It’s all about the gonads!

There’s a headline grabbing title if ever one was needed!

The eagle eyed among you have been in touch to ask a fantastic question about a point made on Winterwatch… That only chequer pigeons breed in winter.

Many have asked if pigeons prefer mating partners that look different from themselves, how does any winter breeding happen at all?

In winter, won’t the non-chequered partner be in non-breeding condition?

The truth of the matter is down to something called ‘gonadal regression’. Most temporal breeding birds breed seasonally in order that there is lots of food available for their nestlings. In order to be ready to breed at the right time, such birds use day length (known as photoperiod) as an indicator to make sure their bodies are ready for reproduction at the correct time. This is why it is common to hear of some seasonally breeding animals breeding too soon, when spring comes ‘late’.  Day length tells them it is time to breed, despite a ‘late’ spring occurring.

But what about pigeons? I hear you ask.

Pigeons have an extra problem to overcome – the production of ‘pigeon milk’. Pigeon milk is produced by BOTH male and female pigeons and is regulated by a hormone called prolactin. In order that both male and female pigeon produce pigeon milk at the correct time (when the young hatch), each pigeon’s own hormone production has to be influenced by the other bird. In that way, they can synchronise their hormones.

Female pigeons are heavily influenced by the behaviour of their mate – if the male pigeon of the pair is ready to reproduce, his courtship behaviour helps to stimulate the female to be in the same mood.

Here we see a male pigeon cooing to his mate

So what about the gonads?

In winter, as the days get shorter, BLUE BAR male pigeons recognise this and their testes begin to shrink – this is known as gonadal regression. This shrinking reduces their circulating testosterone levels and stops the pair reproducing. As an upside, these males are then free to develop fat reserves to help them survive the cold months of winter.

CHEQUER birds on the other hand are not as influenced by changing day length; their testes remain large and continue to produce lots of testosterone in the depths of winter. As such, these virile males are able to continue to stimulate their female partners into reproducing. This comes at a cost though; these males are unable to lay down fat reserves – however, in our busy cities there is usually so much food to eat that this does not cause them any problem.

Therefore, despite a pair only having one male chequer individual, the pair can still breed in winter, when those with a blue bar male will generally not.

My current results show chequer males are the most common plumage type – a true sign that pigeons have evolved to take advantage of our wasteful lifestyle.

Is there a link between human obesity and feral pigeon productivity?

Pigeons appear to have an uncanny ability to defy all attempts at eradication by people who mistakenly believe they are a risk to human health.  In order for a population to remain stable, each pair need only produce two surviving offspring to eventually replace themselves. However, no individual can know the probability of any of its offspring surviving and therefore, must attempt to produce as many offspring as it can. Feral pigeons show iteroparity, that is, multiple breeding attempts over a life time and are known to pair for life. As R selected organisms, feral pigeons can respond quickly to favourable circumstances, producing offspring in a short space of time, which in turn can breed within 6 months.  In fact, the strong pair bond of adult birds allows clutches to overlap, with the female sitting on a new clutch of eggs whilst the male tends to the newly fledged youngsters.  Clearly, this highlights the very reason lethal control methods are ineffective.

Young feral pigeon
Feral pigeon youngsters look very similar to the adults – the beak and eyes often give the game away. Youngsters also squeak to the adults, hence young feral pigeons are often referred to as squeakers.

Schein’s 1954 study of feral pigeon recruitment found interesting results. Two sites were studied, a church steeple with multiple nesting platforms and a hospital. Nests were monitored from point of lay to fledging and the number of fledged young was recorded. The majority of the nests on the hospital study site were located within a central heating tower where temperatures remained warm and stable, whereas the ambient temperature of the church site fluctuated,  however, results showed very little difference between the survival probability of both flocks.

Of 152 eggs laid at the church site, 85 hatched, and of these, 47 fledged , giving a fledging probability of 0.309.

Similarly, of 293 eggs laid at the hospital site, 149 hatched and of these, 83 fledged giving a fledging probability of 0.283.

Given that temperature was shown to have little effect on the fledging probability of individuals of either flock and that both flocks showed similar figures, it can be suggested that other causal factors are involved.

Which brings me to the strange title of this blog…

Given the r selected nature of feral pigeons, food is a factor in the rate of offspring production, so I wonder how the changing face of the human diet affects this rate.

The fact that we are eating greater amounts of fast food is not in question, obesity levels have rocketed since this study was conducted. In 1950, 9.7% of Americans were obese, compared to 30.5% today.  Has the food available to feral pigeons today subsequently increased in both quantity and calorific content? If this is the case, what effect has this had on the probability of offspring fledging today in comparison to those in 1950?

A repeat of this study today may yield interesting results as it may be found that our increasing demand for higher calorie, convenience foods is causing a subsequent increase in one of the most adaptable species on the planet.

REF

Schein.W (1954) Survival records of young feral pigeons, The Auk,  Vol. 71, No. 3 (Jul., 1954), pp. 318-320