Recursos naturales

8.1 Introduction

In this and the following two chapters, various aspects of the behaviour of the black-headed gulls on their territories are examined: the time they spend on the territory, incubating eggs and brooding chicks, their reactions towards intruders on their territories, and their behaviour while feeding their chicks. All these behaviours play an important role in the determination of the breeding success of the gulls, and all are forms of parental investment since they require time and energy from the parents which might decrease the possibility of them investing in future offspring.

Before their eggs are laid, when they are establishing territories in the breeding colony, black-headed gulls must spend enough time on the territory to maintain it and to prevent other pairs of gulls from displacing them or stealing nesting material. When there are eggs and young chicks in the nest at least one of the gulls must be present at all times to protect the eggs and chicks from predators, as crows and coots are quick to take advantage of any unattended nests (see Chapter 5). Thus, it is obvious that the time that individual gulls spend on the territory and the time that they spend incubating their eggs and brooding their chicks could be an important factor for the breeding success of the gulls.

The budgetting of various aspects of parental care is an important part of the overall breeding strategy, and so an examination of the apportioning of nest attendance between males and females was also made in this chapter.

8.2 Methods

The arrival and departure times, on the territories, of all the adult gulls under observation were recorded throughout the breeding season in 1983 and 1984. Using this information, calculations were made of the time each gull had spent on the territory during the observation period, and the amount of time that both gulls were present on the territory together and that both gulls were absent from the territory together.

In 1984 the times that gulls started to and stopped incubating and brooding were also recorded and so in this year it was also possible to calculate the time each bird had spent sitting on eggs or brooding chicks and the amount of time that the eggs were left uncovered. The number of periods of presence on the territory and periods of incubation or brooding during the observation period were also calculated.

The periods of observation of groups of nests were not always the same length, and so the time spent on the territory and the time spent incubating and brooding were calculated as a percentage of the time the territory was watched. The mean values then calculated were the means of the percentage- attendance values of individual pairs of gulls throughout the period in question. The number of periods of presence on the territory and the number of periods of incubation and brooding were calculated as the number of periods per hour observed.

8.3 Results

8.3.1 Time Spent on the Territory

The results were divided into four sections as follows:

1) No-egg - the results acquired from pairs of gulls during the pre-egg-laying stage which failed to lay eggs and subsequently left the study area.

2) Pre-egg - the results acquired from gulls during the pre-egg-laying stage which did subsequently lay eggs on the territory.

3) Egg - The results obtained from pairs of gulls with eggs in their nests. 4) Chick - The results obtained from gulls with chicks.

The mean percentage of time spent on the territory, and the mean numbers of periods of presence per hour are shown in Appendix 16 and plotted in Figures 8.1 to 8.4. In 1983 there were only five records obtained from no-egg gulls and 13 from pre-egg gulls and so analysis of these stages was not possible in most cases for this year.

In 1984 (Figure 8.3) there was no significant difference between the amount of time male and female gulls from either the no-egg or pre-egg groups spent on the territory (F = 3.622, df= 1,188, p>0.5), although there was a tendency for both males and females from pre-egg pairs to spend more time on the territory than no-egg gulls (F = 4.720, d f - 1,188, p < 0.05; however Tukey p>0.05). Both guUs of a pair tended to spend less time together on the territory in the no-egg group than the pre-e^ group (F=3.928, df=1,94, p just > 0.05) and there was a significant difference in the amount of time that both gulls were absent from the territory, with no-egg gulls being absent together more often than pre-egg gulls (F = 7.313, df=l,94, p<0.01).

No-egg males spent their time on the territory in more periods (ie. they went to and from the territory more often) than their partners (F = 16.501, df= 1,188, p<0.01; Tukey p< 0.05) (Figure 8.4) and there

was an interaction between the sex of the gull and the group (F = 4.555, df = 1,188, p < 0.05) with no-egg males going to and from the territory more often than pre-egg males (Tukey p < 0.05) while the females from the two groups did not differ (Tukey p > 0.05).

The comparison between gulls observed during the pre-egg and egg stages of 1984 (Figure 8.3) shows that there was no significant difference in the time males and females spent on the territory between the two groups (F = 2.985, d f= 1,356, p>0.05) but during the egg stage male gulls spent significantly more time on the territory than females (F =9.051, df= 1,356, p < 0.01; Tukey p<0.05). Both gulls spent significantly more time on the territory together during the pre-egg stage than they did during the egg stage (F = 44.161, df= 1,178, p<0.01) and the territory was left unattended more often during the pre-egg stage than the egg stage (F = 19.770, df = 1,178, p<0.01). This last result was also found in 1983 (Figure 8.1) (F = 42.283, df= 1,155, p<0.01) but there were no other significant differences found in that year during these stages.

In both 1983 (Figure 8.2) and 1984 (Figure 8.4) both males and females went to and from the territory more often during the pre-egg stage than the egg stage (1983 - F = 10.233, df= 1,310, p<0.01; 1984 - F =37.780, df=1,356, p < 0.01; Tukey p < 0.05). During the egg stage of 1984 only, males were found to go to and from the territory significantly more often than females (1984 - F =9.839, df= 1,356, p < 0.01; Tukey < 0.05; 1983 - F = 0.154, df= 1,310, p > 0.05).

In 1983 (Figure 8.1) a comparison between the results obtained from gulls observed during the egg stage and the chick stage shows that both males and females spent more time on the territory during the egg stage than the chick stage (F = 10.568, df= 1,558, p < 0.01; Tukey p < 0.05) but there is no significant difference between the time males and females spent on the territory during either stage (F = 2.054, df= 1,558, p>0.05). In 1984 (Figure 8.3), however, there is an interaction between the sex of the territory holder and the season (F = 6.209, df=1,320, p<0.05). Females spent less time on the territory during the egg stage than their partners but more time during the chick stage (Tukey p < 0.05)