Bird intelligence
The difficulty of defining or measuring intelligence in non-human animals makes the subject difficult to study scientifically in birds. In general, birds have relatively large brains compared to their head size. Furthermore, bird brains have two-to-four times the neuron packing density of mammal brains, for higher overall efficiency. The visual and auditory senses are well developed in most species, though the tactile and olfactory senses are well realized only in a few groups. Birds communicate using visual signals as well as through the use of calls and song. The testing of intelligence in birds is therefore usually based on studying responses to sensory stimuli.
The corvids and parrots are often considered the most intelligent birds, and are among the most intelligent animals in general. Pigeons, finches, chickens, and birds of prey have also been common subjects of intelligence studies.
Studies
Bird intelligence has been studied through several attributes and abilities. Many of these studies have been on birds such as quail, domestic fowl, and pigeons kept under captive conditions. It has, however, been noted that field studies have been limited, unlike those of the apes. Birds in the crow family as well as parrots have been shown to live socially, have long developmental periods, and possess large forebrains, all of which have been hypothesized to allow for greater cognitive abilities.Counting has traditionally been considered an ability that shows intelligence. Anecdotal evidence from the 1960s has suggested that crows can count up to 3. Researchers need to be cautious, however, and ensure that birds are not merely demonstrating the ability to subitize, or count a small number of items quickly. Some studies have suggested that crows may indeed have a true numerical ability. It has been shown that parrots can count up to 6.
Cormorants used by Chinese fishermen were given every eighth fish as a reward, and found to be able to keep count up to 7. E.H. Hoh wrote in Natural History magazine:
Many birds are also able to detect changes in the number of eggs in their nest and brood. Parasitic cuckoos are often known to remove one of the host eggs before laying their own.
Associative learning
Visual or auditory signals and their association with food and other rewards have been well studied, and birds have been trained to recognize and distinguish complex shapes. This may be an important ability which aids their survival.Associative learning is a method often used on animals to assess cognitive abilities. Bebus et al. define associative learning as "acquiring knowledge of a predictive or causal relationship between two stimuli, responses or events." A classic example of associative learning is Pavlovian conditioning. In avian research, performance on simple associative learning tasks can be used to assess how cognitive abilities vary with experimental measures.
Associative learning vs. reversal learning
Bebus et al. demonstrated that associative learning in Florida scrub-jays correlated with reversal learning, personality, and baseline hormone levels. To measure associative learning abilities, they associated coloured rings to food rewards. To test reversal learning, the researchers simply reversed the rewarding and non-rewarding colours to see how quickly the scrub-jays would adapt to the new association. Their results suggest that associative learning is negatively correlated to reversal learning. In other words, birds that learned the first association quickly were slower to learn the new association upon reversal. The authors conclude that there must be a trade-off between learning an association and adapting to a new association.Neophobia
Bebus et al. also showed that reversal learning was correlated with neophobia: birds that were afraid of a novel environment previously set up by the researchers were faster at reversal learning. The inverse correlation, where less neophobic birds performed better on the associative learning task, was measured but was not statistically significant. Opposite results were found by Guido et al., who showed that neophobia in Milvago chimango, a bird of prey native to South America, negatively correlated to reversal learning. In other words, neophobic birds were slower at reversal learning. The researchers suggested a modern explanation for this discrepancy: since birds living near urban areas benefit from being less neophobic to feed on human resources, but also benefit from being flexible learners, perhaps low neophobia coevolved with high reversal learning ability. Therefore, personality alone might be insufficient to predict associative learning due to contextual differences.Hormones
Bebus et al. found a correlation between baseline hormone levels and associative learning. According to their study, low baseline levels of corticosterone, a hormone involved in stress response, predicted better associative learning. In contrast, high baseline levels of CORT predicted better reversal learning. In summary, Bebus et al. found that low neophobia and low baseline CORT levels predicted better associative learning abilities. Inversely, high neophobia and high baseline CORT levels predicted better reversal learning abilities.Diet
In addition to reversal learning, personality, and hormone levels, further research suggests that diet may also correlate with associative learning performance. Bonaparte et al. demonstrated that high-protein diets in zebra finches correlated with better associative learning. The researchers showed that high-diet treatment was associated with larger head width, tarsus length, and body mass in the treated males. In subsequent testing, researchers showed that high-diet and larger head-to-tarsus ratio correlated with better performance on an associative learning task. The researchers used associative learning as a correlate of cognition to support that nutritional stress during development can negatively impact cognitive development which in turn may reduce reproductive success. One such way that poor diet may affect reproductive success is through song learning. According to the developmental stress hypothesis, zebra finches learn songs during a stressful period of development and their ability to learn complex songs reflects their adequate development.Contradicting results by Kriengwatana et al. found that low food diet in zebra finches prior to nutritional independence enhanced spatial associative learning, impaired memory, and had no effect on neophobia. They also failed to find a correlation between physiological growth and associative learning. Though Bonaparte et al. focused on protein content whereas Kriengwatana et al. focused on quantity of food, the results seem contradictory. Further research should be conducted to clarify the relationship between diet and associative learning.