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EFFECTS OF SPRING LEAF-FALL ON COMPOSITION AND DENSITY OF BREEDING BIRDS IN TWO SOUTHERN ARIZONA WOODLANDS RUSSELL P. BALDAl Department of Zoology University of Illinois Champaign, Illinois 61820 Nesting bird composition and population den- tailed information on these plant associations sities reflect the suitability of a given plant in southern Arizona may be found in Shreve community for providing the requisites for ( 1915), Wallmo ( 1955), Marshall ( 1957), and survival and reproduction, and also, the selec- Lowe (1961,1964). tion of these suitable habitats by the birds. The oaks mentioned above are often re- Two important features in habitat selection ferred to as “live” oaks because they retain are availability of food and nest-sites (Hilden ’ their leaves throughout the winter months. 1965). Open-nesting species depend on foli- Marshall (1957) noted that oak leaves turned age for concealment and protection of the nest brown and/or were dropped during drought from harsh weather and predators (Nice 1957). conditions, and that new leaves appeared More than ample foliage for nest-sites is usu- when more mesic conditions followed. Shreve ally available to these species, whereas a major (1915) mentioned that these oaks shed most, problem confronting hole-nesting species is if not all, leaves in late April or May. A bisea- locating suitable sites (von Haartman 1957). sonal rainfall pattern exists in this area. Heavy Tree-foliage nesting birds of the oak wood- leaf-fall occurs during a spring drought period lands in southeastern Arizona may be limited which follows the winter rainy season. Sum- in the same manner as hole-nesters because mer rains usually begin in late June or early a portion of the tree foliage is absent during July. a period in spring and summer when most Few observations have been made on the birds initiate nesting. birds of these woodlands except by Marshall These woodlands of the desert mountains ( 1957), who analyzed the pine-oak avifauna, can be subdivided into two plant associations: and Balda ( X%9), who studied foliage use evergreen or live oak woodland, and pine-oak by birds. Marshall concluded that both oak woodland. The oak woodland lies on gentle woodland and pine-oak woodland are climax slopes and flat sandy plains below the pine-oak communities, but that the avifauna of the woodland. The lower limit of oak woodland is pine-oak woodland is a unique mixture of bird partly dependent on slope exposure and soil species from the oak woodland below and type but it is generally at about 4509 ft, where ponderosa pine forest above. He did not com- this woodland merges with grassland, forming ment on the effects of spring leaf-fall of the a savanna. The upper limit of the pine-oak oaks on the breeding birds of either woodland. woodland occurs at about 7800 ft, where pon- The pattern of leaf-fall, flowering, and ap- derosa pine (Pinus ponderosa) becomes the pearance of new oak leaves exerts two prin- dominant and predominant tree. These, or cipal effects on the diversity and density of very similar woodlands, extend well into breeding birds in these two woodlands in the Mexico (Leopold 1950). In the Chiricahua Chiricahua Mountains of Arizona. First, the Mountains, Cochise County, Arizona, Emory loss of oak leaves may result in a loss of poten- oak (Quercus emoryi) and Arizona oak (Q. tial nest-sites for birds that nest in tree foliage. a&o&a) predominate. The pines of the Second, the flowering and growth of new higher woodland are Chihuahua pine (Pinus leaves attracts large numbers of insects that serve as an abundant source of food for birds leiophyllu) and Apache pine (Pinus engel- that nest in other sites. mannii), with a scattering of ponderosa pine. The woodlands also contain relatively high METHODS AND MATERIALS densities of various shrubs and succulents. Observations were made in two woodlands of the Alligator juniper (Juniperus ckppeanu) varies Chiricahua Mountains in June 1964 and February- in abundance in the two woodlands. More de- August 1965. Detailed plant sampling in each wood- land was done in conjunction with a bird censusing program to obtain an understanding of some of the 1 Present address: De arhnent of Biological Sciences, North- ern Arizona University, P lagstaff, Arizona 86001. basic plant-bird relationships that exist in these wood- TheCondor, 72:325-331, 1970 r32.51 326 RUSSELL P. BALDA FIGURE 1. The oak woodland study area showing well scattered Quercus enroryi in full leaf during the month of February. lands. Trees three or more inches in DBH were sam- RESULTS AND DISCUSSION pled with the point-quarter method (Cottam and Cur- tis 1956). A total of 30 points (120 trees) was sam- OAK WOODLAND pled in both woodlands. Detailed notes were kept This 36-acre study area was within the Chiri- on the growth and flowering of all predominant plants. Birds were censused with the spot-map cahua National Forest and was used for graz- method ( Kendeigh 1944) to obtain an estimate of the ing by the local rancher. The plot was 9 mi. absolute breeding population of each species. Den- SW of Apache, New Mexico and 0.8 mi. NW sities of birds are expressed as numbers of breeding of the Glass Ranch on an unpaved road lead- pairs per 100 acres. Non-territorial species were. cen- sused on the basis of average number seen. The areas ing into Price Canyon. were thoroughly searched for nests, and details such Vegetation. The vegetation was park-like in as nest location, time of egg laying, etc., were re- appearance, with scattered trees of almost uni- corded. Eleven complete daylight counts and four night counts were taken in the oak-juniper-pine wood- form height (fig. 1). There were 68 trees per land study plot, and 10 daylight and three night acre, with Quercus emoyi predominant (table counts were made in the oak woodland. Data pre- 1). The absolute density of oaks was 66 indi- sented herein were analyzed primarily with reference viduals per acre. The shrubby and succulent to the numbers of pairs and species that place their nests in various locations. understory of this area contained predomi- TABLE 1. Tree species composition, density, and dominance of the two woodlands in the Chiricahua Moun- tains. Oak woodland Oak-juniper-pine wnodland R&t~$ Relative ye;; Relative SDecies dominance ’ dominance Quercus emolyi 95.0 96.5, 20.8 10.7 Quercus arizonica 2.9 2.7 17.4 11.0 Juniperus deppeana 1.4 0.5 33.3 GO.1 Juniperus monosperma 0.7 0.2 - Pinus leiophyllu - - 21.7 lF9 Pinus cembroides - - 1.8 0.3 EFFECTS OF SPRING LEAF-FALL ON BREEDING BIRDS 327 nately the following: Fallugia paradoxa, foliage nesters, two nested in trees and two in Brickellia sp., Rhus microphyllu, Calliandra shrubs and succulents. sp., and Nolina microcarpa. Total shrub and Three important tree-foliage nesters (con- succulent density was 308 individuals per acre. tributing 41 per cent of nests in tree foliage) The phenology of oak leaf-fall, and replace- apparently were able to nest successfully in ment is shown in figure 2. The two species of the area because of their ability to use the oaks which contribute 99 per cent of the foliage dense foliage of Juniperus deppeanu for at (see dominance figures, table 1) in this com- least a portion of their nest-sites. The Chip- munity were in the process of replacing their ping Sparrow had three of five nests in juniper. leaves during most of the spring and early Five of six nests of the Black-throated Gray summer. It is of interest to note that Quercus Warbler were located in juniper. The sixth emoryi did not complete new leaf growth un- nest was built in Quercus emoryi, but the in- til the advent of the summer rains in July. cubating female deserted shortly after the Breeding birds. This plant community sup- leaves fell. Three of seven nests of the Black- ported 36 species of nesting birds, with a total chinned Hummingbird were in juniper. A density of 224 pairs per 100 acres (table 2). female Mexican Jay whose nest was located in The four most abundant species (Bewicks’ an oak also deserted after incubating for three Wren, Ash-throated Flycatcher, Bridled Tit- days, during which time the nest was com- mouse, Lark Sparrow) nested either in natural pletely exposed as leaf-fall was complete. cavities, old woodpecker holes, or on the Thus, some species attempted to nest in oak ground. at the very time leaf-fall was underway, and In most areas where cavity nesting species there were strong indications that many failed. are common, the holes are provided by wood- It is also of interest to note that the number peckers. In this area, however, woodpeckers of singing male Chipping Sparrows on the and old woodpecker holes were noticeably area until the second week in April indicated rare. Most cavities were located at decayed a nesting density of 16 pairs per 100 acres, or places on oaks where large branches had six more than actually nested there. One male broken off the trunk. Some of the oaks sam- Western Wood Pewee and one Gray Vireo pled in this area had hollow trunks because of also appeared to leave established territories heartwood rot. Thus, after the branch stub about mid-April. It is possible that these birds rotted away, potential nest-sites became avail- deserted their territories when oak leaf-fall able but certainly not plentiful or obvious. Of occurred. Thus, some individuals may leave the 24 apparently suitable cavities located in their territories without attempting to nest the study plot and permanently marked and in- once leaf-fall is underway.