers wintered in Colorado should differ from that in Cali- in the foothills along Hardscrabble Creek from about fornia both in species of competitors and in- 5 miles NE of Wetmore, Colorado (elevation 5800 tensity of competition. ft), to about 3 miles SW of the town (elevation Although almost totally migratory in Mon- 6200 ft). Narrow-leafed cottonwoods (Pop&s tana, Idaho, eastern Oregon, and eastern angustifolia) and willows (Saliz spp.) were domi- nant elements of the riparian vegetation; the cotton- Washington (Bock 1970), and almost totally woods providing food storage sites for all but two resident in the Salt Lake region of Utah Asyndeimus. Those two birds stored in a power pole (Snow, unpubl. data, quoted in Bock 1970)) and a dead Gambels’ oak. A belt of Gambels’ oaks Asyndesmus is only partially resident on the covered the edges of the river valley on both sides throughout the wintering grounds. At the northeastern eastern plains of Colorado. With the onset of extreme, the upland surrounding the creek was grass- cold weather about half of these birds, which land used for cattle ranching, but southwest of Wet- commonly breed in clumps of cottonwood more the agricultural lands gave way to rugged lime- ( Populus sargentii) surrounding ranches, mi- stone ridges covered with Gambels’ oaks, ponderosa pine ( Pinus ponderosa), and juniper (Juniperus grate west to the foothills of the Rocky Moun- amek~n~) This oak-pine-juniper complex gradually tains and amass stores of acorns from Gambels’ was replaced by a dense Douglas fir (Pseudotsugu

[2101 The Condor 75:210-224, 1973 WINTER ECOLOGY OF LEWIS ’ WOODPECKERS 211

sturdy wintered on land belonging to Mrs. E. Branstine (designated Branstines’ below) at the eastern edge of Wetmore, Colorado (R. 69 W., T. 21 S., sec. 10, Custer County; fig. 10). The storage cottonwood (designated Lw, figs. 3, ? ) was70 m from the nearest unoccupied cottonwood, on the edge of a beaver pond. A garden of several acres, east of the storage 06 tree, contained four clumps of Gambels’ oaks (fig. 3). 0 The nearest conspecific wintered 84 m from tree Lw 7 in a power pole (designated PI, fig. 3). Two additional observation sites were vegetatively similar to that already described, but the density of wintering Asymlesmus was quite different. At a site 1 mile SW of Wetmore, eight birds wintered in a stand of riparian cottonwoods 76 m long, with three of them defending stores in the same tree. At Morlans’ Ranch on the extreme northeastern edge of the sturdy area, one Asyntlesmus wintered in an oak -=I0 M HM clump, 200 m from its nearest conspecific. Neither Asyntlesmus nor Gamhels’ oaks were present dnwn- FIGURE 1. Lewis ’ Woodpecker study area at Kings’ stream from this location. Center. Numbered circles represent mature cotton- woods; corn kernels were stored in lN, lM, lS, 2,4; METHODS NCL is a clump of immature locusts; HM is an un- I used standard field techniques in this study, i.e., rrsed hay mill. observing behavior with a Swift spotting scope of 20, 40, or 60 power and a pair of Swift 8.5 x 44 mensiesii) forest above 7000 ft, but south-facing binoculars, and recording results in a field notebook. slopes retained vestiges of the oak complex up to at Observations were made either from a parked car least 8000 ft elevation. or from the shelter of available buildings, always at The pair of Asyndesmu.~ selected for intensive least 40 m from a given storage tree. Feeding be-

FIGURE 2. Kings’ Center photographed from the NE at a location corresponding to the ripper right-hand corner of fig. 1. 212 HARLO H. HADOW

concentrating where insects are abundant in 7 the spring and where mast is abundant in the NCL‘ NORTH fall. Furthermore, migrants do not necessarily use the same wintering ground in successive P‘ -= IO M years; none has returned to the Hardscrabble S‘ drainage in either of the two winters since my study. Asyndesmus populations that breed on the plains of Colorado are only partly migratory (Bailey and Niedrach 1967). In June 1970, the locations of 34 pairs of Lewis ’ Wood- peckers which bred in Crowley County, Colo- rado, were plotted on topographical maps as part of a study of their sympatry with the Red-headed Woodpecker ( Melanerpes erythrocephalus) (Bock et al. 1971). I cen- sused the area twice, on 27 and 29 November 1970, when I5 pairs were found amassing corn stores at their nesting sites. No Asyndesmus was found at sites other than those noted in the June census. Eight of 18 pairs that nested PO‘ @@#@me PI less than 100 m from cornfields remained Hcl resident, compared with 7 of 16 pairs that /I\ nested more than 100 m from cornfields. This suggests that even though Asyndesmus re- FIGURE 3. Foothills study area at Branstines.’ Open quires storable resources to winter in an area, circles represent storage sites of Asyndesmus during something other than proximity to cornfields the winter of 1969-76; solid circles represent unused determined which birds migrated. Intra- mature cottonwoods. A fence (light solid line) de- specific aggression might be involved, though limits a garden containing four clumps of Gambels’ oaks (solid clusters); ch is the chicken house from this seems unlikely in view of the variation in which the pair was observed. densities seen in the foothills. If Asyndesmus pair before leaving their wintering grounds, there may be a genetic explanation such as haviors were recorded continuously to the nearest 0.5 Howell (1953) found in Spyrapicus. This min as gleaning, flycatching, storing, or working problem demands further study. stores, based upon criteria explained in table 2. Hawking flights and other specific behavioral seg- In 1969, migrant Lewis ’ Woodpeckers ar- ments were timed to the nearest 0.1 min with an rived at Wetmore in early October. I walked Elgin stop watch. For aggressive encounters, the through the Hardscrabble Creek drainage each species of intruder, time of encounter, and behavior weekend in September, seeing no Asyndesmus of the defending Lewis ’ Woodpecker were recorded; there until 30 September when two were seen the location of the encounter was plotted on a field map of the study area. Weather conditions were perched on a dead cottonwood across the noted at hourly intervals during the day. Since con- road from Branstines.’ There was no evidence tinuously timed observations do not logically lend that the birds had selected this as a storage themselves to statistical treatment, statistical tests tree, and no Asyndesmus stored there in 1969. were limited. These might have been transients on the ad- FALL MIGRATION vanced edge of the migration. On 4 October, I observed about 300 Lewis ’ Woodpeckers in The migratory patterns of Lewis ’ Wood- small loose flocks moving up the valley from peckers are highly variable. Bock (1970) northeast to southwest. Most had not selected listed the following reasons for this variabil- storage sites at that time, but some had done ity: (1) the distance from nesting sites to so and were beginning to amass and defend the wintering grounds may vary from O-100 stores. The greatest concentration of migrants miles or more; (2) some Asyndesmus are nomadic during the late summer, and spend was at Branstines,’ with the upper limit about several weeks wandering in the mountains at 200 m W of Wetmore, and the eastern ex- up to 10,000 ft elevation after leaving the treme at Morlans’ Ranch. The upper limit breeding ground and before arriving at the advanced 3 miles by the next morning to the wintering ground; (3) Asyndesmus opportun- highest elevation that Asyndesmus wintered istically select wintering and breeding grounds, in 1969. WINTER ECOLOGY OF LEWIS’ WOODPECKERS 213

FIGURE 4. Photograph of Branstine’s study area, taken at Hcl (fig. 3) facing northwest. LW is on the ex- treme left, NCL is the distant cottonwood in the center, oak clump M is on the extreme right.

The behavior of these migrants differed territory, and the bird at Morlans’ was estab- from that described by Adams (1941) and lished and encountering few transients, I feel Smith (1941). Instead of flying in a straight certain that this series represented (1) a line like the birds they observed, these mi- transient with no stores, (2) a recently estab- grants circled the area in small flocks of up lished pair, and (3) a bird that had been to 20 individuals, sometimes landing en masse established for several days, respectively, on on a cottonwood or power pole, only to fly 5 October. Observations were made on 5 Oc- off minutes later and circle again. Yet there tober, beginning at the Tough Teat Dairy at was a net movement toward the lower con- 12:03, at Morlans’ at 13:00, and at Branstines’ centration of birds upstream, and it was as at 15:47. The results are summarized in table if the birds were diffusing to equilibrium, 1. While the data are limited, there appears with movement in all directions, but a net to be a correlation between storing and ag- movement upstream. By 11 October, all but gressive behavior. about 50 of the birds were gone; these 50 had Adams ( 1941) and Smith (1941) were established territories and were accumulating probably observing migrants on the way to stores. the wintering grounds since the dates given To quantify behavioral changes that might are within the time period that Bock (1970) occur while the birds were setting up their listed for “leaving the breeding ground,” are winter territories, I timed observations on one 2 weeks early for “arrival on the wintering bird at the upper extreme (Tough Teat grounds,” and their birds were not in normal Dairy), one at the lower extreme ( Morlans),’ winter habitat when observed. If so, mine are and the pair at Branstines’ in the middle of the first published observations of Lewis ’ the wintering grounds. Since on 4 October Woodpeckers arriving at their wintering the bird at Tough Teat Dairy was not present, grounds and setting up mast stores. the pair at Branstines’ had not established a From the data of Adams ( 1941), Smith 214 HARLO H. HADOW

TABLE 1. Changes in behavior upon establishing winter territories in migratory Asyndesmus EeuSs.

Per cent feeding behavior Location Flycatching Gleaning Storing Work. St.” Enc./h+ BM’ _- -- Morlans’ 39.0 0.0 54.2 6.0 2 83 Branstines’ 0.0 0.0 100.0 0.0 4 178 Tough Teat 62.8 37.2 0.0 0.0 0 45 .______a Working stores. b Aggressive encounters per 60 bird-minutes of observation. p Number of bird-minutes observation at a specified location.

( 1941), and this study, the following picture if aggressive encounters were occurring, one of Asyndesmus migration emerges : (1) mi- would expect to hear chatter-calling (Bock grants travel to and arrive at the wintering 1970; Bock et al. 1971). grounds more or less as a group; (2) upon arrival, the large flock (Smith counted 101s WINTER FEEDING ECOLOGY migrants passing over him in 30 min) dis- Bock (1970) described the feeding behavior perses into smaller groups that explore the of wintering Lewis ’ Woodpeckers, and my area until individuals establish territories; (3) terminology follows his, except that I split his when all of the suitable territories are claimed, “working acorn stores” into storing and work- the remaining transients leave or perish. The ing stores. This split seemed necessary since spacing of birds on the wintering grounds is these activities involved different behavior probably determined by some combination of patterns and competitors, and Asyndesmus food or storage-site availability and competi- usually did not defend unstored acorns. Con- tion, as Bock suggested (1970). tinuous observations of behavior were made Apparently, the behavior of Asyndesmus for 4004 bird-minutes at Branstines,’ and 6277 changes when they establish territories. Before bird-minutes at Kings’ Center. Behavior pat- doing so, they feed mainly on insects ob- terns based upon the criteria in table 2 were tained by flycatching or gleaning, and they identified and recorded. The importance of are not the aggressors in any encounters; these behaviors for obtaining food are dis- upon acquiring a territory they set about cussed below. amassing stores as rapidly as possible, and The per cent of time that Asyndesmus spent aggressive encounters are frequent (table 1). When the territorial boundaries are more performing the various behaviors differed be- firmly established and the transients have tween the foothills and plains study areas and moved on, the residents are freed to leave changed throughout the winter (fig. 5). their stores to feed on insects or to rear- Working stores made up a greater percentage range the stores (see feeding section below). of total feeding behavior at Branstines,’ while Smiths observation (1941) that the migrants flycatching and storing were much more im- traveled silently supports the above, because portant (percentage-wise) at Kings’ Center.

TABLE 2. Description of Lewis ’ Woodpecker wintering behavior patterns based upon Bock ( 1970) and this study.

Behavim Behavioral objective Characteristicsused to identify behavior

Flycatching or hawking Capture air-borne insects Erratic, short duration flights from an elevated perch; variable periods of scanning, character- ized by an alert posture and rapid head move- ment. Gleaning Capture surface insects Vertical and peripheral movement over tree surface, probing, light tapping, visual observa- tion of tree surface, no drilling. Storing Obtain, prepare, store mast Plucking of acorns or corn kernels; removal of items acorn shells; positioning of acorn meats or intact corn kernels in bark crevices; tapping into place. Working stores Feeding upon and movement Similar to gleaning, but done in areas known to of stored items contain stores; items identifiable as corn or acorn meats; movement from lower to higher in the tree. Perching Maintenance Inactivity, feathers fluffed, preening, etc. WINTER ECOLOGY OF LEWIS’ WOODPECKERS 215

Gleaning was observed only at Kings’ Center. 100 2 = Storing behavior generally decreased as the q q winter progressed in both locations, but it q Z decreased more markedly at Branstines.’ z z Working stores was most important during 1 q = the mid-winter months at Branstines,’ de- = q creasing as flycatching became more impor- 50 = tant in the spring. q The stores were used more at Branstines’ than at Kings’ Center, probably because harsh weather conditions prevented greater use of insects as a food source in the foothills. z Thirty-one per cent of my observations at rz 0 I Branstines’ were made on days when the tem- perature never rose above freezing and the k! ground was completely covered with snow. The pair spent 99.0% of its feeding time work- i ing stores on such days, as opposed to 58.0% when conditions were less harsh. At Kings’ 50 Center, there were no days when the ground was completely covered with snow, and few days when it did not warm up to more than 0°C. Asyndesmus would discontinue other behaviors to flycatch if insects became active at either location. I conclude that the stores 0 I serve primarily as a reserve food supply and 0 N D J that insects are preferred when available. F M Asyndesmus stored all winter at Kings’ FIGURE 5. Distribution of Asyndesmus feeding tinle Center, but very little at Branstines’ after in flycatching ( stipple ), gleaning ( hashed lines ), November (fig. 5). After November, almost working stores (white), and storing (solid black); at King’s Center (above) and Branstine’s (below). no acorns were available in the foothills, either Data are for 4004 and 6277 bird-minutes of obser- because they were covered by snow or be- vation at Branstine’s and King’s Center dnring the cause they had been eaten or stored by other winter of 1968-70. organisms. Corn was available all winter at Kings’ Center. Bock (1970) commented that in California Asyndesmus stored as long as unavailable for gleaning. The pair at Kings’ there were storable items available, even Center frequently left their stores unattended though not all of the stores were used during for a half-hour or more while gleaning in the the winter. Storing increased in spring at locust stand or around the hay mill (fig. 1)) both Colorado locations (fig. 5) because a and flickers (Colaptes cafer x C. auratus in- few acorns were made available when the tergrades) sometimes worked over the stores snow melted at Branstines,’ and because a pair at such times unmolested. The birds at of Sparrow Hawks (F&o spawerius) took Branstines’ never left their stores unattended over the storage tree at Kings’ Center for a for more than a few minutes. Competition nesting site in late February. The evicted and spacing, then, did not permit gleaning at Lewis ’ Woodpeckers amassed new stores in Branstines’ because no gleaning areas were tree 4 (fig. l), part of which were corn available nearby, and the birds could not kernels removed from the stores in tree 1M “afford” to leave the stores unattended while gleaning some distance away. when the Sparrow Hawks were not present. Amassing and defending mast stores seems The importance of gleaning at Kings’ Cen- to be common behavior in melanerpine wood- ter may have been an artifact of my definition peckers. Red-headed (Kilham 1958a,b) and of gleaning, but it probably was caused by Acorn (Ritter 1921; Macroberts 1970) Wood- the spacing of birds at the two locations. peckers also store mast during the winter, but Since the birds at Kings’ Center were the only the three species differ in specific details of Asyndesmus within at least a mile, they could their storing behavior. Acorn Woodpeckers glean in any of the trees near the hay mill. communally store and defend acorns, which At Branstines,’ conspecifics defended stores are placed intact into individual holes exca- in the nearby cottonwoods, making those trees vated by the woodpeckers to fit one acorn 216 HARLO H. HADOW

- -_ FIGURE 6. Aggressive displays of the Lewis ’ Woodpecker: A. Bill-up display given to a Black-billed ; B. Bill-up display to the birds’ mate: C. Wings-out display to a conspecific intruder; D. Circle- flight to a conspecific &&der. each (Ritter 1921; Bock 1976; Macroberts competition is indirect, there is one source of 1970). The Red-headed Woodpecker may direct evidence: the physical conflict that oc- store large quantities of intact acorns in curs when two individuals contend for some cavities within the trunk of a tree or may component of the environment. Bock (1970) store acorn meats in the cracks of bark, further reasoned that if direct conflict over covering them with wedges of wet bark after a food source is evidence of competition for a rain (Kilham 1958a,b). This covering is that commodity, then the frequency and in- adaptive, since Red-headed Woodpeckers dis- tensity of aggressive encounters should be a perse shelled acorn meats throughout their quantitative measure of the intensity of that winter territory. Both Asyndesmus and the competition. Macroberts (1970) disagreed Acorn Woodpecker defend concentrated mast with the above, suggesting that not all of the stores. Red-headed Woodpeckers eat corn birds displaced by his Acorn Woodpeckers kernels when available, and may store them were actual competitors, since the birds in (Kilham 1958a,b), but this has not been de- his study supplanted any intruder that landed scribed for Acorn Woodpeckers. in their storage tree, irrespective of proximity to the stores. This did not seem to be the COMPETITION case in my study because of the differential Orians and Willson (1964) explained that aggressiveness (described below) with which while most of the evidence for interspecific Asyndesmus defended their mast stores. There- WINTER ECOLOGY OF LEWIS ’ WOODPECKERS 217 fore, I feel that the encounters observed dur- the Picidae. Asyndesmus gave it in conflict ing my study occurred because of competition situations (attack vs. the pair-bond with the for the stores, and the quantification of com- mate; attack vs. escape with large corvids; petition to follow is based on the assumptions attack vs. ignore when an intruder was not of Orians and Willson (1964) and Bock close enough to “warrant” attack). (1970). The circle-flight (fig. 6-D) and wings-out When an intruder landed near or ap- display (fig. 6-C) are both primarily intra- proached defended stores, the resident specific displays involving wing elevation and Asyndesmus would respond in one of five display of the rosy breast feathers. These ways: ( 1) it would ignore the intruder; ( 2) feathers are probably species-recognition char- the resident would respond with a bill-up dis- acters (Bock 1970). Either display could be play, frequently accompanied by one or more given to the mate when it returned to the squeak notes; (3) it would respond with a stores from outside the storage tree. (See wings-out display; (4) it would respond with Bock for a more complete description of these a circle-flight display (both elements of the displays.) display usually accompanied by chatter calls) ; An intrapair circle-flight begins with the or (5) it would displace the intruder aggres- displaying bird actually diving at the mate sively by flying at it, usually chattering loudly as though to supplant it, but circling out at and evicting it. Which response occurred in the last moment into a glide with wings held a given situation was determined partly by abnormally high. The display ends with the factors extrinsic to the defending Asyndesmus, displayer either landing on the stores or next such as proximity to the stores, species, sex to the mate. In the latter event, the mate (if conspecific), and partly by the intrinsic assumes a mating-invitational posture, with aggressiveness of the defender. In the fol- wings slightly spread and tail elevated. The lowing paragraphs, “aggressive encounter” or displaying bird mounts and copulates, and “displacement” refers to bouts in which sometimes they reverse positions and copulate aggressive behavior by a defending Lewis ’ again. Short (1971) suggested that pseudo- Woodpecker evicted an intruder from a storage copulation may reduce intrapair aggression in tree or its immediate vicinity. Picoides. Ligon (1970) stated that copulation The bill-up display was given to con- and during the nonbreeding season was trig- heterospecific intruders and was sometimes gered by unsettling conditions, and may given when the mate returned to the stores have strengthened the pair-bond in the from outside the store tree. The bill was Red-cockaded Woodpecker ( Dendrocopos pointed at the intruder (fig. 6-A), and the bore&s). Pseudo-copulation (or the mating- back flattened so that the tail, back, neck, invitational posture) apparently enabled mate and head were essentially a straight line recognition or reduced intrapair aggression in terminating in the bill. The bill seemed to be Asyndesmus. the character emphasized in this display. A Two vocalizations frequently accompanied bill-up display was identical when given to displacements or displays. The chatter call the mate except that the body was oriented usually accompanied wings-out displays and away from the stimulus (fig. 6-B). Usually, circle-flights, and was sometimes given as a when this display was given, it was the only defending bird flew to supplant an intruder. indication that an intrusion was noticed by It often was sufficient warning to displace the resident Asyndesmus, but it could be fol- intruders without an attack from the defend- lowed by more intense aggressive behavior, ing bird. The squeak note is a single, sub- especially if the intruder moved toward the dued note of a chatter call (Bock et al. 1971). stores. In several encounters against Black- Bock (1970) called it a warning note, but in billed ( pica) and Common my study it was given either simultaneously Crows ( Corzjus hrnchyrh ynchos) , an Aslln- with, or under similar conditions as, the desmus gave a bill-up display as it moved bill-up display. It was not usually sufficient toward the intruder, and (in the case of the to evict an intruder by itself. [See Bock magpie) the intruder was supplanted without (1970) or Bock et al. (1971) for further de- the resident taking flight. Short (1971) de- scriptions of these vocalizations.] scribed similar displays for four species of The bill-up, wings-out, and circle-flight dis- his genus Picoides, and I have seen Red- plays appear to be points on a continuum of headed Woodpeckers display similarly when increasingly aggressive responses between the defending nest holes from conspecifics. Ap- end points of “ignore” and “attack,” much like parently, the bill-up display is common within the attack-flight continuum of Balph and 218 HARLO II. HADOW

TABL,E 3. Frequency of aggressive encounters between Asynde.smus lewis and intruders at the stores at Bmnstines’ and kings’ Cen&.a

Branstines’ Kings’ Center

Specie.\ N Enc./hr yfototal N Enc./hr % total

Lewis ’ Woodpecker ( Asyndesmus lewis) 36 0.54 37.9 1 0.009 3.6 Flicker intergrade (Colaptes cufer x auratus) 9 0.13 9.5 21 0.20 75.0 Black-billed Magpie (Pica pica) 15 0.27, 15.8 3 0.03 10.7 Starling ( Sturnus oulgaris) 9 0.13 9.5 0 0.00 0.0 Stellers’ ( Cyanocitta stelleri) 11 0.16 11.6 0 0.00 0.0 ( Cyunocitta cristata) 3 0.04 3.2 0 0.00 0.0 Oregon Junco (Jurhco o~egunus) 2 0.03 1.1 0 0.00 0.0 Belted Kingfisher (Meguceryle ulcyon) 1 0.01 1.1 0 0.00 0.0 Common Crow ( hrachyrhynchos) 6 0.09 6.3 0 0.00 0.0 House Sparrow (Passer domesticus) 0 0.00 0.0 3 0.03 10.7 Unidentified small ’ 3 0.04 3.2 0 0.00 0.0

Total 95 1.41 100.2 28 0.26 100.0

a Based upon 4004 bird-minutes at Branstines’ and 6277 at Kings’ Center.

Stokes (1963). This continuum exists only tion on the continuum, their relative positions for intraspecific encounters, since the wings- on the continuum may be used to compare the out and circle-flight displays are primarily aggressiveness of the two displays. intraspecific displays. The circle-flight is Aggressive encounters in defense of the essentially an aerial wings-out display, and a stores occurred more frequently at Branstines,’ wings-out display is essentially a bill-up dis- and there were more species of intruders there play with elevated wings and movement than at Kings’ Center (table 3). At Branstines,’ toward the intruder. Since energy expended, intraspecific encounters outnumbered inter- probability that the display will supplant the specific encounters with any one species, but intruder, and intensity of accompanying vo- interspecific encounters outnumbered intra- calizations all increase with advancing posi- specific encounters overall. Only one intra- specific encounter occurred all winter at Kings’ Center, and flickers were the most commonly 4 encountered heterospecific. An encounter-fre- quency curve (fig. 7) shows two peaks (Octo- ber and December) at Branstines,’ but only t one December peak at Kings’ Center. The r I frequency of encounters declined for the rest 3 I‘ of the winter at both locations. I The frequency of aggressive encounters at \ Branstines’ increased when the ground was I 1 \ covered with snow, and temperatures were \ below 0°C (designated “snowy” days; table 2 -- \ 3 4) over days when the ground was at least \ I' \ I' partly bare and/or temperatures were above \ \ freezing (designated “nonsnowy” days; table \ I' '0 II . 4). Blue Jays (Cyanocitta cristntu), Stellers’ 1 -- \ I '3 Jays (Cyanocittu stelleri), and flickers were ? displaced more frequently under the “snowy” conditions, intraspecific encounters were more important under “nonsnowy” conditions (table 4). On “snowy” days all intruders 0, I I I I I I I were evicted from the storage tree, flickers were frequently supplanted from the small ~NDJ FMA cottonwoods in the creek bottom up to 40 m FIGURE 7. Frequency of aggressive encounters from tree LW and a neighboring Astydesmus between Asyndesmrcs and intruders at the stores was supplanted from tree P (fig. 3), 70 m at Branstines’ (solid line) and Kings’ Center from LW. The creek bottom and P (a large, (dashed line), compared by months from October 1969 to April 1970, based on 4004 bird-minutes of dead cottonwood) were defended at no other observation at Branstines’ and 6274 at Kings’ Center. time during my study. Passerines were usu- WINTER ECOLOCY OF LEWIS ’ WOODPECKERS 219

TABLE 4. The frequency of aggressive encounters between rlsr~nt/esm1u Lewis and intruders at the stores at Branstines,’ compared between snowy and nonsnowy days.” ._ Snowy days Nonsnowy days

Species N EIX./hr % total N EIlC./hr % total

Lewis ’ Woodpecker 14 0.66 33.3’ 22 0.48 41.5 Flicker intergrade 7 0.33 16.7 2 0.04 3.8 Black-billed Magpie 4 0.19 9.5 11 0.24 20.8 Starling 2 0.09 4.8 7 0.15 13.2 Stellers’ Jay 10 0.47 23.8 1 0.02 1.9 Blue Jay 3 0.14 7.1 0 0.00 0.0 Oregon Junco 2 0.09 4.8 0 0.00 0.0 Belted Kingfisher 0 0.00 0.0 1 0.02 1.9 Common Crow 0 0.00 0.0 6 0.13 11.3 Unidentified small passerines 0 0.00 0.0 3 0.07 5.7

Total 42 1.97 100.0 53 1.17 100.1

il The observations on snowy days total 1278.5 bird-minutes, those for nonsnowy days total 2759.5. Enc./hr z encounters per hour.

ally not evicted from tree LW, and often compared the distribution of intensity factors flickers and conspecifics were not supplanted between “snowy” and “non-snowy” days (fig. from the top of LW on “nonsnowy” days. S). The Lewis ’ Woodpeckers were signifi- There were no days at Kings’ Center when cantly more aggressive on “snowy” days (fig. the ground was completely covered with S), and the aggressiveness of intrapair en- snow, and few days when the temperature did counters appears to correlate with the degree not rise above 0°C so a similar comparison of their reliance upon the stored mast (fig. 9). of aggressiveness was impossible there. If the frequency and intensity of aggressive A potentially good measure of intrinsic encounters that occurred between Asyndesmus aggressiveness was the intensity with which and intruders at the storage site are indic- a defending Asyndesmus responded to the ative of the degree of competition for these return of its mate to the stores from outside stores, then competition was more intense at of the storage tree. Since species and sex of Rranstines’ than at Kings’ Center (table 3, the intruder and proximity of the intruder fig. 7) ; competition was most intense when to the stores was constant in intrapair en- stores first were established in the foothills counters, differences in the responses would be due to changes in intrinsic aggressiveness of the defender. The defender never actually 100 displaced its mate from the stores, but could respond with any of the four other responses on the “ignore-attack continuum” described above. I quantified the aggressiveness of the defender in intrapair encounters by assigning t- z intensity factors of O-3 to such bouts, based w 50 upon the criteria outlined in table 5, and u” w a

TABLE 5. Behavioral characteristics used to define intensity factors of intrapair aggression, based upon the response of a Lewis ’ Woodpecker on its stores to the return of its mate. Larger intensity factors are thought to indicate higher aggression levels within 0 the pair. t-SNOWY----+ &NON -SNOWY--i Intensity factor Behavior of defendin= Lewis ’ Woodpecker FIGURE 8. Distribution of intensity factors of Lewis ’ No response. Woodpecker intrapair encounters ( stipple), and per Bill-up display, squeak notes. cent of feeding time working stores (solid black) on Wings-out display, chatter call, no “snowy” and “nonsnowy” days at Branstines.’ The movement toward mate. mean intensity was 1.17 for “snowy” days (N = 48), Circle-flight, aggressive chatter, fre- and 0.22 for “nonsnowy” days (N = 247). This quently accompanied by pseudo- difference is significant: Chi square = 45.0, d.f. = copulation. 3, P < 0.001. 220 HARLO H. HADOW

The opportunistic Lewis ’ Woodpeckers spent 99.0% of their feeding time at their stores on such days, suggesting that other foods were not available (see also the feeding section of this paper). If not available to Asyndesmus, these foods probably would also be obscured from flickers and jays, and these birds would be forced to compete with Asyndesmus for the available food: the Lewis ’ Woodpeckers ’ stores. The apparent increase in competition resulted in an increased frequency of aggres- 0 N D J F M sive encounters, probably due in part to in- (Ill) (33) (29) (40) (66) (20) creased attempts to rob the stores, and to FIGURE 9. Correlation of mean intensity factor increased tenacity on the part of the robbers; (dashed line) and the per cent of time Lewis ’ Wood- but it was due also to increased aggressiveness peckers spent working stores (solid line) at Branstines’ on the part of the defending Asyndesmus. The during the Deriod October 1969 to March 1970. same explanation would apply for the Decem- NumbYrs in parentheses are the number of intrapair ber peak in aggressive-encounter frequency at encounters upon which that months’ mean was based. Kings’ Center (fig. 7), though to a lesser degree since there was never sufficient snow (fig. 7); and competition in the foothills was to cover the ground. The inactivation of in- dynamic, becoming more intense on days sects by cold weather apparently caused some when the ground was covered with snow and increased competition with flickers for the temperatures were low (table 4). By defining stores. Bock (1970) found a similar, though competition as two individuals attempting to not so pronounced, increase in the frequency gain control of a commodity when there is of aggressive encounters during January in not enough of that commodity for both, the California, but attributed the increase in en- variability in competition can be explained. counter frequency to increased numbers of At Kings’ Center, the birds continued to amass competitors during mid-winter rather than to stores all winter (fi g. 5), suggesting that the increased aggressiveness on the part of Asyn- food supply was not exhausted or in short desmus. He used interspecific intensity fac- supply. Thus there was little competition for tors to quantify aggressiveness, while I used the corn stores, since other birds could obtain intrapair measures. Also the weather condi- corn kernels from the same locations as the tions were apparently not as harsh in Cali- pair obtained its stores. At Branstines,’ acorns fornia, and changes in aggressiveness would apparently were abundant for only a short be Iess apparent. time in the fall (fig. 5). There was not Apparently, selection has favored those enough of this commodity for all; thus there Asyndesmus that supplanted any bird or was competition. mammal that came close to the stores, but Competition at Branstines’ was most intense defended a larger area against woodpeckers. when stores were first established (fig. 7). A likely identifying characteristic of an in- This probably reflects competition for storage truding woodpecker is the vertical perching sites as well as competition for the stores, posture on the side of a tree. Birds that can since less than a sixth of the migrants estab- perch verticalIy are a greater potential threat lished stores in the Hardscrabble Creek drain- to the stores than are those that cannot, be- age (see migration section above). However, cause an acorn meat stored anywhere in the the frequency of aggressive encounters rose tree could be taken by the former, while only in November (fig. 7), the month in which those meats stored on horizontal surfaces storing began at Kings’ Center, emphasizing could be taken easily by the latter. Differ- again the correlation between amassing of ential defense of stores conserves energy since stores and aggressive encounters. the greatest effort is spent against the most There was a December peak at both places important competitors. No energy is spent on that corresponded with the period of most less important competitors unless they come inclement weather. Competition was most close enough to the stores to be a threat, and intense on “snowy” days in the foothills be- more energy is spent defending stores on cause snow obscured foods on the ground, “snowy” days than when competition is po- such as weed seeds and remaining acorns, and tentially less intense. Differential defense of the low temperatures made insects inactive. stores is certainly adaptive in view of the great WINTER ECOLOGY OF LEWIS ’ WOODPECKERS 221

TABLE 6. Response of Lewis ’ Woodpeckers to avian predators at Kings’ Center.

Predator Predator’s behavior

Sparrow Hawk (F&o sparverius) Lands in storage tree Ducked under branch Sparrow Hawk Lands in storage tree Flies to denser tree Sparrow Hawk Lands in storage tree No response Sparrow Hawk Circles above tree Alarm cry, flight Sparrow Hawk Flies throngh tree Freezes in place Sparrow Hawk Lands in storage tree Freezes in place Red-tailed Hawk ( RuteoN jamaicensis) Circles above tree Crawls under limb Marsh Hawk (Circus cyaneus) Flies nearby, low Flies up, circles Marsh Hawk Circles above tree Crawls under limb

variety of competitors and weather conditions attempts by a Sharp-shinned Hawk were that Asyndesmus faces throughout its exten- thwarted in the same fashion. Apparently, sive winter range. the keen eyesight and great maneuverability that are so adaptive for flycatching also aid PREDATION the Lewis ’ Woodpecker in escaping predation. Very little is known about predation upon Asytiesmus; the only reference I could find WINTER RANGE EXTENSION OF was one reporting that feathers from one THE LEWIS ’ WOODPECKER Lewis ’ Woodpecker were found in a regurgi- This study suggested that it was adaptive for tation pellet from a Red-tailed Hawk (Buteo adult Lewis ’ Woodpeckers that bred on the jamaicensis) , along with the remains of plains to remain resident. Competition was eight Acorn Woodpeckers, three Red-shafted less intense on the plains, due to the pres- Flickers, and assorted other birds and mam- ence of fewer species of competitors, greater mals (Fitch et al. 1946). I observed 28 pred- spacing between conspecifics, and more stor- ator-prey interactions between five species of able mast per individual. As a result, less avian predators and Asyndesmus; these are energy was spent defending stores. More summarized in tables 6 and 7. The attack by types of food were available on the plains the Coopers’ Hawk (Accipiter cooperii) and during the winter since the milder weather the first three attacks by the Sharp-shinned permitted greater insect activity, and food Hawk ( Accipiter striatus) at Rranstines’ were supplies on the ground were not obscured by definitely attempted predations. None was snow. Resident Asyndesmus spent no energy successful. on migration or the establishment of a new Lewis ’ Woodpeckers reacted in four ways winter territory, and they retained nesting to the presence of an avian predator: they sites for the next breeding season. Accipiter “froze” in place; crawled to the underside of predators in the foothills probably took wood- a branch; sought denser cover; or flew into peckers more effectively than did the buteos the air and out-maneuvered the predator. The of the plains. Asyndesmus must have been first three behaviors could occur any time that more vulnerable to predation during migra- the birds detected an avian predator, the last tion than when established on breeding or occurred only when the birds were startled wintering grounds. If it was maladaptive for at close range. The Coopers’ Hawk was adult plains-breeding Asyndesmus to migrate, within less than a meter of the Lewis ’ Wood- perhaps the reason that some did was related pecker when the latter took flight after giving to the species ’ recent range extension onto the a loud cry of “fright,” a call that was heard eastern plains of Colorado. only one other time; and the Lewis ’ Wood- Apparently, at the beginning of the 20th pecker easily out-maneuvered the hawk which century (fig. lo), Lewis ’ Woodpeckers seldom left after only one attempt. Three predation bred or wintered farther east in Colorado than

TABLE 7. Response of Lewis ’ Woodpeckers to avian predators at Branstines.’ _ Predator Predator’s behavior Asyndesmus behavior N

Cooper’s Hawk ( Accipiter cooperii) Attempted predation Alarm, circles tree 1 Sparrow Hawk (F&o sparuerius) Flies by, SO m away Flight, circles tree 1 Sharp-shinned Hawk ( Accipiter striutus ) Attempted predation Circles tree 3 Sharp-shinned Hawk Flies over tree No reaction 1 222 HARLO H. HADOW

River valley 20 miles NE of Denver, citing this as evidence of a range extension. He found one Asyndesmus wintering in Denver (Bergtold 1920), the first in 8 years of winter observation. Henderson (1920) found one Asyndesmus nest in the Arkansas valley near Boone, Colorado, in June 1917, and cited it as further evidence for a range extension. I found Asyndesmus resident in large numbers on the plains near Ordway, Colorado, from 1969 through 1972, and with C. E. Bock and others saw over 100 in the “cedars” of south- western Baca County [actually a Gambels’ oak, pinyon pine (Pinus edulis), juniper com- plex] in April and October 1971, and in March 1972. We also found six Asyndesmus winter- ing on the outskirts of Lamar, Colorado, in October 1971. I feel that the preceding two paragraphs constitute evidence for a large extension, at least in winter range, since the turn of the century. The maturation of cottonwoods around ranch buildings, which provided stor- NEW MEXICO OKLAHOMA age and nesting sites, and irrigated corn, which provided storable mast, probably were 8 qLW PRIOR 1910 @ =LW POST 1910 at least part of the cause. We found that

@ =RH PRIOR 1910 @ =RH POST 1910 Asyndesmus bred almost exclusively in cotton- wood clumps around ranch buildings (Bock FICURE 10. Location of some Lewis ’ (LW) and et al. 1971), but only if the trees contained Red-headed Woodpecker (RH) sitings on a map of dead branches or rotten areas where nest southeastern Colorado. The locations were cited in holes could easily be excavated. Asyndesmus the following studies: ( 1) this study (Kings’ Cen- ter); (2) this study (Branstines);’ (3) Dille, 1903; is poorly adapted for drilling (Spring 1965; (3) Richards, 1908; (5) Hersey and Rockwell, 1909; Bock 1970). Corn is an essential component ( 6) Warren, 1906; this study; (7) Henderson, 1920; of the winter habitat because it is the only (8) Bergtold, 1919; (9) Bergtold, 1920; (10) C. E. storable mast found east of the foothills in the Bock, pers. comm., about 1970. Arkansas valley. While I feel that Asyndesmus has only re- the eastern Rocky Mountain foothills. In cently become resident on the plains of south- Colorado Asyndesmus were common near eastern Colorado, there are two other possi- Durango (Gilman 1907), in Mesa County bilities: ( 1) that Asyndesmus resided at their (Rockwell 1908), near Steamboat Springs present locations, but were overlooked by ob- (Warren 1908), and in southeastern Montrose servers in the early 1900s; or (2) that current County (Warren 1908). Hersey and Rockwell populations of Asyndesmus represent tempo- (1909) found no Lewis ’ Woodpeckers in rary concentrations in areas of high food Adams County, but Red-headed Woodpeckers density which were absent prior to about were common summer residents. Dille ( 1903) 1920. The first of these possibilities seems un- and Richards (1908) found them breeding likely because populations such as I have commonly in coniferous forest down to about described are very conspicuous, especially 5500 ft elevation in the eastern foothills of the when the birds are flycatching. Isolated Rockies. Warren (1906) found no Asyndesmus breeding pairs of Lewis ’ Woodpeckers might in Baca County, Colorado, at Lamar, Spring- have been overlooked on the edge of riparian field, or in the “cedars” at the extreme south- woodlands, but they could not have wintered western edge of the county. there until recent agriculture provided corn Asyndesmus apparently began to extend its for mast stores. Concentrated winter popula- breeding range eastward onto the plains along tions in the riparian woodlands along the the Platte and Arkansas river drainages soon Platte and Arkansas rivers prior to modern after 1910 (fig. 10). Bergtold (1919) found agriculture were unlikely for the same reason. a Lewis ’ Woodpecker nesting in the Platte Although the existence of temporary breeding WINTER ECOLOGY OF LEWIS ’ WOODPECKERS 223 concentrations is hard to disprove, it seems (1919) found them breeding in telegraph unlikely that such concentrations would have poles in New Mexico and suggested that they gone unnoticed, or that riparian woodlands might have dispersed along the route of the would be suitable habitat for such concen- railroads across the open plains to near Albu- trations (Rock et al. 1971) . querque, New Mexico. We found them to be There were probably a few transients and quite common near Ordway, Colorado (Bock perhaps a few breeding birds in the area prior et al. 1971), but they bred almost exclusively to the range extension, since there are a few in riparian woodlands where Asyndesmus was specimens from western Kansas (Tordoff absent, and never in small cottonwood clumps 1956) and Cimarron County, Oklahoma around ranches which were preferred Asyn- (Sutton 1967) collected during the 1920s. desmus habitat. Asyndesmus had the advan- Transients are not unknown for the Lewis ’ tage in nest-site competition because residents Woodpecker, as one was found wintering in maintained nesting sites all year; migrant central Wisconsin in January 1972 (Donald Asyndesmus established nesting territories 1972). This also probably explains the female before migrant Red-headed Woodpeckers ar- specimen in the University of Kansas Museum rived; and breeding Red-headed Wood- (KU 7890), taken near Lawrence, Kansas, in peckers were almost always subordinant to November 1908 (Tordoff 1956) and other Asyndesmus in aggressive encounters (Bock Kansas specimens from this period. et al. 1971). These circumstantial lines of The situation may be quite different in the evidence suggest that Asyndesmus has dis- “cedars” of southwestern Baca County, Colo- placed M. erythrocephalus from the uplands rado. This area is immediately adjacent to to the riparian vegetation where flycatching Cimarron County, Oklahoma, where Sutton is difficult, thus placing the former at a (1967) reported scattered sightings of Lewis ’ disadvantage. Woodpeckers “during all seasons from 1921 to 1931.” Those birds, like those which I SUMMARY studied in Baca County, could have been resi- Lewis ’ Woodpeckers were partially migratory dent since they could use acorns for winter in southeastern Colorado. About half of the mast stores. Cottonwoods. present in thin birds that bred on the plains remained resi- stands along the small streams of the region, dent, storing corn kernels during the winter. would have provided more suitable nesting The others migrated west to the foothills of habitat for Asyndesmus than would the the Rocky Mountains in October and amassed dense riparian woodland along the Platte stores of Gambels’ acorns. Birds in both loca- and Arkansas rivers. Asyndesmw numbers tions defended their stores both inter- and have been shown to fluctuate dramatically, intraspecifically. probably in response to fluctuating acorn Plains-wintering Asyndesmus relied less crops (e.g., Bock 1970; migration section, this upon their stores as a source of winter food study). This may explain why Warren ( 1906) (% vs. 74 of total feeding time) and more found no Asyndesmus in Baca County even upon flycatching and gleaning for insects. though they were in nearby Cimarron Probably insects were the preferred food at County, Oklahoma, less than 20 years later. both locations, but were more frequently Possibly, Asyndesmus populations in south- available on the plains than in the foothills eastern Colorado are derived from two due to milder weather. Corn kernels were sources: the eastward movement along the available and stored all winter on the plains; major east-west river drainages; and a north- acorns were seldom stored after November in east movement which originated in New the foothills. Mexico and spread north and east through Competition for the Lewis ’ Woodpeckers’ the northwest tip of Oklahoma into Baca mast stores, quantified by measuring the fre- County, Colorado. quency and intensity of aggressive encounters The range-extension of Asyndesmus along at these stores, was more intense in the foot- the Platte and Arkansas rivers appears to hills than on the plains. There were more have restricted the breeding range of the species of competitors (10 vs. 4), greater fre- ecologically similar Red-headed Woodpecker quency of aggressive encounters (1.41 vs. in southeastern Colorado. The latter appar- 0.26 per hr), and greater density of con- ently bred commonly on the plains in the specifics in the foothills. Competition was early 1900s (Warren 1906; Hersey and Rock- intense when stores were first established in well 1909), and into the transition zone of the foothills, and during December at both the eastern Rockies (Dille 1903). Leopold locations. Aggressive-encounter frequency was 224 HARLO H. HADOW

higher on “snowy” days than on “non-snowy” BOCK C. E., HAWW, H. H., AND P. SOMERS. 1971. days in the foothills because of an increased Relations between Lewis ’ and Red-headed Wood- rate of intrusion by heterospecifics and in- peckers in southeastern Colorado. Wilson Bull. 83~237-248. creased aggressiveness of the defending Asyn- DILLE, F. M. 1903. Nesting dates for birds in the clesmus. Lewis ’ Woodpeckers responded more Denver District, Colorado. Condor 5:73-74. aggressively to flickers and conspecifics than DONALD, M. 1972. The Badger Birder (monthly to intruders. Differential aggres- newsletter of the Wisconsin Society for Ornithol- ogy), no. 103. February, 1972. siveness in defense of stores is adaptive since FITCH, H. S., SWENSON, AND D. F. TILLOTSON. 1946. the greatest energy expenditure is made de- Behavior and food habits of the Red-tailed fending stores against the greatest potential Hawk. C#ondor 48:205-237. competitors, and under the conditions of most GILMAN, M. F. 1907. Some birds of southwest intense competition. Colorado. Condor 9: 152-158. HENDERSON, J. 1920. Further Colorado notes. It would seem adaptive for Lewis ’ Wood- Condor 22 ~42. peckers nesting on the plains to remain resi- HERSEY, L. V., AND R. B. ROCKWELL. 1909. An dent because of milder weather, reduced annotated list of the birds of the Barr Lake dis- competition, increased food availability, and trict, Adams County, Colorado. Condor 11: 154- reduced energy expenditure in migration and 164. HOWELL, T. R. 1953. Racial and sexual differences the establishment and defense of winter terri- in migration in Sphyrapicus varies. Auk 70:118- tories. Approximately half of a population of 126. Asyndesmus breeding near Ordway, Colorado, KILHARI, L. 1958a. Sealed-in winter storeIs of Red- was migratory, and this may be related to a headed Woodpeckers. Wilson Bull. 70: 107-113. range extension of this species eastward along KILHAM, L. 1958b. Territorial behavior of winter- ing Red-headed Woodpeckers. Wilson Bull. 70: the Arkansas River. This extension may have 347-358. occurred since the turn of the century, per- LEOPOLD, A. 1919. A breeding record for the Red- mitted by conditions that include the matura- headed Woodpecker in New Mexico. Condor tion of cottonwoods around ranches providing 21: 173-174. LIGON, D. J. 1970. Behavior and breeding biology nesting and storage sites, and the cultivation of the Red-cockaded Woodpecker. Auk 87:255- of corn providing storable mast for the birds. 278. Competition between Lewis ’ and Red-headed MACROBERTS, M. H. 1970. Notes on the food Woodpeckers, the result of contact following habits of the Acorn Woodpecker. Condor 72: the range extension, seems to have reduced 196-204. ORIANS, G. H., AND M. F. WILLSON. 1964. Inter- the breeding range of the latter in the area specific territories of birds. Ecology 45:737-745. of contact to riparian woodlands where the RICHARDS, G. 1908. An unusual nesting locality for former is at a disadvantage because flycatch- the Rockv Mountain Nuthatch. Condor 10:194- ing is difficult in dense vegetation. 197. ’ RITTER, W. E. 1921. Acorn-storing by the Cali- fornia Woodpecker. Condor 23:3-14. ACKNOWLEDGMENTS ROCKWELL, R. B. 1908. An annotated list of the I thank the following people for their contributions birds of Mesa County, Colorado. Condor 10: to this study: C. E. Bock for assistance throughout; 152-180. R. E. Tones and Olwen Williams for readinz and SI-IORT, L. L. 1971. Systematics and behavior of criticizing the manuscript; and Mrs. Branstine for some North American Woodpeckers, genus coffee, cookies, and hospitality, especially during the Picoides ( Aves). Bull. Mus. Nat. Hist. 145:1- “snowy” days at Wetmore. 118. SMITH, C. F. 1941. Lewis ’ Woodpecker migration. LITERATURE CITED Condor 43 :76. SPRIX, L. W. 1965. Climbing and perching ADAMS, L. 1941. Lewis ’ Woodpecker migration. adaptations of some North American wood- Condor 43:119. peckers. Condor 67:457-488. BAILEY, A. M., AND R. J. NIEDRACH. 1967. Pictorial SUTTON, G. M. 1967. Oklahoma birds; their ecol- checklist of Colorado birds. Den. Mus. Nat. ogy and distribution with comments on the Hist., Denver, Colo. 168 p. avifauna of the southern Great Plains. Univ. BALPH, D. F., AND A. W. STOKES. 1963. On the Okla. Press, Norman. 674 p. ethology of a population of Uinta ground squir- TOR~OFF, H. B. 1956. Check list of the birds of rels. Amer. Midland Nat. 69:106-126. Kansas. Univ. Kansas Publ. Mus. Nat. Hist. 8: BERGTOLD, W. H. 1919. Denver region. Bird Lore 307359. 21:370. WARREN, E. R. 1906. A collecting trip to south- eastern Colorado. Condor 8: 18-24. BERGTOLD, W. H. 1920. Denver region. Bird Lore WARREN, E. R. 23: 108. 1908. Northwestern Colorado bird notes. Condor 10: 18-26. BOCK, C. E. 1970. The ecology and behavior of the Lewis ’ Woodpecker ( Asyndesmus lewis) . Univ. California Publ. Zool. 92: l-100. Accepted for publication 11 August 1972.