Discerning the Relationship Between Cardinalis cardinalis Populations and Water Proximity in McKinney Falls State Park
Ellington Tough, David Podbielski, and Owen Frank
Liberal Arts and Science Academy, Austin, TX
Spring 2025
Abstract
This study surveyed Northern Cardinal (Cardinalis cardinalis) populations in McKinney Falls State Park, a vegetated area in the Southeast of Austin, Texas. Three transects between 900 and 1010 meters with widths of 250 meters were established along walkable paths in the park, covering 72 hectares in total. Researchers walked these transects, recording observations of Northern Cardinals to determine how many were present in each transect. The study originally hypothesized that more Northern Cardinals would be found near Onion Creek, the only significant source of water in the area, than far from it, an expectation supported by its findings. Although a consistent correlation between population and proximity to water is not conclusively enforced by these data, which report higher populations far from water than at a medium distance from water, it is strongly supported when accounting for observation factors. No correlation is acknowledged between population estimates and external factors, meaning population significantly relates only to water proximity. However, temperature does correlate with birds singing more than calling, likely because of cardinals’ breeding preferences. Human activity correlates with an increase in observations of cardinals by sound rather than by sight, likely because they are temporarily leaving the areas closest to the trail.
Introduction
Purpose
This study aims to identify any correlation between distance to water and Northern Cardinal populations in McKinney Falls State Park, Austin, Texas.
Hypothesis
This study hypothesizes that the population of Northern Cardinals have a positive correlation with distance to water.
Background Information
The Northern Cardinal, or Cardinalis cardinalis, is a songbird that is known for its bright red plumage which occurs in its males. Other identifying features of the cardinal include a distinctive crest and black mask (Britannica). They also have a short and thick orange bill and a relatively long tail (Britannica). The bird’s numbers have grown by about 0.32% each year since 1966 (North American Breeding Bird Survey) to a current total population of approximately 130 million (Partners in Flight Databases). Their range has been expanding from the southern United States and Mexico to more parts of the U.S. (Scenic Hudson). Northern Cardinals have been found to exist in greater abundance in cities than in rural areas, highlighting the significance of this study. This is because invasive fruits such as the Amur Honeysuckle abound in cities and supply them with Vitamin C (Staengl, Ezra & Vondrasek, Joanna, 2024).
The invasive species near the creek should not present themselves as a possible dependent variable to additionally affect and correlate birds’ closeness to water, as no correlation has been found between location and the percentage of cardinalis’ diet consisting of invasive species. This ratio remains constant between urban and rural parks and likely remains constant between areas adjacent to bodies of water and areas distant from bodies of water (Staengl, Ezra & Vondrasek, Joanna, 2024).
There exist several subspecies within the Cardinalis genus and cardinalis species, which are spread throughout North America. The subspecies cardinalis is the focus of this study and is found throughout the eastern United States and most of Mexico, with a range stretching along its gulf until the Yucatan Peninsula and as far inland as Chihuahua (Smith et al., 2011).
This same subspecies is omnivorous, feeding on mainly plants during the winter and insects during the breeding season. Its lack of migration means it must be able to find and digest a range of food sources (Staengl, Ezra & Vondrasek, Joanna, 2024). This may distribute the population of Cardinalis cardinalis throughout a geographic region as individual birds avoid each other’s competition. Within the area of this study, however, the water-adjacent areas can be clearly ecologically distinguished from those farther away from water. Additionally, water has the effect of supporting insects and plants and increasing their populations.
Emberizoids like the Northern Cardinal have been found to be cognitively advanced and adept at colonizing new areas and then growing in population (Lefebvre, et al, 2016). From this it is inferred that the bird of this study’s focus should be an opportunistic species able to take advantage of varying landscapes and water availability. This equalizing factor may contribute to similar population densities of Northern Cardinals between water-adjacent areas and non-water-adjacent areas.
Now, the total geographic range of the Northern Cardinal species is from southern areas of the coast of the Gulf of Mexico to the Eastern coast and Northern border of the United States. The range goes as far west as Southern Arizona, Northern Mexico, and most of Texas. The Northern Cardinal's preferred habitat is “backyards, parks, woodlots, and shrubby forest edges” (All About Birds). Northern Cardinals like to sit low in shrubs and trees where they can forage near or on the ground. They are also known for traveling in pairs (All About Birds). The Northern Cardinal has a melodious song that people identify by the mnemonic, “birdie, birdie, birdie”. Their call is a loud, metallic chip sound (All About Birds).
It has been found that Cardinals have a higher density of population near riparian vegetation (Dow, 1970). Riparian vegetation is on wetlands or near banks of rivers, showing that Northern Cardinals are more likely to inhabit areas close to water. According to the study, rivers can act as guidelines when cardinals disperse from the nest they were born in. Because of this many individual birds will end up close to rivers when they create their nests, making their population higher in riverside areas. The sedentary nature of cardinals implies that despite natal dispersal of a few hundred feet, they should remain around these riparian areas.
Site Description
Location
McKinney Falls is located in southeast Austin 4.9 kilometers west of the southernmost point of the Austin-Bergstrom International Airport (Google).
Size and Shape
The area of McKinney Falls State Park is 259.40 hectares (Visit Austin). The park is shaped similarly to a vertically aligned rectangle.
Topography
The maximum elevation of McKinney Falls State Park is 199 meters. The minimum elevation of the park is 143.26 meters (Peakbagger). The park is generally flat but slopes towards Onion Creek. The slope is steepest within 30 meters of elevation of the creek, and becomes more gradual radially from it.
Geology
McKinney Falls State Park covers a diverse strip of land in Central Texas.
The area of the park is almost completely dominated by the Ko geologic layer, which is the geologic code for the Ozan Formation (USGS). The Ozan Formation is a Gulfian preserver that runs as far east as Arkansas, and is thicker in the Austin area, with about 600 feet of thickness. It makes contact with the younger Annona chalk layer and is primarily composed of clay and marl basally and quartz, calcite, glauconite, phosphate, hematite and pyrite more upward (USGS).
Several fossils have been found on the grounds of the park, including many inoceramids, a saltwater clam, which became extinct in the Cretaceous Period (Raney, 1997). This is evidence for the Western Interior Seaway that covered Central Texas from the Cretaceous Period to the Paleogene (Marfa Public Radio).
Areal Extent and General Type of Vegetation
The park is almost entirely wooded, likely 90%. The remainder of the land is almost equally grassland and the creek itself, roughly 5% each. One prevalent plant species that can be found in the area is the Bald Cypress, found lining the creek itself. Other plant species that can be easily found within McKinney falls are Ashe Juniper and Live Oak.
Availability of Water
Onion Creek, a tributary stream of the Colorado River, runs through the west side of the park flowing northeast (Texas State Historical Association). The level of flow within the park is high and large amounts of water can be seen flowing quickly through and over the rock formations (Field visit 2/16/25). A tributary stream of Onion creek, Williamson Creek, branches off towards the central western border of McKinney falls, running along the north western edge, leaving the park.
Proximity to Urban or Industrial Areas
Located in the urban area of Austin, Texas, McKinney Falls State Park is surrounded by proximal human settlement. A golf course is adjacent to the bend of the creek, which demarcates the westernmost border of the park (Google).
History
McKinney Falls State Park was first acquired by the City of Austin in 1971 (Austin City Records). The area has been in use since the 18th century, causing indentations in the rock (National Park Service). It was first used as a ranch by a number of landed men in the area, and eventually fully developed by Thomas Freeman McKinney, who ranched and bred horses there.
In 2013, a flood damaged 1,200 homes near the creek and inundated the bathrooms and visitor’s center with water (Garrett, 2015).
Research Plan
Design of Study
Lengths:
Orange (Owen): 1147.65 meters
Purple (David): 947.59 meters
Yellow (Ellington): 899.41 meters
The percentage of the total study area from which data will be collected is 28.86%.
This study aims to discern the relationship between Northern Cardinal populations and large bodies of water. The three transects covered here are of varying distances from a major body of water, to which they are all mostly parallel. This means that the transects should vary in Northern Cardinal populations depending on how far they are to water.
The northernmost transect runs along a trail directly adjacent to Onion Creek, an approximately 20-meter-diameter river (Google). This trail is within 10 meters of the creek, ideal for observing and recording the population of cardinals closest to bodies of water. Additionally, the trail is surrounded by trees that do not obstruct vision, which aids in identifying birds.
500 meters south of this transect is the middle transect in terms of latitude. This transect runs from the northernmost point of the Picnic Trail’s parking lot to the path to the Youth Group Campsite, ranging from within 110 meters of the water to more than 400 meters from it. The vegetation cover is clear along the transect due to the transect being along a road which is then interrupted by densely wooded vegetation.
The southernmost transect runs from the southernmost turn of the Onion creek trail to its northernmost end, ending at the campground loop road. The vegetation cover is dense wooded area. This trail is 575 meters away from water on the northernmost end and 700 meters from water at the southernmost end.
Materials
Field journals (Unison)
Binoculars (Bushnell)
Merlin Bird ID (Cornell Lab)
Procedure
3 Transects within 200 meters of 1 kilometer established at varying distances from water source.
Transects surveyed 6 times over different weeks.
Surveyors will walk at 1 km/hr for the length of the transect and record every instance of Northern Cardinal either seen or heard by call or song. Surveyors will record every Northern Cardinal once by the way they first observed the Northern Cardinal at the distance the Northern cardinal is from them, marked on a sheet with sections in 5 meter increments going to 35 meters, then moving in 30 meter increments to 125 meters.
The total amount of Northern Cardinals of every transect added up to create a total for that transect.
The amount of Northern Cardinals on either transect are compared to find which transect have a greater amount of Northern Cardinals.
Respective full detection strips will be created for each transect visit by finding a drop of 20% from the average of all previous 5 meter sections to the next.
Population estimate will be calculated by a predetermined formula that accounts for error
Coefficient of detection is determined by dividing the total calculated population within the 125 meter area around the 1 km transect by the number of total Northern Cardinals detected by surveyor.
A correlation or lack thereof between Northern Cardinal populations and distance to water is identified.
Results
Differences between population estimates of transects of all distances from water are all significant. That between medium and close to water is most significant (p = 0.00), followed by between close and far (p = 0.00) and medium and far (p = 0.01). The river-adjacent (30-40 meters from water) transect possessed the largest average population estimate, representing 56.0% of the total population (Fig. 1).
Figure 1. Percentage of total population of Northern Cardinals between the respective population estimates of each transect.
The farthest transect (550-650 meters from Onion Creek) has a midrange percentage, closest to the mean of the three, with 27.2% of the total Northern Cardinal population. The moderate distance transect (300-350 meters from Onion Creek) has the smallest percentage of total individuals at 16.9% (Fig. 1).
Past ~150 meters from the river, the correlation between proximity and Northern Cardinal population seems to be diminishing (Fig. 1). This is also reinforced by the fact that the total number of Cardinals detected within the intermediate transect is higher than the far transect.
The total number of birds observed is more consistently proportional to proximity to water. The average total number of observations is 16.00, 15.00, and 11.50 birds for the close, intermediate distant and far transects, respectively.
Temperature, wind speed, and human activity had no remarkable correlation with population (Fig. 2). The correlation between population and distance from water is much more marked (Fig. 1). All four variables seem to be independent of each other, most relevantly population estimate.
Figure 2. Temperature, wind speed, human activity, and population estimate lack correlation between visits.
The ratio of observations by sound to by sight positively correlates with temperature (Fig. 3). The ratio of observations by song to by call correlates positively with temperature as well (Fig. 3).
Figure 3. Temperature’s correlation with ratios of observations by sound to by sight and by song to by call.
Although the ratio of observations by song to by call exhibits no correlation with amount of human activity, that by sound to by sight does (Fig. 4).
Figure 4. As humans increase in prevalence, so does the ratio of observations by sound to observations by sight. The ratio of sound observations by song to by call shows no correlation with these two variables.
Discussion
This study’s initial hypothesis was that Northern Cardinal populations are inversely proportional to distance from substantial water sources. This hypothesis can be supported by the present data, although they do not present a conclusive affirmation, nor a conclusive nullification.
Studies such as the one done by Barbosa, Karlla Vanessa de Camargo, et al. support this hypothesis. Although the study done by Barbosa is done from a much greater distance from water on average than this study, the conclusion that resident bird populations (such as Northern Cardinals) have a greater density closer to water is supported. One conclusion found by this study is that there may be a drop-off off of effectiveness of proximity to water determining Northern Cardinal population at 150 meters. Barbosa, et al. would disagree with these findings, determining that farther distances have similar effects on population of bird species (Barbosa, et al.).
A study of Arizona cardinals reached similar conclusions. These birds preferred to nest at riparian zones (Powell and Steidl). Our study took place during the Northern Cardinal’s nesting period, meaning this may have influenced their populations to move closer to the river as the riparian foliage there attracted them.
The data may have been affected by human methods of collection; not error, but observation methods likely biased transects in dense forest to have lower full detection strips as foliage obscured both hearing and vision for distances upwards of 50-100 meters.
At the same time, the transect at a moderate distance from water ran along a road with a width of 9 meters. When adjoined to an adjacent parking lot, this width reaches over 15 meters. Studies have shown that asphalt plain dissuades bird species from inhabiting the immediately proximal area (Miller, et. al). Additionally, this road was lightly trafficked; car activity is correlated to a decrease in nearby bird populations (Parris and Schneider). These data were recorded during March and April, the beginning of Cardinalis cardinalis’ breeding season.
The full detection strips reported for the moderately distanced transect were therefore high, as few birds were available to observe within several dozen meters of the transect’s median. The formula used to calculate population estimate is below. All distance measurements are in meters.
(Strip Transect Width/Full Detection Strip Width) * # of detections in Full Detection Strip
Area of transect
Because the full detection strip for the transect along the paved road was higher, its population estimates were divided by a greater dividend and thus became smaller. This may have produced a population estimate for the moderately distanced transect skewed to reflect a lower number of birds than is actually present.
The dense foliage of the most distant transect obscured sight and sound of more distant birds, biasing the full detection strip to be low and the population estimated to be high. Northern Cardinals prefer to inhabit forests and densely wooded areas (Merkord, et. al). This may also explain why they were least abundant along the human boulevard and more prevalent near the riparian undergrowth and tree cover and the dense cedar and oak cover further from the creek.
The total number of individuals recorded reinforces this notion. On average, more birds were consistently seen close to water. This clearly does not account for the distance variables the estimates include but is a notable consistency for consideration.
The positive correlation between song and call ratios and temperature suggests that as temperature increases, birds vocalize more through song than through call. Correlation is not direct causation; birds sing more often during the breeding season to attract mates and defend territory and breeding season is usually in spring, when temperatures become warmer (Slagsvold). Warmer temperatures allow for more plant growth, which facilitates all breeding behaviors, from increased sexual proclivity to nest building to incubation to the rearing of hatchlings.
The lack of correlation between population estimates and temperature, wind speed, and human activity (Fig. 2) implies that those factors had no effect on the methods of study nor on actual Cardinalis populations. Due to the temporal limitations of this study, no conclusion can be drawn concerning the long-term effects of these stressors upon Northern Cardinals. Over the span of years, increasing temperatures have affected Northern Cardinal populations across the United States (Beddall).
Overall, this lack of correlation coincides with other findings (Safi, et. al). It simply suggests that these factors did not affect the comparative populations of different transects at different times; estimates therefore should not vary greatly temporally although they may change over time. This means that most likely, the number of Northern Cardinals living in an area will not change between weeks despite changes in other external factors. This is supported by other studies that have found Northern Cardinals to be non-dispersing (Conner, et. al).
This is important because it reinforces the idea that these results show the correlation between proximity to water and Northern Cardinal populations, and not the correlation of any other data. Although temperature and proximity to water are closely linked, this connection was therefore not relevant to the outcome of the present study.
Human activity may be correlated with ratios of observation by sound to by sight for a number of reasons. Systematic error could explain this by suggesting seeing people distracted the data collectors from seeing avians, but such reasoning is discounted by human activity’s lack of correlation with population estimates illustrated in Figure 2. A more reasonable explanation is that human beings frightened birds in their immediate proximity as they walked the trail that the transect’s center was upon, a pattern observed in other studies (Brewer). In this theoretical situation, nearby birds that could have otherwise been seen were pushed several dozen meters away and instead heard. Because the birds were still observed, this discovery does not conflict with any of this study’s findings.
Further research is needed to discern the extent of these correlations. More robust survey methods, such as microphones that detect birdsong or trackers on individual birds, would produce results of greater accuracy. In combination with local weather statistics from external sources, including sky condition, rainfall and temperature, such methods would elucidate the correlation between song to call ratios and climatic factors.
A small sample size increases the likelihood of inaccuracy (Hertzog). Recommended sample sizes are as much as 10% of the total population. Considering the total estimated population often exceeded 100, but the total number of observations was similarly often under 10, this sample size was not achieved by the limited scope of the present study. More conclusive research will include more of the total population, reducing random error and producing more specificity of results.
The human methods used in this study involved walking down the middle of the transect from which data was recorded; human beings often frighten birds and temporarily decrease their immediately proximal populations (Pruteanu et al.).
Tracking Cardinalis populations over multiple years was out of the scope of this study. Future studies would reach more holistic conclusions by collecting sample data over many years, decreasing this factor’s ability to affect statistical outcomes.
Studies of 0.05 as an effective p-value threshold have found that its arbitrariness decreases its ability to relay significance. Null hypotheses are not well supported by p-values under 0.10 (Kwak). Future studies could expand the significance of smaller but more consistent differences by modifying their definition of significance.
Competing Interests
The researchers declare no competing interests.
Acknowledgments
The researchers thank Ms. Cong for her support.
References
Barbosa, Karlla Vanessa de Camargo, et al. “Noise Level and Water Distance Drive Resident and Migratory Bird Species Richness within a Neotropical Megacity.” Landscape and Urban Planning, vol. 197, 1 May 2020, p. 103769, www.sciencedirect.com/science/article/pii/S0169204619313453, https://doi.org/10.1016/j.landurbplan.2020.103769. Accessed 15 May 2025.
This study, conducted in Mexico City, shows that both resident and migratory bird richness declined with increased distance to water and increased noise level. They conducted this study by using point-count methods, which meant they counted birds, including defining them as migratory or resident at specific areas. At these points, they also took noise level data, distance to water, human abundance, and tree cover. Their findings are similar to this study’s findings, as both suggest a greater population of birds with decreased distance to water.
Beddall, Barbara G. “Range Expansion of the Cardinal and Other Birds in the Northeastern States.” The Wilson Bulletin, vol. 75, no. 2, 1963, pp. 140–158. JSTOR, www.jstor.org/stable/4159151, https://doi.org/10.2307/4159151. Accessed 13 May 2025.
This article summarizes long-term changes in Northern Cardinal populations relative to increasing continental temperatures. The widely observed phenomenon of global warming is linked to a shift and expansion of their range, which could account for changes out of the scope of this study.
Brewer, Jenna. “DIVERGENT RESPONSE TO ANTHROPOGENIC CHANGE WITHIN THE SONGBIRD TAXON: INSIGHTS FROM THE RANGE EXPANSION OF THE NORTHERN CARDINAL AND MITIGATION MECHANISMS FOR MIGRATORY BIRD-BUILDING COLLISIONS.” Digitalcommons.mtu.edu, Michigan Technological University, 2024, digitalcommons.mtu.edu/cgi/viewcontent.cgi?article=2924&context=etdr. Accessed 15 May 2025.
This study touches on important anthropogenic factors triggering population change in Northern Cardinals outside of secondary effects of human activities such as industrial activity. It relates to the non-dependent variable involved in the researchers’ study of proximity to man made synthetic development. This was not accounted for throughout the course of data collection, meaning it may present unanticipated weightedness in any final conclusions. The article provides a more sophisticated understanding of what this relationship can and does look like.
Cen, Christina, et. al. “Bird community interactions at water sources at Hastings Natural History Reservation.” California Ecology and Conservation Research, Fall 2020, https://ucnrs.org/wp-content/uploads/2020/12/Bird-community-interactions-at-water-sources.pdf, https://doi.org/10.21973/N3MW98. Accessed 15 May 2025.
Cen and colleagues reviewed the behavior of several birds in relation to water. The fact that there was no noted observation of Northern Cardinals limited the extent to which this article was useful for the purposes of the study.
Conner, Richard, et. al. “Relationships among Territory Size, Habitat, Song, and Nesting Success of Northern Cardinals.” The Auk, January 1986, https://academic.oup.com/auk/article-abstract/103/1/23/5191379, https://doi.org/10.1093/auk/103.1.23. Accessed 15 May 2025.
This article guided the study’s understanding of normal dispersal patterns of Northern Cardinals. It reinforced the concept of Cardinalis’ non-dispersing lifestyle, aiding in the understanding of how their population can be expected to change from one day to another.
“Data & Results.” www.pwrc.usgs.gov/bbs/, 2021,
https://www.pwrc.usgs.gov/bbs/results/.
This is the official website of a survey group, the North American Breeding Bird Survey. It contains data of recorded bird populations since 1966, offering statistical information on changes in their populations since then. These data include counts of the Northern Cardinal.
Dow, Douglas D. “Distribution and Dispersal of the Cardinal, Richmondena Cardinalis, in Relation to Vegetational Cover and River Systems.” The American Midland Naturalist, vol. 84, no. 1, 1970, pp. 198–207. JSTOR, https://doi.org/10.2307/2423736. Accessed 13 Feb. 2025.
This study of Northern Cardinal dispersal showed where Cardinals are most likely to end up after breeding dispersal. This gives reasonable data to say that cardinalis distribution has a positive correlation to the presence of bodies of water, specifically river systems and similar waterways. The conclusion of this study greatly influenced the hypothesis of this study, which was tested.
Garrett, Stephen. “Park Pick: After the Flood.” Texas Parks and Wildlife, 2015, https://tpwmagazine.com/archive/2015/jan/scout4_parkpick_mckinneyfalls/
This article offered a reliable retelling of the flooding event that occurred in McKinney Falls in 2013.
Guest Blogger. “Exploring McKinney Falls in Austin.” austintexas.org, Visit Austin, 2024 https://www.austintexas.org/austin-insider-blog/post/mckinney-falls/#:~:text=With%20641%20acres%20and%2081,fishing%2C%20and%20mountain%20biking%20trails.
This blog article provided the acreage of the park that is the focus of this study. It also detailed human activity that takes place within the park, a crucial piece of information for the purposes of this study.
James, Jurassic. “Llano Uplift: Geology of the Texas Hill Country | Jurassic James.” Jurassicjames.com, 2018, jurassicjames.com/geology-of-the-texas-hill-country/. Accessed 1 Mar. 2025.
This source offered information on some geologic phenomena observable at McKinney Falls today.
Kwak, Sang Gyu. “Are Only P-Values Less than 0.05 Significant? A P-Value Greater than 0.05 Is Also Significant!” Journal of Lipid and Atherosclerosis, vol. 12, no. 2, 1 Jan. 2023, pp. 89–89, pmc.ncbi.nlm.nih.gov/articles/PMC10232224/, https://doi.org/10.12997/jla.2023.12.2.89. Accessed 13 May 2025.
This paper explains the arbitrariness of 0.05 as a significant p-value. It supports the recommendations for further study by suggesting that p-values higher than 0.05 are acceptably significant.
Land, C E. “Statistical limitations in relation to sample size.” Environmental Health Perspectives, vol. 42, 1 December 1981, p. 15-20, https://ehp.niehs.nih.gov/doi/abs/10.1289/ehp.814215, https://doi.org/10.1289/ehp.814215. Accessed 12 May 2025.
This paper discussed the limitations of small sample sizes in reaching significant statistical conclusions. It explained how a few outliers can create bigger changes with undue weight. This supported the recommendations for further research by specifying that it should be more broad and far-reaching.
Lefebvre, Louis, et al. “Feeding innovations in a nested phylogeny of Neotropical passerines.” https://royalsociety.org/journals/, The Royal Society, https://royalsocietypublishing.org/doi/full/10.1098/rstb.2015.0188
This study of the Emberizoid class of birds, in which the family Cardinalidae is classified, shines light on the adept colonization birds such as the cardinals have performed. Northern Cardinals are common birds throughout much of the northern New World. This success points to Northern Cardinals being highly adaptable and possessing a varied diet of diverse species.
“McKinney Falls State Park.” https://www.nps.gov/index.htm, National Park Service, https://www.nps.gov/elte/learn/photosmultimedia/mckinney-falls-state-park.htm
This source gives information about early McKinney falls history, including the first recordings of McKinney falls by Isidro Félix Esty Espinosa and how it has evidence of Native Americans and early Americans using the area as a trail.
“McKinney Falls State Park High Point, Texas.” https://www.peakbagger.com/Default.aspx, Peakbagger, https://www.peakbagger.com/Peak.aspx?pid=-53846#:~:text=653%20feet%2C%20199%20meters
This is a profile of McKinney Falls’ topography and offers a maximum elevation. It also allows for minimum elevation to be calculated through the topographic map embedded within the site.
Merkord, Christopher, et. al. “Vegetation associations of riparian birds in successional woodlands along the regulated Missouri River.” Avian Conservation & Ecology, vol. 18, Jan. 2023, https://ace-eco.org/vol18/iss2/art9/, https://doi.org/10.5751/ACE-02492-180209. Accessed 14 May 2025.
This in-depth study reviewed the populations of several songbirds in relation to riparian vegetation of the Missouri River. Being a midwestern central North American river, the Missouri River bears several similarities to Onion Creek, allowing for parallels between the fauna and flora of the two to be drawn. This study found that while cardinal populations increase near water, it was because they prefer the forest associated with rivers, suggesting that the more dense forest was home to more individuals because of its wooded nature. It also directly supports the original hypothesis of this study.
Miller, Colleen R., et al. “Bill Size Variation in Northern Cardinals Associated with Anthropogenic Drivers across North America.” Ecology and Evolution, vol. 8, no. 10, 17 Apr. 2018, pp. 4841–4851, www.jstor.org/stable/4159151, https://doi.org/10.1002/ece3.4038. Accessed 14 May 2025.
This article discusses how human impact is affecting cardinal populations. It is a critical question that more robust studies may take into account when focusing on this same dilemma of water proximity to population.
“Northern Cardinal | Bird | Britannica.” Www.britannica.com, 31 Jan. 2025, www.britannica.com/animal/northern-cardinal.
This online resource describes cardinals’ phenotypic characteristics. It was used to describe specific physical information which would have been difficult or impossible to include in such detail solely from observation.
“Northern Cardinal Identification, All about Birds, Cornell Lab of Ornithology.” Allaboutbirds.org, 2017, www.allaboutbirds.org/guide/Northern_Cardinal/id.
This webpage provided critical information concerning the geographical distribution and common habits of the bird of this study’s focus, the Northern Cardinal.
“Onion Creek (Blanco County).” Texas State Historical Association, 2025, www.tshaonline.org/handbook/entries/onion-creek-blanco-county.
This site provided the basic history of the study area.
Parris, Kirsten M, and Angela Schneider. “Impacts of Traffic Noise and Traffic Volume on Birds of Roadside Habitats.” Ecology and Society, vol. 14, no. 1, 2009. JSTOR, www.jstor.org/stable/26268029, https://doi.org/10.2307/26268029. Accessed 15 May 2025.
This source enabled the consideration of the external factor of anthropogenic development on Northern Cardinal populations. Although this factor was never studied with intent, it was relevant because of the different proximity to urban buildup of different areas in McKinney Falls State Park.
“Pocket Texas Geology.” https://www.usgs.gov/, United States Department of the Interior, https://webapps.usgs.gov/txgeology/.
This interactive website revealed the underlying geological formations covering the park in which this study’s data was collected.
Powell, Brian F. and Steidl, Robert J. “Habitat Selection by Riparian Songbirds Breeding in Southern Arizona.” The Journal of Wildlife Management, Oct. 2002, https://www.jstor.org/stable/3802940, https://doi.org/10.2307/3802940. Accessed 15 May 2025.
The similitude of the above study and that at present was notably helpful. This study found that Cardinalis cardinalis prefers to nest in riparian trees adjacent to rivers, such as the zones explored by the researchers conducting this study. Their results support the original hypothesis that more Northern Cardinals will be found closer to water.
Pruteanu, Augustina, et al. “Review of Effectiveness of Visual and Auditory Bird Scaring Techniques in Agriculture.” 22nd International Scientific Conference Engineering for Rural Development Proceedings, 24 May 2023, www.tf.lbtu.lv/conference/proceedings2023/Papers/TF056.pdf, https://doi.org/10.22616/erdev.2023.22.tf056. Accessed 14 May 2025.
Pruteanu and colleagues explain birds’ reaction to human beings, deciding that songbirds usually fear humans and physically avoid them upon realization of their proximity. This implies that humans along the trails on which data was collected may have scared away nearby Northern Cardinals, making it impossible to observe them by sight and biasing them to be observed instead by sound.
Raney, Jay. “Down to Earth at McKinney Falls State Park, Texas.” Utexas.edu, 2025, store.beg.utexas.edu/de/291-de0001.html. Accessed 1 Mar. 2025.
This book provided for more in-depth geologic history of McKinney Falls.
Safi, Kamran, et al. “Flying with the Wind: Scale Dependency of Speed and Direction Measurements in Modelling Wind Support in Avian Flight.” Movement Ecology, vol. 1, no. 1, 3 July 2013, https://doi.org/10.1186/2051-3933-1-4.
Although this source was never directly cited as evidence for any one conclusion, it can be used to relay the idea that wind direction and speed did not correlate meaningfully with population estimates.
Slagsvold, Tore. “Bird Song Activity in Relation to Breeding Cycle, Spring Weather and Environmental Phenology.” Rapport, Universiteit i Trondheim, 1977, www.ntnu.no/c/document_library/get_file?uuid=95ec81e7-3c35-4b64-be14-0af397bc31cd&groupId=10476. Accessed 15 May 2025.
This study assisted in the comprehension of birds’ temporal breeding habits. It provided useful information about birds’ common breeding patterns, times and behaviors.
Smith, Brian, et al. “The role of historical and contemporary processes on phylogeographic structure and genetic diversity in the Northern Cardinal, Cardinalis cardinalis.” https://www.biomedcentral.com/, Springer Nature, https://bmcecolevol.biomedcentral.com/articles/10.1186/1471-2148-11-136.
This paper discusses the genetic diversity of different Northern Cardinal subpopulations. It finds that the Northern Cardinal subpopulation throughout the central and eastern United States is the cardinalis subspecies, allowing inference from this mitochondrial lineage (implying a cardinalis mitochondrial Eve) that other articles on Northern Cardinals in the United States will cover the same subspecies of genetic similarity. This eliminates the possibility of this study being misinformed by articles covering separate subspecies.
Staengl, Ezra & Vondrasek, Joanna. “Winter Diet of the Northern Cardinal (Cardinalis cardinalis) in Central Virginia: A Comparison of a Rural and an Urban Site.” researchgate.net, The Raven Vol. 95,
This paper is an in-depth analysis of the diet of Northern Cardinals, with a focus on the variation of diet between rural and urban parks. Although the original hypothesis that urban cardinalis ate more non-native plants by percentage of diet than rural cardinalis was proved incorrect, the study developed critical ideas concerning the seasonal diet of Northern Cardinals. Most notably, the cardinals were found to consume diets of 30% non-native foods, which has further implications about the reliance of Northern Cardinals on native species. Further, separate study must be held on the percentage of invasive plant cover in the LASA Preserve and the effects it may have on Northern Cardinal population to understand how this would affect total population of cardinalis throughout the park. Such an analysis is out of the scope of this study. By cross-referencing this study with that from Springer Nature, it is found that the subspecies analyzed here is the same subspecies observed at LASA Preserve.
“Western Interior Seaway.” Marfa Public Radio, Radio for a Wide Range., www.marfapublicradio.org/show/nature-notes/2015-06-08/western-interior-seaway.
This source gave clear and concise information about the ancient geology of Central Texas.