Geology topics

B.A. Dowell

Publications and source records attributed to B.A. Dowell.

21 records · Page 2Linked to original sources

USGS1987

Comparison of neotropical winter bird populations in isolated patches versus extensive forest

Wintering birds were captured with mist nets at 12 pairs of forested sites in the New World tropics in 1984 and 1985 to compare populations in small isolated woodlands (generally 5-50 ha) with those in extensive forests (> 1,000 ha). Net-hours of effort were similar in large and small sites, as were total birds captured and banded, but species composition was very different. Members of the Todidae, Dendrocolaptidae, Formicariidae and Thraupinae were significantly more common in extensive forest than in small isolated tracts, indicating that these birds are especially vulnerable to effects of forest fragmentation. However, in winter many species of North American migrants, even species that are restricted to extensive forest during the breeding season, were just as common in small forest fragments as in extensive forest. A high percentage of the North American migrants banded in January 1984 (40 to 50 % for some species) were recaptured in 1985.

Acta Oecologica Oecologia Generalis↗

USGS1987

Improving atlas methodology

We are studying a sample of Maryland (2 %) and New Hampshire (4 %) Atlas blocks and a small sample in Maine. These three States used different sampling methods and block sizes. We compare sampling techniques, roadside with off-road coverage, our coverage with that of the volunteers, and different methods of quantifying Atlas results. The 7 1/2' (12-km) blocks used in the Maine Atlas are satisfactory for coarse mapping, but are too large to enable changes to be detected in the future. Most states are subdividing the standard 7 1/2' maps into six 5-km blocks. The random 1/6 sample of 5-km blocks used in New Hampshire, Vermont (published 1985), and many other states has the advantage of permitting detection of some changes in the future, but the disadvantage of leaving important habitats unsampled. The Maryland system of atlasing all 1,200 5-km blocks and covering one out of each six by quarterblocks (2 1/2-km) is far superior if enough observers can be found. A good compromise, not yet attempted, would be to Atlas a 1/6 random sample of 5-km blocks and also one other carefully selected (non-random) block on the same 7 1/2' map--the block that would include the best sample of habitats or elevations not in the random block. In our sample the second block raised the percentage of birds found from 86% of the birds recorded in the 7 1/2' quadrangle to 93%. It was helpful to list the expected species in each block and to revise this list annually. We estimate that 90-100 species could be found with intensive effort in most Maryland blocks; perhaps 95-105 in New Hampshire. It was also helpful to know which species were under-sampled so we could make a special effort to search for these. A total of 75 species per block (or 75% of the expected species in blocks with very restricted habitat diversity) is considered a practical and adequate goal in these States. When fewer than 60 species are found per block, a high proportion of the rarer species are missed, as well as some of the common ones. Similarity indices based on fewer than 60 species per block reflect coverage rather than habitat affinities. Atlas blocks that are ecologically similar should have similarity indices (S) of at least 0.80 to be considered adequately covered. S = 2C/(A + B), where C is the number of species in common and A and B are species totals for each of the two blocks being compared. A series of 15 13-minutes roadside stops yielded more species than 15 off-road stops, but off-road stops always had some species not detected at the roadside stops. A series of timed stops is an excellent way to map relative abundance if the stops are standardized with respect to time of day and weather, and the counts are made by observers of comparable ability. Efforts to estimate Atlas block populations in powers of 10 (as in the French Atlas) have not gained acceptance in U.S.A. Most observers feel unqualified to make estimates. An efficient way to Atlas a block is to make at least 3 early morning visits to 15 or more specific stops. Arrive in the block early enough to check for nocturnal species on at least two days; and after completing the specific stops, search the block for other species and for confirmations.

Acta Oecologica Oecologia Generalis↗

USGS1986

Use of miniroutes and Breeding Bird Survey data to estimate abundance

1. Information on relative abundance is easily obtained and adds greatly to the value of an atlas project. 2. The Breeding Bird Survey (BBS) provides annual counts (birds per 50 roadside stops) that can be used to: (1) map relative abundance by physiographic region within a state or province, (2) map relative abundance on a more local scale by using results from individual routes, or (3) compute estimates of total state populations of a species. Where BBS coverage is too scanty to permit mapping, extra temporary routes may be established to provide additional information for the atlas. Or, if continuing coverage is anticipated, additional permanent random routes can be assigned by the U. S. Fish and Wildlife Service. 3. Miniroutes of 15 or more stops can be established in individual atlas blocks to serve the dual purposes of providing efficient uniform coverage and providing information on relative abundance. Miniroutes can also be extracted from BBS routes to supplement special atlas coverage, or vice versa; but the data from the BBS will not be confined to individual atlas blocks. 4. Advantages of 15- or 20-stop Miniroutes over 25-stop Miniroutes are several: the ability to do two per morning and the lower variability among M1niroute results. Also, many 5-km atlas blocks do not have enough secondary roads to accommodate 25 stops at one-half mile intervals. Disadvantages of 15-stop Miniroutes starting at sunrise are the smaller numbers of birds recorded, missing of the very productive dawn chorus period (Robbins 1981), and missing crepuscular species (rails, woodcock, owls, and goatsuckers). 5. Advantages of recording counts of individuals rather than checking only species presence at Miniroute stops are that: (1) relative abundance can be mapped rather than frequency only (a measure of frequency is already available in the number of blocks recording each species); (2) population change can be measured over a period of years when the next atlas is made; and (3) comparative abundance data are available for habitat correlations and other statistical applications (Luis et ale 1983). Disadvantages of recording counts are that: (1) many observers do not feel they can make accurate counts of individuals; and (2) a few species may be missed while observers are counting (an hypothesis that will be field-tested this summer). While counts of individuals may be more subject to observer differences than are frequency counts, both the numbers of birds and their frequencies are available when birds are counted. 6. Roadside counts produce slightly larger samples and more species than off-road counts. The main advantage of off-road coverage is to sample habitats that cannot be sampled from roadsides. 7. Miniroutes can be recommended to individual atlasers as an efficient means of detecting species and upgrading them from Possible to Probable, even when there is no statewide Miniroute program.

Book chapter↗