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Amateur Naturalist: What Is Happening To The Birds?

on January 3, 2019 - 9:01am
An American robin prepares to drink some water. Photo by Bob Walker
A Broad tailed hummingbird shows why it received its name. Photo by Bob Walker
Los Alamos
There are three sources of information about the bird populations of Los Alamos county.
Each has its own approach to gathering data. One approach considers where and what kinds of birds were breeding throughout the county. This involves dividing the county in to 60 blocks and visiting the blocks over four years.
A second approach involves people reporting the numbers and kinds of birds that come to bird feeders. The counts are done weekly over six months of the year when birds may be migrating in the fall or spring as well as living in the area in the winter. This approach has been in use for 29 years and is continuing.
A third approach involves comparing birds in two specific kinds of habitats for 10 years. One kind of habitat involves areas where trees had been thinned to reduce or prevent future forest fires while the second habitat involves areas that had not been thinned.
The breeding bird survey has resulted in 112 species being identified in 12 kinds of habitats. The habitats range from mountain tops to the river valley. The feeder watch approach has identified 95 species that come to residential/urban areas in search of food. The un-thinned/thinned approach has identified 59 species in what is primarily a juniper/pinon habitat. The three approaches have 26 species in common.
TABLE A shows the distribution of the 26 species among ten of the habitats. A species may be common among all the habitats in terms of nesting. Alternatively, a species may nest primarily in one kind of habitat and so is identified with it. (Note that “primarily” does not mean “exclusively”. Species move about the county in search of food and places to rest.) TABLE A shows that six species are common to all the habitats for breeding purposes while another six species are primarily in the pinon/juniper habitat.
TABLE A: Distribution of Twenty-Six Species Among Ten Habitats
Habitat/Number of Species
All Habitats Combined 6
Pinon/juniper 6
Low Forest 3
Mid Forest 2
High Forest 3
Mixed & Ponderosa 1
Spruce 1
Marsh 1
Cliff 1
Rio Grande 2
TABLE B shows how frequently each of six species in the All Habitats category have been seen at feeders for 27 years. Broad tailed hummingbirds represent one extreme. Hummingbirds may breed throughout the county but are not commonly seen near feeders. Only 4 hummingbirds have seen. Hummingbirds do not remain for the winter, migrating away in the fall and return in spring. The American robin in contrast has been seen 2,049 times at feeders as well as breeding throughout the county.
TABLE B: Frequency count at feeders for species breeding throughout the county
American Robin 2,049
Broad-tailed Hummingbird 4
Northern Flicker 762
Chipping Sparrow 59
Common Raven 531
Western Bluebird 179
Total 3,584
TABLES A and B represent summary snapshots. What has been happening from year to year? Is there a trend over the 27 years of reporting on the birds coming to feeders?
CHART A shows that there has been an increase in the number of birds seen annually from 1990 to 2017. The dotted regression line shows a long-term trend in which the average number of birds seen annually per viewing location has increased from about 20 in the early 1990’s to about 90 in the late 2010’s. This long-term trend line can be divided into two sections. There is little increase from 1990 to 2006. Most of the increase occurs thereafter.
However the Cerro Grande forest fire in 2000 may have contributed to a decrease in the following year. The Las Conchas forest fire in 2011 in contrast may have contributed to an increase. These downward/upward changes are shown in red on the trend line. Draught conditions after 2006 may be the major long-term contributor to change rather than the forest fires.
CHART A shows the combined pattern of the six bird species together. But are they all following the same pattern? Bird species that can breed in variety of habitats may also follow a variety patterns for how frequently that they are seen from year to year. CHARTS B, C and D show three differing patterns.
The dotted regression line in CHART B shows that the American Robin has been seen more frequently over the years. An average of about 5 robins were seen annually in the early 1990’s at feeder locations. This average increased to about 23 robins in the 2010’s. The number of robins declines in the years after the fires. A peak of an average of 59 birds per location occurred in 2011 and then fell to 5 birds over the next three years.
CHART A and B look similar since robins dominate the number of birds seen over the years. The 2,048 robins reported in TABLE A represent 57 percent of the 3,584 birds seen.
The regression line in CHART C shows that the number of common ravens has remained nearly the same from year to year. An average of 8 to 11 are seen each year. The trend line shows a dramatic increase to 60 ravens in 2012, the year after the Las Conchas forest fire in 2011. (This kind of increase is called an irruption.) The trend then decreases to be similar to the other years. The pattern for ravens is the opposite for robins. The increase in ravens in 2012 more than offsets the decrease for robins, resulting in the overall increase from 2011 to 2012 as shown in CHART A.
The Northern flicker provides a third variation. The trend line in CHART D shows that the number of flickers seen annually was very low in the 1990’s. Only 1 to 4 flickers were seen on the average at feeder locations. The number increases to about 12 seen annually in 2007. It then starts to about 4 being seen annually in 2017.
The dotted regression line shows a long-term peak developing in 2007 about half way between the 2000 and 2011 forest fires. It appears that the forest fires have some impact for flickers in the years immediately after fires but not like what occurs for robins and ravens.
Robins increased in numbers in 2007, copying the increase for flickers. Although much less common, Chipping sparrows and Western blue birds have a similar peak 2007-8. Four of the six species together create the second highest peak in 2007 as shown in
CHART A. Why did the raven not follow this pattern for 2007? Why is the raven the only species to increase after 2011 forest fire rather than decrease?