Almost any statement that Russell makes about animals is a projection. Consider, for example, the first line of the book: "Animals are happy so long as they have health and enough to eat." Did he acquire this knowledge through conversation or by telepathy? Animals frequently play for Russell the role that "the noble savage" played for Rousseau; they represent nature unaffected by civilization.
The book contains a number of historical misperceptions that were common among liberal intellectuals of the day. For example, the Soviet Union was thought to be a grand experiment, with no hint of the police state horrors that we are now so well aware of. "The creation of an organization may be of supreme importance. So is the work of those few statesmen who have devoted their lives to producing order out of chaos, of whom Lenin is the supreme type in our day." [page 167] Again, I recommend we simply shake our heads and move on.
Psychological terminology has changed greatly in the last seven decades. Nervous fatigue, for example, refers to a variety of conditions that we now might call depression or stress or chronic anxiety. Fortunately, Russell uses a number of hypothetical examples, so it is usually not difficult to guess what his psychological terminology must mean. Russell wrote this book long before the advent of anti-depressant drugs, and so he cannot be expected to know or discuss the physiological aspects of happiness and unhappiness. (Though he does speculate "Perhaps when biochemistry has made further advances we shall all be able to take tablets that will ensure our having an interest in everything." [page 128]) The extent to which attitude, mood, or temperament is the result of brain chemistry or genetic makeup is something that a man of 1930 could not have understood as well as we do today.
Further caveats are given by Russell in the first chapter: "I shall confine my attention to those who are not subject to any extreme cause of outward misery." [page 17] In other words, if you object that Russell's prescriptions are not adequate to find happiness for people in abject poverty, in great physical pain, or subject to persecution of one sort or another, Russell would probably have agreed with you. He also acknowledges that the causes of much unhappiness "lie partly in the social system, partly in individual psychology -- which, of course, is itself to a considerable extent a product of the social system. I have written before about the changes in the social system required to promote happiness. Concerning the abolition of war, of economic exploitation, or education in cruelty and fear, it is not my intention to speak in this volume." [page 16]