In the past month, two recently published research articles captured my attention. I’ll review these two studies in today’s column.
The first paper documents the ability of Northern Mockingbirds to recognize particular human faces. This work is significant because it is the first research to show a wild animal can recognize individuals of a different species.
The work was done by Douglas Levey and his students at the University of Florida. The Gainesville campus has a large population of nesting mockingbirds. With 51,000 students walking around the campus, a nesting pair of mockingbirds will have around 15,000 humans walking within five meters of a nest during a nesting cycle (about 23 days). Most of these passers-by are ignored by the mockingbirds. Occasionally, a mockingbird will give loud alarm calls, dive bomb or even graze the head of a human. Levey’s team tested the hypothesis that mockingbirds can distinguish threatening humans from people that simply walk past, usually unaware there is even a nest present.
Levey enlisted the assistance of a number of students. The team located mockingbird nests, usually just a few feet above the ground in shrubbery, and set up an experimental protocol. For four days in a row, a student would walk to the nest and stand at the nest for 30 seconds. During the last 15 seconds, the student would put her/his hand on the rim of the nest. The students never touched the eggs or nestlings.
After only two days, the mockingbirds increased their aggressive response to the student even though the student approached the nest from a different direction and wore different clothing. The female mockingbird would leave the nest more quickly, alarm calling would increase and dive-bombing of the student would become more frequent. The intensity of the mockingbirds’ responses increased on the third and fourth days.
Now, the really interesting part. On the fifth day, a different student would approach the nest and stand close for 30 seconds, again with a hand on the nest for 15 seconds. The mockingbirds did not respond aggressively. The birds did not recognize this new intruder as a threat.
The ability of the mockingbirds to perceive their environment in such detail surely helps explain why mockingbirds coexist so well with humans in urban environments.
Wealth has its advantages. Well-to-do parents may send their children to the finest private schools. Presumably, such kids will have advantages in life compared to the rest of us from families with more modest incomes. However, recent work by Liana Zanette of the University of Western Ontario on Song Sparrows challenges this idea.
In particular, Zanette was interested in how food influences nestling growth and development. She began her study by raising Song Sparrow nestlings in the laboratory. Some were given only adequate food and others were given much higher quality food. The nestlings on the better diets grew larger than the nestlings on the poor diets. Importantly, the birds on the better diet developed larger brains and a larger repertoire of songs. We know that female Song Sparrows choose mates with larger song repertoires. So, based on this laboratory work, Zanette expected better-fed nestlings in the field to thrive.
In British Columbia, Zanette mapped the territories of a number of Song Sparrows. She put out bird feeders in the middle of half of the territories. Thus, half of the Song Sparrow pairs in the study had access to extra food. The remaining territories had no feeders and served as an experimental control.
As expected, the parents on the territories with bird feeders had more energy they could devote to their reproductive effort. Zanette expected that those females would lay larger eggs than the females on the control territories. Larger eggs lead to larger nestlings with larger brains and, for males, larger song repertoires.
In fact, just the opposite result happened. Eggs and nestlings from food-supplemented pairs were smaller those from control pairs. Rather than produce higher quality young, the Song Sparrows with supplemental food produced more eggs. The eggs were smaller on average than those produced by females on control territories. The male nestlings grew up to develop smaller song repertoires than males from control territories. The females chose quantity over quality.
The parents on a food-supplemented territory had to spread the food they collected for their nestlings among more individuals. So much for the idea that wealthy parents produce offspring with significant advantages!
Zanette found that food-supplemented parents gave more food to the oldest male nestlings. That preferred treatment allowed the sons to become as large as nestlings from control areas. However, their brain growth never caught up.
[Originally published on June 13, 2009]